Keywords: X chromosome
D. melanogaster meiotic driver Stellate compromises sperm development by impeding a process of nuclear envelope remodeling
35390Xuefeng Meng, Yukiko M Yamashita,, Genetics, 2026-01-23 09:55:19.
Meiotic drive is a phenomenon that violates Mendel’s Law of Equal Segregation, leading to biased transmission of the meiotic driver to the offspring. D. melanogaster Stellate (Ste) is an X-linked meiotic driver that preferentially harms Y-chromosome-bearing spermatids, thereby favoring the transmission of the X chromosome to the next generation. We have recently shown that Ste protein segregates asymmetrically during meiosis I with a strong bias toward the Y-chromosome-inheriting side, leading to the eventual demise of the Y-chromosome-containing spermatids. However, the cellular mechanisms by which Ste protein interferes with spermatid development remain unknown. Here, we show that Ste-containing spermatids are delayed in the process of nuclear envelope remodeling, an essential process during sperm DNA compaction. We show that components of the nuclear lamina (such as Lamin Dm0, and the LEM domain proteins Otefin and Bocks) are rapidly removed during nuclear envelope remodeling during the early stages of normal spermatid development. However, Ste-containing spermatids retained these nuclear lamina proteins for a prolonged time. Their delayed removal is associated with defective formation of the dense complex, which is composed of a bundle of microtubules and serves as a structural support for sperm nuclear morphogenesis. Defective dense complex formation in Ste-containing spermatids led to defective sperm DNA compaction. Together, the present study reveals an unexpected cellular mechanism by which a meiotic driver, Ste, sabotages sperm development.
Pervasive suppressors halt the spread of selfish Segregation Distorter in a natural population
35247Ching-Ho Chang, Tyler Handler, Nick Fuda, et al., bioRxiv, 2025-10-16 09:21:01.
Meiotic drivers are selfish genetic elements that subvert Mendelian inheritance to increase their own transmission, yet they are typically found at low frequencies across natural populations. The factors that limit their spread remain unclear. To investigate this paradox, we studied the Segregation Distorter (SD) system, a selfish coadapted gene complex in Drosophila melanogaster. SD biases its transmission by killing sperm carrying a homologous chromosome bearing a target locus, Responder (Rsp), which appear as satellite repeats. Such selfish killing impairs male fertility and imposes selective pressure on the host genome to evolve resistance, either by deleting Rsp copies or acquiring unlinked suppressors. To characterize the spectrum of Rsp alleles and the frequency of segregating suppressors, we surveyed 90 strains from the Drosophila Genome Reference Panel. Rather than loss of Rsp, we found that over half of the strains (52/90) harbor suppressors located on the X chromosome or autosomes, but not the Y chromosome. The widespread presence of strong suppressors limited the resolution of our genome-wide association mapping; however, recombination analysis identified a strong X-linked suppressor to a ~300 kb interval on the chromosome. Together, our findings suggest that pervasive, multilocus suppression constrains the spread of SD in natural populations.
Rapid protamine evolution suppresses meiotic drive in Drosophila
35159Ching-Ho Chang, Aida Flor de la Cruz, Isabel Mejia Natividad, Alex Noyola, Harmit S. Malik, bioRxiv, 2025-09-22 10:55:08.
Many animal species replace histones with protamines during spermatogenesis. Despite their importance for sperm function, protamines rapidly evolve in many species; the biological causes behind their rapid evolution remain unknown. Here, using in vivo gene replacement, we investigated the causes and consequences underlying the rapid evolution of protamine Mst77F, which is essential for male fertility in D. melanogaster. Mst77F ortholog replacements led to defects in DNA compaction of X-chromosome-bearing sperm compared to Y-chromosome-bearing sperm during spermatogenesis, resulting in fewer X-bearing mature sperm and male-biased progeny. Unlike D. melanogaster, Mst77F is not essential for male fertility in D. yakuba but is still required to suppress sex-ratio distortion. Our results suggest that relentless pressure to suppress sex chromosomal meiotic drive drives the rapid evolution of protamines.
Single-cell consequences of X-linked meiotic drive in stalk-eyed flies
35155Price PD, Parkus SM, Lloyd VJ, Alston BT, Bradshaw SL, Bates S, et al., PLoS Genet, 21. 2025-09-22 10:33:47.
Meiotic drivers, a class of selfish gene, are frequently located on sex chromosomes and have dramatic impacts on gamete development. However, our understanding of their molecular consequences for gamete production and sex chromosome regulation has focused on a handful of model organisms. In this study, we use single-cell RNA-sequencing approaches to produce a single-cell atlas of the testis of the stalk-eyed fly, Teleopsis dalmanni. This species harbours an X-linked meiotic driver where drive males produce more than 90% female offspring. First, we generate a comprehensive profile of the cellular and transcriptional landscape of spermatogenesis. We show limited evidence for meiotic sex chromosome inactivation and unique patterns of dosage compensation across spermatogenesis, relative to both other dipterans and insects in general. Finally, by comparing single-cell expression data between standard and drive males, we show that although there are significant differences in genome regulation, broad expression dynamics in the testis are conserved in the presence of meiotic drive. Notably, we highlight key genes with perturbed expression as a potential consequence of the disruption of spermatogenesis by the X-linked meiotic driver.
Functional validation of a white pupae minimal gene construct in Ceratitis capitata
34835Lucas Henrique Figueiredo Prates, Roswitha A. Aumann, Inga Sievers, et al., Insect Science, 2025-05-12 19:41:58.
Genetic sexing strains (GSS) are important tools for the sterile insect technique (SIT), an environmentally friendly and species-specific insect pest control method. GSS feature sex-specific phenotypes, enabling sex sorting in mass-rearing facilities and male-only releases, which significantly improve the cost-effectiveness and efficiency of SIT programs. In classical GSS, sex linkage of marker gene(s), such as white pupae (wp), is achieved through an irradiation-induced translocation between the marker-carrying autosome and the Y chromosome. However, this approach may render GSS males semisterile. The recently proposed neo-classical GSS concept suggests using genome editing to achieve sex linkage by directly inserting the wild-type marker allele onto the Y chromosome, potentially yielding GSS males with higher fertility. In this study, we examined the Ceratitis capitata wp gene as a genetic marker for the neo-classical GSS concept and developed a minimal, intronless version of this gene, termed mini-wp. We demonstrate that a single copy of mini-wp is sufficient to restore the wild-type brown puparium phenotype and is functional when integrated at various positions within the C. capitata genome, including the X chromosome. Due to its smaller size (4689 bp, including 2000 bp of putative promoter region) relative to the full wild-type wp allele (20868 bp), mini-wp may facilitate its precise insertion into the Y chromosome, representing an important step toward realizing neo-classical GSS. Furthermore, the methodology developed for designing and testing mini-wp in medfly may be adapted to other Tephritid species with an identified wp gene.
A selfish supergene causes meiotic drive through both sexes in Drosophila
34805G.L. Keais,C.M. Saad-Roy,E. Gonzalez-Sqalli, et al., Proceedings of the National Academy of Sciences, 122. 2025-04-25 12:15:11.
Meiotic drivers are selfish genetic elements that bias their own transmission during meiosis or gamete formation. Due to the fundamental differences between male and female meiosis in animals and plants, meiotic drivers operate through distinct mechanisms in the two sexes: In females, they exploit the asymmetry of meiosis to ensure their inclusion in the egg, whereas in males, they eliminate competing gametes after symmetric meiosis. Meiotic drive is commonly reported in males, where it strongly influences the evolution of spermatogenesis, while the few known cases in females have highlighted its crucial role in centromere evolution. Despite a growing number of examples in a wide range of organisms, meiotic drive has so far only been observed in one sex or the other since its discovery nearly 100 y ago. Here, we show that a selfish X chromosome known to cause meiotic drive in male Drosophila testacea flies also causes meiotic drive in females. We find that this X chromosome has supergene architecture, harboring extensive structural rearrangements that suppress recombination between the two X chromosomes. This has contributed to a substantial expansion of its size compared to the wild-type chromosome, partly due to the accumulation of species-specific repetitive elements. Our findings suggest that female meiotic drive may play an important role in the evolutionary dynamics of polymorphic structural variants that suppress recombination, including inversions, translocations, and supergenes.
Reenacting a mouse genetic evolutionary arms race in yeast reveals that SLXL1/SLX compete with SLY1/2 for binding to Spindlins
34550M.F. Arlt,A.N. Kruger,C.M. Swanepoel,& J.L. Mueller, Proceedings of the National Academy of Sciences, 122. 2025-03-04 10:54:14.
In the house mouse, an evolutionary arms race between proteins encoded by the X-linked Slxl1/Slx and Y-linked Sly gene families during spermatogenesis can distort offspring sex ratio, but how these proteins compete remains unknown. We report how SLXL1/SLX competes with SLY1/SLY2 by demonstrating their dose-dependent competitive binding to Spindlins, the key protein domains and rapidly evolving residues and exons that drive the competition, and how the competition is likely between complex multimers. Our findings have broad implications for the mechanics of evolutionary arms and how competition between sex chromosomes influences X- versus Y-sperm fitness and sex ratio.
Meiotic Drive and Speciation
34057Jeremy B. Searle and Fernando Pardo-Manuel de Villena, Annual Review of Genetics, 58:341-363. 2024-12-16 15:22:27.
Meiotic drive is the biased transmission of alleles from heterozygotes, contrary to Mendel's laws, and reflects intragenomic conflict rather than organism-level Darwinian selection. Theory has been developed as to how centromeric properties can promote female meiotic drive and how conflict between the X and Y chromosomes in males can promote male meiotic drive. There are empirical data that fit both the centromere drive and sex chromosome drive models. Sex chromosome drive may have relevance to speciation through the buildup of Dobzhansky-Muller incompatibilities involving drive and suppressor systems, studied particularly in Drosophila. Centromere drive may promote fixation of chromosomal rearrangements involving the centromere, and those fixed rearrangements may contribute to reproductive isolation, studied particularly in the house mouse. Genome-wide tests suggest that meiotic drive promotes allele fixation with regularity, and those studying the genomics of speciation need to be aware of the potential impact of such fixations on reproductive isolation. New species can originate in many different ways (including multiple factors acting together), and a substantial body of work on meiotic drive point to it being one of the processes involved.
Stalk-eyed flies carrying a driving X chromosome compensate by increasing fight intensity
32712Kimberly A. Paczolt, Macy E. Pritchard, Gabrielle T. Welsh, et al., Frontiers in Ethology, 3. 2024-10-22 17:39:40.
Exaggerated ornaments provide opportunities to understand how selection can operate at different levels to shape the evolution of a trait. While these features aid their bearer in attracting mates or fending off competitors, they can also be costly and influenced by the environment and genetic variation. The eyestalks of the stalk-eyed fly, Teleopsis dalmanni, are of interest because eyestalk length is the target of both intra- and intersexual selection and is also reduced by loci on a highly-divergent sex ratio X chromosome (XSR), a meiotic driver accounting for up to 30% of wild X chromosomes. Male stalk-eyed flies fight to control access to females and over food using a combination of low-intensity displays and high-intensity physical fights. We staged, filmed, and scored contests between pairs of eyespan-matched males to evaluate whether X chromosome type impacts the behavior and outcome of aggressive interactions. While our results broadly match expectations from previous studies, we found that XSR males used more high-intensity behaviors than males carrying a non-driving, standard X chromosome (XST), particularly when their eyestalks were of similar size or smaller than their opponents. Additionally, we found that when XSR males use high-intensity behaviors, they win more bouts than when they use low-intensity behaviors. Taken together, these results suggest that XSR impacts male aggressive behavior to compensate for the shorter eyestalks of XSR males and may help to explain how this selfish chromosome is maintained.
Non-Mendelian transmission of X chromosomes: mechanisms and impact on sex ratios and population dynamics in different breeding systems
31181Sally Adams; Andre Pires-daSilva, Biochemical Society Transactions, 2024-08-25 20:35:26.
The non-Mendelian transmission of sex chromosomes during gametogenesis carries significant implications, influencing sex ratios and shaping evolutionary dynamics. Here we focus on known mechanisms that drive non-Mendelian inheritance of X chromosomes during spermatogenesis and their impact on population dynamics in species with different breeding systems. In Drosophila and mice, X-linked drivers targeting Y-bearing sperm for elimination or limiting their fitness, tend to confer unfavourable effects, prompting the evolution of suppressors to mitigate their impact. This leads to a complex ongoing evolutionary arms race to maintain an equal balance of males and females. However, in certain insects and nematodes with XX/X0 sex determination, the preferential production of X-bearing sperm through atypical meiosis yields wild-type populations with highly skewed sex ratios, suggesting non-Mendelian transmission of the X may offer selective advantages in these species. Indeed, models suggest X-meiotic drivers could bolster population size and persistence under certain conditions, challenging the conventional view of their detrimental effects. Furthering our understanding of the diverse mechanisms and evolutionary consequences of non-Mendelian transmission of X chromosomes will provide insights into genetic inheritance, sex determination, and population dynamics, with implications for fundamental research and practical applications.
Could X Centromeric Drive Underpin PCOS, Miscarriage and Other Age-Related Reproductive Issues?
31176Rhianna-lily Smith, Technology Networks, 2024-08-25 20:13:11.
One of the most prevalent conditions is polycystic ovary syndrome (PCOS), affecting 8-13% of women of reproductive age. Characterized by irregular menstrual cycles, excess androgen levels and ovarian cysts, PCOS is one of the leading causes of infertility. Despite its commonality, the underlying causes of the condition remain poorly understood. Dr. Tom Moore, a principal investigator in the School of Biochemistry and Cell Biology at University College Cork, has proposed a new hypothesis that may explain why human female reproduction is prone to multiple disorders. The X chromosome’s involvement in meiotic drive mechanisms could be a key factor in understanding why certain reproductive disorders are so common and persistent. Meiotic drive refers to a genetic phenomenon where certain genes manipulate the process of meiosis to increase their transmission to the next generation, often at the expense of other genes. In normal meiosis, genes have a 50% chance of being passed on to offspring, but meiotic drive skews this process, allowing "selfish" genes to be inherited more frequently than would be expected by chance. Due to genetic hitchhiking, this can introduce traits that may have detrimental effects on individual fitness. Antagonistic selection is an evolutionary concept where a genetic trait, which is beneficial in one context but harmful in another, may be passed on to a future generation. This typically occurs when a gene has different effects at different stages of life or under different environmental conditions. In the context of human evolution, antagonistic selection can help explain why certain reproductive disorders, like PCOS, might persist.
The haplolethal gene wupA of Drosophila exhibits potential as a target for an X-poisoning gene drive
28845Clancy D. Lawler, Ana Karla Parra Nuñez, Natalia Hernandes, Soumitra Bhide, Isabelle Lohrey, Simon Baxter, Charles Robin, bioRxiv, 2024-01-28 12:54:44.
A synthetic gene drive that targets haplolethal genes on the X-chromosome can skew the sex ratio towards males. Like an ‘X-shredder’ it does not involve ‘homing’ and that has advantages including the reduction of gene drive resistance allele formation. We examine this ‘X-poisoning’ strategy by targeting four of the 11 known X-linked haplolethal/haplosterile genes of Drosophila melanogaster with CRISPR/Cas9. We find that targeting the wupA gene during spermatogenesis skews the sex ratio so fewer than 14% of progeny are daughters. That is unless we cross the mutagenic males to X^XY female flies that bear attached-X chromosomes, which reverses the inheritance of the poisoned X chromosome so that sons inherit it from their father; in which case only 2% of the progeny are sons. These sex ratio biases suggests that most of the CRISPR/Cas9 mutants we induced in the wupA gene are haplolethal but some are recessive lethal. The males generating wupA mutants do not suffer from reduced fertility rather the haplolethal mutants arrest development in the late stages of embryogenesis well after fertilized eggs have been laid. This provides a distinct advantage over genetic manipulation strategies involving sterility which can be countered by the remating of females. We also find that wupA mutants that destroy the nuclear localization signal of shorter isoforms are not haplolethal as long as the open reading frame remains intact. Like D. melanogaster wupA orthologs of D. suzukii and Anopheles mosquitos are found on X chromosomes making wupA a viable X-poisoning target in multiple species.
Introgression of a synthetic sex ratio distortion transgene into different genetic backgrounds of Anopheles coluzzii
23760P. Pollegioni, T. Persampieri, R. L. Minuz, A. Bucci, A. Trusso, S. Di Martino, C. Leo, M. Bruttini, M. Ciolfi, A. M. Waldvogel, F. Tripet, A. Simoni, A. Crisanti and R. Müller, Insect Molecular Biology, 2022-10-17 06:14:10.
The development of genetically modified mosquitoes (GMM) and their subsequent field release offers innovative approaches for vector control of malaria. A non-gene drive self-limiting male-bias Ag(PMB)1 strain has been developed in a 47-year-old laboratory G3 strain of Anopheles gambiae s.l. When Ag(PMB)1 males are crossed to wild-type females, expression of the endonuclease I-PpoI during spermatogenesis causes the meiotic cleavage of the X chromosome in sperm cells, leading to fertile offspring with a 95% male bias. However, WHO states that the functionality of the transgene could differ when inserted in different genetic backgrounds of An. coluzzii which is currently a predominant species in several West-African countries and thus a likely recipient for a potential release of self-limiting GMMs. In this study, we introgressed the transgene from the donor Ag(PMB)1 by six serial backcrosses into two recipient colonies of An. coluzzii that had been isolated in Mali and Burkina Faso. Scans of informative SNP markers and whole-genome sequencing analysis revealed a nearly complete introgression of chromosome 3 and X, but a remarkable genomic divergence in a large region of the chromosome 2 between the later backcrossed (BC6) transgenic offspring and the recipient paternal strains. These findings suggested to extend the backcrossing breeding strategy beyond BC6 generation and increase the introgression efficiency of critical regions that have ecological and epidemiological implications through the targeted selection of specific markers. Disregarding of differential introgression efficiency, we concluded that the phenotype of the sex ratio distorter is stabile in the BC6 introgressed An. coluzzii strains. This article is protected by copyright. All rights reserved.
Anopheles homing suppression drive candidates exhibit unexpected performance differences in simulations with spatial structure
23734S. E. Champer, I. K. Kim, A. G. Clark, P. W. Messer and J. Champer, eLife, 11:e79121. 2022-10-14 06:37:55.
Recent experiments have produced several Anopheles gambiae homing gene drives that disrupt female fertility genes, thereby eventually inducing population collapse. Such drives may be highly effective tools to combat malaria. One such homing drive, based on the zpg promoter driving CRISPR/Cas9, was able to eliminate a cage population of mosquitoes. A second version, purportedly improved upon the first by incorporating an X-shredder element (which biases inheritance towards male offspring), was similarly successful. Here, we analyze experimental data from each of these gene drives to extract their characteristics and performance parameters and compare these to previous interpretations of their experimental performance. We assess each suppression drive within an individual-based simulation framework that models mosquito population dynamics in continuous space. We find that the combined homing/X-shredder drive is actually less effective at population suppression within the context of our mosquito population model. In particular, the combined drive often fails to completely suppress the population, instead resulting in an unstable equilibrium between drive and wild-type alleles. By contrast, otherwise similar drives based on the nos promoter may prove to be more promising candidates for future development than originally thought.
Hoisted with his own petard: How sex-ratio meiotic drive in Drosophila affinis creates resistance alleles that limit its spread
23471W. J. Ma, E. M. Knoles, K. B. Patch, M. M. Shoaib and R. L. Unckless, J Evol Biol, 2022-08-23 07:13:52.
Meiotic drivers are selfish genetic elements that tinker with gametogenesis to bias their own transmission into the next generation of offspring. Such tinkering can have significant consequences on gametogenesis and end up hampering the spread of the driver. In Drosophila affinis, sex-ratio meiotic drive is caused by an X-linked complex that, when in males with a susceptible Y chromosome, results in broods that are typically more than 95% female. Interestingly, D. affinis males lacking a Y chromosome (XO) are fertile and males with the meiotic drive X and no Y produce only sons-effectively reversing the sex-ratio effect. Here, we show that meiotic drive dramatically increases the rate of nondisjunction of the Y chromosome (at least 750X), meaning that the driver is creating resistant alleles through the process of driving. We then model how the O might influence the spread, dynamics and equilibrium of the sex-ratio X chromosome. We find that the O can prevent the spread or reduce the equilibrium frequency of the sex-ratio X chromosome, and it can even lead to oscillations in frequency. Finally, with reasonable parameters, the O is unlikely to lead to the loss of the Y chromosome, but we discuss how it might lead to sex-chromosome turnover indirectly.
Slow and steady wins the race: spatial and stochastic processes and the failure of suppression gene drives
23117J. F. Paril and B. L. Phillips, Molecular Ecology, 2022-07-05 08:47:10.
Gene drives that skew sex ratios offer a new management tool to suppress or eradicate pest populations. Early models and empirical work suggest that these suppression drives can completely eradicate well-mixed populations, but models that incorporate stochasticity and space (i.e., drift, and recolonization events) often result in loss or failure of the drive. We developed a stochastic model to examine these processes in a simple 1-dimensional space. This simple space allows us to map the events and outcomes that emerged and examine how properties of the drive's wave of invasion affect outcomes. Our simulations, across a biologically-realistic section of parameter space, suggest that drive failure might be a common outcome in spatially explicit, stochastic systems, and that properties of the drive wave appear to mediate outcomes. Surprisingly, the drives that would be considered fittest in an aspatial model were strongly associated with failure in the spatial setting. The fittest drives cause relatively fast moving, and narrow waves that have a high chance of being penetrated by wild-types leading to wild-type recolonization, leading to failure. Our results also show that high rates of dispersal reduce the chance of failure because drive waves get disproportionately wider than wild-type waves as dispersal rates increase. Overall, wide, slow-moving drive waves were much less prone to failure. Our results point to the complexity inherent in using a genetic system to effect demographic outcomes and speak to a clear need for ecological and evolutionary modelling to inform the drive design process.
Hoisted with his own petard: how sex-ratio meiotic drive in <em>Drosophila affinis</em> creates resistance alleles that limit its spread
23231W.-J. Ma, E. M. Knoles, K. B. Patch, M. M. Shoaib and R. L. Unckless, bioRxiv, 2022.02.14.480432. 2022-06-19 06:28:31.
Meiotic drivers are selfish genetic elements that tinker with gametogenesis to bias their own transmission into the next generation of offspring. Such tinkering can have significant consequences on gametogenesis and end up hampering the spread of the driver. In Drosophila affinis, sex-ratio meiotic drive is caused by an X-linked complex that, when in males with a susceptible Y chromosome, results in broods that are typically more than 95% female. Interestingly, D. affinis males lacking a Y chromosome (XO) are fertile and males with the meiotic drive X and no Y produce only sons - effectively reversing the sex-ratio effect. Here, we show that meiotic drive dramatically increases the rate of nondisjunction of the Y chromosome (at least 750X), meaning that the driver is creating resistant alleles through the process of driving. We then model how the O might influence the spread, dynamics and equilibrium of the sex-ratio X chromosome. We find that the O can prevent the spread or reduce the equilibrium frequency of the sex-ratio X chromosome and it can even lead to oscillations in frequency. Finally, with reasonable parameters, the O is unlikely to lead to the loss of the Y chromosome, but we discuss how it might lead to sex-chromosome turnover indirectly.Competing Interest StatementThe authors have declared no competing interest.
Selective targeting of biting females to control mosquito-borne infectious diseases
22953B. B. Kojin, A. Compton, Z. N. Adelman and Z. Tu, Trends in Parasitology, 2022-06-13 06:48:00.
Mosquitoes are vectors for a number of infectious diseases. Only females feed on blood to provision for their embryos and, in doing so, transmit pathogens to the associated vertebrate hosts. Therefore, sex is an important phenotype in the context of genetic control programs, both for sex separation in the rearing facilities to avoid releasing biting females and for ways to distort the sex ratio towards nonbiting males. We review recent progress in the fundamental knowledge of sex determination and sex chromosomes in mosquitoes and discuss new methods to achieve sex separation and sex ratio distortion to help control mosquito-borne infectious diseases. We conclude by suggesting a few critical areas for future research.
Finding the strongest gene drive: Simulations reveal unexpected performance differences between Anopheles homing suppression drive candidates
21566S. E. Champer, I. K. Kim, A. G. Clark, P. W. Messer and J. Champer, bioRxiv, 2022.03.28.486009. 2022-03-28 12:29:28.
Recent experiments have produced several Anopheles gambiae homing gene drives that disrupt female fertility genes, thereby eventually inducing population collapse. Such drives may be highly effective tools to combat malaria. One such homing drive, based on the zpg promoter driving CRISPR/Cas9, was able to eliminate a cage population of mosquitoes. A second version, purportedly improved upon the first by incorporating an X-shredder element (which biases inheritance towards male offspring), was similarly successful. Here, we re-analyze the data of each of these gene drives and suggest an alternative interpretation of their performance. We assess each suppression drive within an individual-based simulation framework that models mosquito population dynamics in continuous space. We find that the combined homing/X-shredder drive is actually less effective at population suppression within the context of our mosquito population model. In particular, the combined drive often fails to completely suppress the population, instead resulting in an unstable equilibrium between drive and wild-type alleles. By contrast, otherwise similar drives based on the nos promoter may prove to be more promising candidates for future development due to potentially superior performance.Competing Interest StatementThe authors have declared no competing interest.
CRISPR-Cas9 effectors facilitate generation of single-sex litters and sex-specific phenotypes
19469C. Douglas, V. Maciulyte, J. Zohren, D. M. Snell, S. K. Mahadevaiah, O. A. Ojarikre, P. J. I. Ellis and J. M. A. Turner, Nature Communications, 12:6926. 2021-12-03 15:06:30.
Animals are essential genetic tools in scientific research and global resources in agriculture. In both arenas, a single sex is often required in surplus. The ethical and financial burden of producing and culling animals of the undesired sex is considerable. Using the mouse as a model, we develop a synthetic lethal, bicomponent CRISPR-Cas9 strategy that produces male- or female-only litters with one hundred percent efficiency. Strikingly, we observe a degree of litter size compensation relative to control matings, indicating that our system has the potential to increase the yield of the desired sex in comparison to standard breeding designs. The bicomponent system can also be repurposed to generate postnatal sex-specific phenotypes. Our approach, harnessing the technological applications of CRISPR-Cas9, may be applicable to other vertebrate species, and provides strides towards ethical improvements for laboratory research and agriculture.
Flavors of Non-Random Meiotic Segregation of Autosomes and Sex Chromosomes
18819F. Pajpach, T. Wu, L. Shearwin-Whyatt, K. Jones and F. Grützner, Genes, 12. 2021-08-28 15:33:36.
Segregation of chromosomes is a multistep process occurring both at mitosis and meiosis to ensure that daughter cells receive a complete set of genetic information. Critical components in the chromosome segregation include centromeres, kinetochores, components of sister chromatid and homologous chromosomes cohesion, microtubule organizing centres, and spindles. Based on the cytological work in the grasshopper Brachystola, it has been accepted for decades that segregation of homologs at meiosis is fundamentally random. This ensures that alleles on chromosomes have equal chance to be transmitted to progeny. At the same time mechanisms of meiotic drive and an increasing number of other examples of non-random segregation of autosomes and sex chromosomes provide insights into the underlying mechanisms of chromosome segregation but also question the textbook dogma of random chromosome segregation. Recent advances provide a better understanding of meiotic drive as a prominent force where cellular and chromosomal changes allow autosomes to bias their segregation. Less understood are mechanisms explaining observations that autosomal heteromorphism may cause biased segregation and regulate alternating segregation of multiple sex chromosome systems or translocation heterozygotes as an extreme case of non-random segregation. We speculate that molecular and cytological mechanisms of non-random segregation might be common in these cases and that there might be a continuous transition between random and non-random segregation which may play a role in the evolution of sexually antagonistic genes and sex chromosome evolution.
Red queen’s race: rapid evolutionary dynamics of an expanding family of meiotic drive factors and their hpRNA suppressors
17957J. Vedanayagam, C.-J. Lin and E. C. Lai, bioRxiv, 2021.08.05.454923. 2021-08-06 16:51:10.
Meiotic drivers are a class of selfish genetic elements that are widespread across eukaryotes. Their activities are often detrimental to organismal fitness and thus trigger drive suppression to ensure fair segregation during meiosis. Accordingly, their existence is frequently hidden in genomes, and their molecular functions are little known. Here, we trace evolutionary steps that generated the Dox meiotic drive system in Drosophila simulans (Dsim), which distorts male:female balance (sex-ratio) by depleting male progeny. We show that Dox emerged via stepwise mobilization and acquisition of portions of multiple D. melanogaster genes, including the sperm chromatin packaging gene protamine. Moreover, we reveal novel Dox homologs in Dsim and massive, recent, amplification of Dox superfamily genes specifically on X chromosomes of its closest sister species D. mauritiana (Dmau) and D. sechellia (Dsech). The emergence of Dox superfamily genes is tightly associated with 1.688 family satellite repeats that flank de novo genomic copies. In concert, we find coordinated emergence and diversification of autosomal hairpin RNA/siRNAs loci that target subsets of Dox superfamily genes across simulans clade species. Finally, an independent set of protamine amplifications the Y chromosome of D. melanogaster indicates that protamine genes are frequent and recurrent players in sex chromosome dynamics. Overall, we reveal fierce genetic arms races between meiotic drive factors and siRNA suppressors associated with recent speciation.Competing Interest StatementThe authors have declared no competing interest.
Mice Plague Eastern Australia in Record Numbers
17751B. Nogrady, The Scientist, 2021-07-12 13:50:17.
Just before Christmas last year, Julie Leven and her husband Des took their camper up to visit their son in northern New South Wales, Australia. Driving back at night to their home in Gilgandra, around 430 kilometers northwest of Sydney, they saw masses of white spots moving across the dark road surface. The spots, they soon realized, were mice. Once they reached their house, the Levens saw a scene of rodent devastation. Mice had invaded their home in such numbers that it was unlivable. The creatures had gnawed their way into the pantry and ruined all the food they could get into. Their droppings and pungent urine were spread from one end of the dwelling to the other, across soft furnishings and bedding. The rodents had even eaten the insulation around the engine wiring in two tractors and ruined their harvested hay bales.
Selfish DNA: how new gene technology could stop the advance of mice
17450M. McMillan, Tentenfield Star, 2021-06-15 17:20:08.
It used to be that seeing a mouse in the house was a rare occurrence. Now, it's rarely a day that goes by where we aren't seeing or hearing the little vermin. Current methods of baiting and trapping are struggling to control the plague of mice spreading across regional Australia. But a $1.8 million investment from the NSW government might soon give us a new weapon in the war. The government is investing in research into the use of gene drives, or "selfish DNA" - a genetic tool that can help us to control pests. How? Well, to understand gene drives we first need to understand the normal way in which genes are inherited. Mice, like humans, have two copies of each gene, one inherited from their mother and one from their father. We call these copies alleles, and they can be exactly the same or slightly different from each other. Normally, there is a 50/50 chance as to which allele will be passed on to any offspring. If one allele carries some sort of mutation, there is a 50 per cent chance that it will be passed on.
Victory: NSW Government Invests in Humane Mice Control!
17224PETA Australia, PETA Australia, 2021-06-05 16:21:19.
Just two weeks after calling us “brainless” for suggesting that the state government invest in more ethical, eco-friendly methods of mice control – Minister for Agriculture Adam Marshall announced a $1.8 million package to “fast-track the delivery of next generation ‘gene drive’ technology to control future plagues”. The money will fund a three-year programme of genetic biocontrol research, led by the University of Adelaide, CSIRO, and the Centre for Invasive Species Solutions, to identify fast-acting gene drives designed to spread an inherited characteristic through a population. The research will test two strategies for population control, including an approach which eliminates sperm carrying the X chromosome, producing more male than female offspring, and a second approach of making female mice infertile. PETA has been talking about immunocontraception methods of controlling invasive animal populations for years, so we’re pleased to see the government is finally using science to tackle this problem in a more ethical and eco-friendly manner. Had it acted sooner, millions of small animals, including non-target species, would have been spared slow and agonising deaths. Last month, the New South Wales government announced it would use a new, strong poison and spruiked it as “napalm” for mice. Leading rodent experts questioned the plan, warning that the poison’s use came with a high risk of killing native and domestic animals as well. Dr Peter Brown, leader of the rodent management research team at CSIRO, told The Guardian, “The anti-coagulants can accumulate up through the food chain, and so birds of prey or other animals can be feeding on dead mice and they could potentially get a lethal dose themselves through secondary poisoning.”
Gene drive could be a game changer for future mouse control.
17214Anonymous, Centre for Invasive Species Solutions, 2021-06-03 16:02:28.
We are proud to announce we will be coordinating a brand new, three-year program of genetic biocontrol research, which will identify fast acting gene drives designed to spread an inherited characteristic through a population at higher-than-normal rates. Using targeted gene drives, scientists aim to interrupt the breeding cycle of mice and potentially other ferals, which could keep populations at manageable levels. The $1.8 million research program will be led by Professor Paul Thomas at our partner organisation the University of Adelaide in collaboration with our member organisation CSIRO. The NSW Minister for Agriculture, Adam Marshall said cutting edge solutions meant future mouse plagues could be extinguished before they begin. This specific research funding will test two strategies for population control and recommend at least one for future suppression of mice. The ‘X-shredder’ approach eliminates sperm carrying the X chromosome, producing more male than female offspring. The ‘female infertility’ approach spreads a genetic modification that would eventually make females infertile. We look forward to seeing the outcomes of this world-class innovative research being led by Aussie scientists.
“Gene Drive” Technology To Control Mouse Invasions | Liverpool City Champion
17208T. Carrington, Liverpool IL, 2021-06-03 15:45:57.
As western New South Wales faces a devastating mouse plague, the government is investing in groundbreaking genetic biocontrol research that could transform pest management in Australia. Agriculture Minister Adam Marshall said the NSW government will provide $ 1.8 million for the project to accelerate delivery of next-generation ‘gene drive’ technology to control future plagues. “The government has invested $ 50 million in a range of supportive measures, not only to mitigate the impacts of the mice that are currently crawling across much of New South Wales, but also to create options to reduce l ‘impact of future population peaks, “he said. Thursday. Until now, farmers had to rely on baiting and trapping to control mouse infestations, but the government is now “accelerating critical research to bring mouse control into the 21st century,” he said. declared. The three-year genetic biocontrol research program will identify fast-acting gene drives that are designed to spread an inherited trait through a population at above normal rates.
Australia plots biological warfare to eradicate rampaging ‘mouse plague’
17204J. Smyth, Financial Times, 2021-06-03 15:37:11.
Australia is home to some of the world’s most fearsome creatures. But none is more destructive than the humble house mouse, a plague of which is rampaging across vast swaths of farmland and terrorising countryfolk. Farmers in New South Wales, the worst affected state, warned the furry critters could cost them A$1bn ($765m) in lost crops and poison baits this season. Residents in rural towns have been fighting a six-month battle against the army of wild house mice, which has gnawed through wiring on home appliances, polluted water supplies and even bitten patients in hospital beds. Scientists said the plague was bolstered by favourable weather conditions after years of drought and the nation’s second biggest grain harvest on record. State authorities have proposed “napalming” the mice by allowing farmers to use the poison bromadiolone against the mice, which has ignited a furious debate over its environmental impact.
Mouse plague control hopes raised with funding for genetic biocontrol research
17289Anonymous, From Press, 2021-06-03 10:36:25.
As communities and farmers continue to battle the mouse plague, a funding announcement for genetic biocontrol research could be a potential game changer for future plagues. The New South Wales government has today announced a $50 million mouse control package which includes $1.8 million dollars in funding for genetic control of mice populations. The project aims to fast-track the delivery of next generation "gene drive" technology to control plagues of the future. Researchers have welcomed the announcement, including Australia's lead researcher Professor Paul Thomas from the University of Adelaide. He said the technology is only relatively new, having been developed to some extent for insects and malaria control, but has not yet been applied to mammals. "So effectively it just uses the natural mating processes to spread a gene though a population that will cause, [and] what we are trying to cause, female [mouse] infertility," he said. "We have modelled it already and that should cause the population to crash over time. "This boost of funding will enable us to move much faster on these projects."Another control approach will also be investigated, the "X-shredder" approach, which eliminates sperm carrying the X chromosome, producing more male than female offspring.
‘Gene drive’ tech to control mice plagues
17219AAP, Countryman, 2021-06-02 16:11:50.
As western NSW deals with a devastating mouse plague the government is investing in breakthrough genetic biocontrol research that could transform pest management in Australia. Agriculture Minister Adam Marshall said the NSW government would provide $1.8 million to the project to fast-track the delivery of next generation 'gene drive' technology to control future plagues. "The government has invested $50 million in a range of support measures, not only to mitigate the impacts of the mice currently crawling across so much of NSW, but also to create options to ensure we reduce the impact of future population spikes," he said on Thursday. Until now farmers have had to rely on baiting and trapping to control mouse infestations but the government was now "fast-tracking critical research to bring mouse control into the 21st century", he said. The three-year program of genetic biocontrol research will identify fast acting gene drives which are designed to spread an inherited characteristic through a population at higher-than-normal rates. Mr Marshall said it would also investigate the transferability of the technology to other pest species such as black rats, rabbits and feral cats using advanced computer modelling.
Major fly pest genetically modified in lab to produce more males
16835H. Dunning, Imperial College London, 2021-04-23 14:29:07.
It has been predicted that the world's population will increase to over nine billion people by 2050, and that global food production will need to increase by around 70 percent to match this rate of change. Lead researcher Dr Angela Meccariello, from the Department of Life Sciences at Imperial, said: “Currently, medfly infestations are estimated to contribute towards a loss of up $298 US million annually due to crop damage across a wide geographic area and in over 250 different species of fruits and vegetables. “Due to factors such as climate change, the spread of invasive species and pesticide resistance, there is potential for the negative impact of the medfly on global agriculture to increase if left unchecked. We therefore need new technologies to fight these pests, and our modification could be one such tool.” The team’s modification works by using a DNA-cutting enzyme to destroy the X chromosome during the production of sperm, leading to predominantly male offspring, as females require two Xs. In their experiments, they managed to produce populations of the flies that were 80% male.
Engineered sex ratio distortion by X-shredding in the global agricultural pest Ceratitis capitata
16837A. Meccariello, F. Krsticevic, R. Colonna, G. Del Corsano, B. Fasulo, P. A. Papathanos and N. Windbichler, BMC Biology, 19:78. 2021-04-16 14:29:21.
Genetic sex ratio distorters are systems aimed at effecting a bias in the reproductive sex ratio of a population and could be applied for the area-wide control of sexually reproducing insects that vector disease or disrupt agricultural production. One example of such a system leading to male bias is X-shredding, an approach that interferes with the transmission of the X-chromosome by inducing multiple DNA double-strand breaks during male meiosis. Endonucleases targeting the X-chromosome and whose activity is restricted to male gametogenesis have recently been pioneered as a means to engineer such traits.
Modeling impact and cost-effectiveness of gene drives for malaria elimination in the Democratic Republic of the Congo
16477N. Metchanun, C. Borgemeister, G. Amzati, J. von Braun, M. Nikolov, P. Selvaraj and J. Gerardin, medRxiv, 2020.06.29.20142760. 2021-02-22 13:51:04.
Using a spatially explicit, agent-based model of malaria transmission in eight representative provinces of the Democratic Republic of the Congo, we predict the impact and cost-effectiveness of integrating driving-Y gene drive mosquitoes in malaria elimination strategies that include existing interventions such as insecticide-treated nets and case management of symptomatic malaria. Gene drive mosquitoes could eliminate malaria and were the most cost-effective intervention overall if the drive component was highly effective with at least 95% X-shredding and associated cost of deployment below 7.17 $int per person per year. Suppression gene drive could be a cost-effective supplemental intervention for malaria elimination, but tight constraints on drive effectiveness and cost ceilings may limit its feasibility.
The New Yorker Magazine: Gene Drives as a Tool for Saving Nature
16277E. Heber, Island Conservation, 2021-01-03 17:07:04.
In a recent New Yorker Magazine article, entitled “CRISPR and the Splice to Survive,” journalist and best-selling author Elizabeth Kolbert dives into the world of gene drive research. She touches on aspects of gene drive research from altering the toxin produced by cane toads to recovering nearly-extinct trees to eradicating invasive mice through attrition, all to understand the possibilities this tool could hold.
Selfish genetic elements and male fertility
15183R. L. Verspoor, T. A. R. Price and N. Wedell, Philosophical Transactions of the Royal Society B-Biological Sciences, 375:7. 2020-10-20 14:40:03.
Selfish genetic elements (SGEs) are diverse and near ubiquitous in Eukaryotes and can be potent drivers of evolution. Here, we discuss SGEs that specifically act on sperm to gain a transmission advantage to the next generation. The diverse SGEs that affect sperm often impose costs on carrier males, including damaging ejaculates, skewing offspring sex ratios and in particular reducing sperm-competitive success of SGE-carrying males. How males and females tolerate and mitigate against these costs is a dynamic and expanding area of research. The intense intra-genomic conflict that these selfish elements generate could also have implications for male fertility and spermatogenesis more widely. This article is part of the theme issue 'Fifty years of sperm competition'.
Role of gene drives in malaria elimination strategy: modeling impact and cost-effectiveness in the Democratic Republic of the Congo
12729N. Metchanun, C. Borgemeister, J. von Braun, M. Nikolov, P. Selvaraj and J. Gerardin, medRxiv, 2020-06-29 13:15:39.
The tremendous burden of malaria has led to renewed efforts on malaria elimination and the development of novel tools for application where existing tools fall short. Gene drive mosquitoes, where transgenes and their associated phenotypes are efficiently propagated to future generations, are under development to suppress vector populations or render vectors incapable of malaria transmission. However, the role of gene drives in an integrated elimination strategy is underexplored. Using a spatially explicit agent-based model of malaria transmission in the Democratic Republic of the Congo, we describe the impact of integrating a population suppression driving-Y gene drive into malaria elimination strategies. We find that as long as the driving-Y construct is extremely effective, releases of gene drive mosquitoes can eliminate malaria, and we identify a cost ceiling for gene drive to be cost-effective relative to existing tools. Vector control via gene drive is worth considering as a supplemental intervention when the construct parameters and costs are suitable.
Genetic Biocontrol – An Overview (video 13.13 min)
11398GeneConvene Global Collaborative, 2020-04-25 12:41:00.
This video explains what genetic biocontrol is and surveys various technologies that can be consider genetic biocontrol technologies. It offers a conceptual organization of the various technologies based on the potential of genetic biocontrol organisms to persist and spread in an environment. This video was produced by the GeneConvene Global Colaborative.
Autosomal suppression and fitness costs of an old driving X chromosome in Drosophila testacea
7363G. Keais, S. Lu and S. Perlman, Journal of Evolutionary Biology, 2020-01-28 21:58:18.
Driving X chromosomes (XDs) are meiotic drivers that bias their own transmission through males by killing Y-bearing gametes. These chromosomes can in theory spread rapidly in populations and cause extinction, but many are found as balanced polymorphisms or as ?cryptic? XDs shut down by drive suppressors. The relative likelihood of these outcomes, as well as the evolutionary pathways through which they come about, are not well-understood. An XD was recently discovered in the mycophagous fly, Drosophila testacea, presenting the opportunity to compare this XD with the well-studied XD of its sister species, Drosophila neotestacea. Comparing features of independently evolved XDs in young sister species is a promising avenue towards understanding how XDs and their counteracting forces change over time. In contrast to the XD of D. neotestacea, we find that the XD of D. testacea is old, with its origin predating the radiation of three species: D. testacea, D. neotestacea, and their shared sister species, Drosophila orientacea. Motivated by the suggestion that older XDs should be more deleterious to carriers, we assessed the effect of the XD on both male and female fertility. Unlike what is known from D. neotestacea, we found a strong fitness cost in females homozygous for the XD in D. testacea: a large proportion of homozygous females failed to produce offspring after being housed with males for several days. Our male fertility experiments show that while XD male fertility is lower under sperm depleting conditions, XD males have comparable fertility to males carrying a standard X chromosome under a free mating regime, which may better approximate conditions in wild populations of D. testacea. Lastly, we demonstrate the presence of autosomal suppression of X chromosome drive. Our results provide support for a model of XD evolution where the dynamics of young XDs are governed by fitness consequences in males, whereas in older XD systems, both suppression and fitness consequences in females likely supersede male fitness costs.
Fitness consequences of a non-recombining sex-ratio drive chromosome can explain its prevalence in the wild
5797Dyer, K. A. and D. W. Hall, Proceedings of the Royal Society B: Biological Sciences, 286:20192529. 2019-12-19 14:27:34.
Understanding the pleiotropic consequences of gene drive systems on host fitness is essential to predict their spread through a host population. Here, we study sex-ratio (SR) X-chromosome drive in the fly Drosophila recens, where SR causes the death of Y-bearing sperm in male carriers. SR males only sire daughters, which all carry SR, thus giving the chromosome a transmission advantage. The prevalence of the SR chromosome appears stable, suggesting pleiotropic costs. It was previously shown that females homozygous for SR are sterile, and here, we test for additional fitness costs of SR. We found that females heterozygous for SR have reduced fecundity and that male SR carriers have reduced fertility in conditions of sperm competition. We then use our fitness estimates to parametrize theoretical models of SR drive and show that the decrease in fecundity and sperm competition performance can account for the observed prevalence of SR in natural populations. In addition, we found that the expected equilibrium frequency of the SR chromosome is particularly sensitive to the degree of multiple mating and performance in sperm competition. Together, our data suggest that the mating system of the organism should be carefully considered during the development of gene drive systems.
The potential for a released autosomal X-shredder becoming a driving-Y chromosome and invasively suppressing wild populations of malaria mosquitoes
5648Alcalay, Y., S. Fuchs, R. Galizi, F. Bernardini, R. E. Haghighat-Khah, D. B. Rusch, J. R. Adrion, M. W. Hahn, P. Tortosa and P. A. Papathanos, bioRxiv, 2019:860551. 2019-12-17 17:51:02.
Synthetic sex-ratio distorters based on X-chromosome shredding are predicted to be more efficient than sterile males for population suppression of malaria mosquitoes using genetic control. X chromosome shredding operates through the targeted elimination of X-chromosome-bearing gametes during male spermatogenesis, resulting in males that have a high fraction of male offspring. Strains harboring autosomal constructs containing a modified endonuclease I-PpoI have now been developed in the malaria mosquito Anopheles gambiae, resulting in strong sex-ratio distortion towards males. Data are being gathered for these strains for submission of regulatory dossiers for contained use and subsequent field release in West Africa. Since autosomal X shredders are transmitted in a Mendelian fashion and can be selected against their frequency in the population is expected to decline once releases are halted. However, any unintended transfer of the X-shredder to the Y-chromosome could theoretically change these dynamics: This could lead to 100% transmission of the newly Y-linked X-shredder to the predominant male-biased offspring and its insulation from negative selection in females, resulting in its potential spread in the population and ultimately to suppression. Here, we analyze plausible mechanisms whereby an autosomal X-shredder could become linked to the Y-chromosome after release and provide data regarding its potential for activity should it become linked to the Y-chromosome. Our results strongly suggest that Y-chromosome linkage through remobilization of the transposon used for the initial genetic transformation is unlikely, and that, in the unexpected event that the X-shredder becomes linked to the Y-chromosome, expression and activity of the X-shredder would likely be inhibited by meiotic sex chromosome inactivation. We conclude that a functioning X-shredding based Y-drive resulting from a naturally induced transposition or translocation of the transgene onto the Y-chromosome is unlikely.
Genetic Control of Mosquitoes
5588Alphey, L., Annual Review of Entomology, 59:205-224. 2019-12-16 19:01:55.
Genetics can potentially provide new, species-specific, environmentally friendly methods for mosquito control. Genetic control strategies aim either to suppress target populations or to introduce a harm-reducing novel trait. Different approaches differ considerably in their properties, especially between self-limiting strategies, where the modification has limited persistence, and self-sustaining strategies, which are intended to persist indefinitely in the target population and may invade other populations. Several methods with different molecular biology are under development and the first field trials have been completed successfully.
A Y-chromosome shredding gene drive for controlling pest vertebrate populations
3938Prowse, TAAA, F.; Cassey, P.; Thomas, P.; Ross, J. V., eLife, 8:19. 2019-01-16 00:00:00.
Self-replicating gene drives that modify sex ratios or infer a fitness cost could be used to control populations of invasive alien species. The targeted deletion of Y sex chromosomes using CRISPR technology offers a new approach for sex bias that could be incorporated within gene-drive designs. We introduce a novel gene-drive strategy termed Y-CHromosome deletion using Orthogonal Programmable Endonucleases (Y-CHOPE), incorporating a programmable endonuclease that 'shreds' the Y chromosome, thereby converting XY males into fertile XO females. Firstly, we demonstrate that the CRISPR/Cas12a system can eliminate the Y chromosome in embryonic stem cells with high efficiency (c. 90%). Next, using stochastic, individual-based models of a pest mouse population, we show that a Y-shredding drive that progressively depletes the pool of XY males could effect population eradication through mate limitation. Our molecular and modeling data suggest that a Y-CHOPE gene drive could be a viable tool for vertebrate pest control.
A genetic system for biasing the sex ratio in mice
3952Yosef, IEB, Liat; Globus, Rea; Shlomovitz, Inbar; Munitz, Ariel; Gerlic, Motti; Qimron, Udi, EMBO reports, 20:e48269. 2019-01-10 00:00:00.
Biasing the sex ratio of populations of different organisms, including plants, insects, crustacean, and fish, has been demonstrated by genetic and non-genetic approaches. However, biasing the sex ratio of mammalian populations has not been demonstrated genetically. Here, we provide a first proof of concept for such a genetic system in mammals by crossing two genetically engineered mouse lines. The maternal line encodes a functional Cas9 protein on an autosomal chromosome, whereas the paternal line encodes guide RNAs on the Y chromosome targeting vital mouse genes. After fertilization, the presence of both the Y-encoded guide RNAs from the paternal sperm and the Cas9 protein from the maternal egg targets the vital genes in males. We show that these genes are specifically targeted in males and that this breeding consequently self-destructs solely males. Our results pave the way for a genetic system that allows biased sex production of livestock.
Genetic manipulation of sex ratio in mammals: the Reaper comes for Mickey
3947Smanski, MJZ, David, EMBO reports, 20:e48577. 2019-01-05 00:00:00.
In most animals, sexual reproduction results in a 1:1 ratio of females to males. For several sectors of agriculture, for example, milk or egg production, only a single sex is needed. Biasing the sex ratio so that only offspring of the desired sex are produced has the potential to increase breeding efficiency. In this issue of EMBO Reports, Yosef et al [1] demonstrate a genetic approach to bias the sex ratio in mice by specifically disrupting essential genes in male embryos. Their approach is an important first step toward generating sex-ratio biasing applications for agriculture
Gene drive to reduce malaria transmission in sub-Saharan Africa
3963Burt, AC, Mamadou; Crisanti, Andrea; Diabate, Abdoulaye; Kayondo, Jonathan K., Journal of Responsible Innovation, 5:S66-S80. 2018-01-21 00:00:00.
Despite impressive progress, malaria continues to impose a substantial burden of mortality and morbidity, particularly in sub-Saharan Africa, and new tools will be needed to achieve elimination. Gene drive is a natural process by which some genes are inherited at a greater-than-Mendelian rate and can spread through a population even if they cause harm to the organisms carrying them. Many different synthetic gene drive systems have been proposed to suppress the number of mosquitoes and/or reduce vector competence. As with any control measure, due attention should be paid to the possible evolution of resistance. No gene drive construct has yet been reported that is "field-ready" for release, and when such constructs are developed, they should be assessed on a case-by-case basis. Gene drive approaches to vector control promise to have a number of key features that motivate their continued development, and scrutiny, by all concerned.
Gene drive: Evolved and synthetic
3964Burt, AC, Andrea, ACS Chemical Biology, 13:343-346. 2018-01-02 00:00:00.
Drive is a process of accelerated inheritance from one generation to the next that allows some genes to spread rapidly through populations even if they do not contribute to—or indeed even if they detract from—organismal survival and reproduction. Genetic elements that can spread by drive include gametic and zygotic killers, meiotic drivers, homing endonuclease genes, B chromosomes, and transposable elements. The fact that gene drive can lead to the spread of fitness-reducing traits (including lethality and sterility) makes it an attractive process to consider exploiting to control disease vectors and other pests. There are a number of efforts to develop synthetic gene drive systems, particularly focused on the mosquito-borne diseases that continue to plague us.
Introduction of a male-harming mitochondrial haplotype via ‘Trojan Females’ achieves population suppression in fruit flies
4074Wolff, JNG, N. J.; Tompkins, D. M.; Dowling, D. K., eLife, 6:e23551. 2017-01-12 00:00:00.
Pests are a global threat to biodiversity, ecosystem function, and human health. Pest control approaches are thus numerous, but their implementation costly, damaging to non-target species, and ineffective at low population densities. The Trojan Female Technique (TFT) is a prospective self-perpetuating control technique that is species-specific and predicted to be effective at low densities. The goal of the TFT is to harness naturally occurring mutations in the mitochondrial genome that impair male fertility while having no effect on females. Here, we provide proof-of-concept for the TFT, by showing that introduction of a male fertility-impairing mtDNA haplotype into replicated populations of Drosophila melanogaster causes numerical population suppression, with the magnitude of effect positively correlated with its frequency at trial inception. Further development of the TFT could lead to establishing a control strategy that overcomes limitations of conventional approaches, with broad applicability to invertebrate and vertebrate species, to control environmental and economic pests.
X chromosome drive in a widespread Palearctic woodland fly, Drosophila testacea
4052Keais, GLH, M. A.; Gowen, B. E.; Perlman, S. J., Journal of Evolutionary Biology, 30:1185-1194. 2017-01-10 00:00:00.
Selfish genes that bias their own transmission during meiosis can spread rapidly in populations, even if they contribute negatively to the fitness of their host. Driving X chromosomes provide a clear example of this type of selfish propagation. These chromosomes have important evolutionary and ecological consequences, and can be found in a broad range of taxa including plants, mammals and insects. Here, we report a new case of X chromosome drive (X drive) in a widespread woodland fly, Drosophila testacea. We show that males carrying the driving X (SR males) sire 80-100% female offspring and possess a diagnostic X chromosome haplotype that is perfectly associated with the sex ratio distortion phenotype. We find that the majority of sons produced by SR males are sterile and appear to lack a Y chromosome, suggesting that meiotic defects involving the Y chromosome may underlie X drive in this species. Abnormalities in sperm cysts of SR males reflect that some spermatids are failing to develop properly, confirming that drive is acting during gametogenesis. By screening wild-caught flies using progeny sex ratios and a diagnostic marker, we demonstrate that the driving X is present in wild populations at a frequency of similar to 10% and that suppressors of drive are segregating in the same population. The testacea species group appears to be a hot spot for X drive, and D. testacea is a promising model to compare driving X chromosomes in closely related species, some of which may even be younger than the chromosomes themselves.
B Chromosomes – A matter of chromosome drive
4048Houben, A, Frontiers in Plant Science, 8:210. 2017-01-06 00:00:00.
B chromosomes are supernumerary chromosomes which are often preferentially inherited, deviating from usual Mendelian segregation. The balance between the so-called chromosome drive and the negative effects that the presence of Bs applies on the fitness of their host determines the frequency of Bs in a particular population. Drive is the key for understanding most B chromosomes. Drive occurs in many ways at pre-meiotic, meiotic or post-meiotic divisions, but the molecular mechanism remains unclear. The cellular mechanism of drive is reviewed based on the findings obtained for the B chromosomes of rye, maize and other species. How novel analytical tools will expand our ability to uncover the biology of B chromosome drive is discussed.
Intragenomic conflict produces sex ratio dynamics that favor maternal sex ratio distorters
4099Rood, ESF, S., Ecology and Evolution, 6:8085-8093. 2016-01-17 00:00:00.
Maternal sex ratio distorters (MSDs) are selfish elements that enhance their transmission by biasing their host's sex allocation in favor of females. While previous models have predicted that the female-biased populations resulting from sex ratio distortion can benefit from enhanced productivity, these models neglect Fisherian selection for nuclear suppressors, an unrealistic assumption in most systems. We used individual-based computer simulation modeling to explore the intragenomic conflict between sex ratio distorters and their suppressors and explored the impacts of these dynamics on population-level competition between species characterized by MSDs and those lacking them. The conflict between distorters and suppressors was capable of producing large cyclical fluctuations in the population sex ratio and reproductive rate. Despite fitness costs associated with the distorters and suppressors, MSD populations often exhibited enhanced productivity and outcompeted non-MSD populations in single and multiple-population competition simulations. Notably, the conflict itself is beneficial to the success of populations, as sex ratio oscillations limit the competitive deficits associated with prolonged periods of male rarity. Although intragenomic conflict has been historically viewed as deleterious to populations, our results suggest that distorter-suppressor conflict can provide population-level advantages, potentially helping to explain the persistence of sex ratio distorters in a range of taxa.
Occasional recombination of a selfish X-chromosome may permit its persistence at high frequencies in the wild
4096Pieper, KED, K. A., Journal of Evolutionary Biology, 29:2229-2241. 2016-01-14 00:00:00.
The sex-ratio X-chromosome (SR) is a selfish chromosome that promotes its own transmission to the next generation by destroying Y-bearing sperm in the testes of carrier males. In some natural populations of the fly Drosophila neotestacea, up to 30% of the X-chromosomes are SR chromosomes. To investigate the molecular evolutionary history and consequences of SR, we sequenced SR and standard (ST) males at 11 X-linked loci that span the ST X-chromosome and at seven arbitrarily chosen autosomal loci from a sample of D. neotestacea males from throughout the species range. We found that the evolutionary relationship between ST and SR varies among individual markers, but genetic differentiation between SR and ST is chromosome-wide and likely due to large chromosomal inversions that suppress recombination. However, SR does not consist of a single multilocus haplotype: we find evidence for gene flow between ST and SR at every locus assayed. Furthermore, we do not find long-distance linkage disequilibrium within SR chromosomes, suggesting that recombination occurs in females homozygous for SR. Finally, polymorphism on SR is reduced compared to that on ST, and loci displaying signatures of selection on ST do not show similar patterns on SR. Thus, even if selection is less effective on SR, our results suggest that gene flow with ST and recombination between SR chromosomes may prevent the accumulation of deleterious mutations and allow its long-term persistence at relatively high frequencies.
Rapid evolution of a Y-chromosome heterochromatin protein underlies sex chromosome meiotic drive
4088Helleu, QG, P. R.; Dubruille, R.; Ogereau, D.; Prud'homme, B.; Loppin, B.; Montchamp-Moreau, C., Proceedings of the National Academy of Sciences of the United States of America, 113:4110-4115. 2016-01-06 00:00:00.
Sex chromosome meiotic drive, the non-Mendelian transmission of sex chromosomes, is the expression of an intragenomic conflict that can have extreme evolutionary consequences. However, the molecular bases of such conflicts remain poorly understood. Here, we show that a young and rapidly evolving X-linked heterochromatin protein 1 (HP1) gene, HP1D2, plays a key role in the classical Paris sex-ratio (SR) meiotic drive occurring in Drosophila simulans. Driver HP1D2 alleles prevent the segregation of the Y chromatids during meiosis II, causing female-biased sex ratio in progeny. HP1D2 accumulates on the heterochromatic Y chromosome in male germ cells, strongly suggesting that it controls the segregation of sister chromatids through heterochromatin modification. We show that Paris SR drive is a consequence of dysfunctional HP1D2 alleles that fail to prepare the Y chromosome for meiosis, thus providing evidence that the rapid evolution of genes controlling the heterochromatin structure can be a significant source of intragenomic conflicts.
The ability to gain matings, not sperm competition, reduces the success of males carrying a selfish genetic element in a fly
4105Verspoor, RLH, G. D. D.; Price, T. A. R., Animal Behaviour, 115:207-215. 2016-01-03 00:00:00.
Females are expected to avoid low-quality males fathering their offspring. X chromosome meiotic drive (XCMD) makes males very low-quality mates. XCMDs are X chromosomes that, in males, cause the failure of all Y chromosome sperm, so all functional sperm carry the driving X and produce daughters. This transmission advantage can allow the XCMD to spread through populations. However, XCMD males typically have low fertility, are very poor at sperm competition, only produce daughters, and bear low fitness alleles associated with XCMD. This imposes significant costs on females that mate with these males. Recently, several studies have shown that females can reduce the risk of their offspring inheriting XCMDs by mating with multiple males (polyandry), as XCMD males typically lose out to normal males in sperm competition. Hence it has been suggested that increased polyandry may be likely to evolve whenever a costly XCMD is common in a population, and that polyandry may be a key factor in preventing XCMDs spreading through populations. We tested this by examining the fruit fly Drosophila subobscura, in which females are known to mate only once in European populations where XCMDs are absent. However, in North African populations an XCMD, referred to as 'SRs', occurs. If the association between XCMDs and polyandry is true, then these North African populations should have evolved polyandry. However, we found no evidence of polyandry in North African D. subobscura populations. Instead, we found some evidence that males that carry SRs are slightly less successful at gaining matings in noncompetitive situations. These results show that polyandry does not necessarily evolve in response to the presence of harmful X chromosome drive. With both sperm competition and female choice both being unlikely to substantially reduce the success of XCMD in D. subobscura, the factors that prevent SRs spreading through these populations remain mysterious. (C) 2016 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Sex chromosome drive
4117Helleu, QG, P. R.; Montchamp-Moreau, C., Cold Spring Harbor Perspectives in Biology, 7:a017616. 2015-01-15 00:00:00.
Sex chromosome drivers are selfish elements that subvert Mendel's first law of segregation and therefore are over represented among the products of meiosis. The sex-biased progeny produced then fuels an extended genetic conflict between the driver and the rest of the genome. Many examples of sex chromosome drive are known, but the occurrence of this phenomenon is probably largely underestimated because of the difficulty to detect it. Remarkably, nearly all sex chromosome drivers are found in two clades, Rodentia and Diptera. Although very little is known about the molecular and cellular mechanisms of drive, epigenetic processes such as chromatin regulation could be involved in many instances. Yet, its evolutionary consequences are far-reaching, from the evolution of mating systems and sex determination to the emergence of new species.
On the origin of sex chromosomes from meiotic drive
4127Ubeda, FP, M. M.; Wild, G., Proceedings of the Royal Society B-Biological Sciences, 282:20141932. 2015-01-05 00:00:00.
Most animals and many plants make use of specialized chromosomes (sex chromosomes) to determine an individual's sex. Best known are the XY and ZW sex-determination systems. Despite having evolved numerous times, sex chromosomes present something of an evolutionary puzzle. At their origin, alleles that dictate development as one sex or the other (primitive sex chromosomes) face a selective penalty, as they will be found more often in the more abundant sex. How is it possible that primitive sex chromosomes overcome this disadvantage? Any theory for the origin of sex chromosomes must identify the benefit that outweighs this cost and enables a sex-determining mutation to establish in the population. Here we show that a new sex-determining allele succeeds when linked to a sex-specific meiotic driver. The new sex-determining allele benefits from confining the driving allele to the sex in which it gains the benefit of drive. Our model requires few special assumptions and is sufficiently general to apply to the evolution of sex chromosomes in outbreeding cosexual or dioecious species. We highlight predictions of the model that can discriminate between this and previous theories of sex-chromosome origins.
Genetic Control of Mosquitoes.
5597Alphey, L., Annual Review of Entomology, 59:205-224. 2014-12-17 15:50:24.
Genetics can potentially provide new, species-specific, environmentally friendly methods for mosquito control. Genetic control strategies aim either to suppress target populations or to introduce a harm-reducing novel trait. Different approaches differ considerably in their properties, especially between self-limiting strategies, where the modification has limited persistence, and self-sustaining strategies, which are intended to persist indefinitely in the target population and may invade other populations. Several methods with different molecular biology are under development and the first field trials have been completed successfully.
Biased transmission of sex chromosomes in the aphid Myzus persicae is not associated with reproductive mode
4160Wilson, ACCD, R. N.; Vorburger, C., PLOS One, 9:1-12. 2014-01-18 00:00:00.
Commonly, a single aphid species exhibits a wide range of reproductive strategies including cyclical parthenogenesis and obligate parthenogenesis. Sex determination in aphids is chromosomal; females have two X chromosomes, while males have one. X chromosome elimination at male production is generally random, resulting in equal representation of both X chromosomes in sons. However, two studies have demonstrated deviations from randomness in some lineages. One hypothesis to account for such deviations is that recessive deleterious mutations accumulate during bouts of asexual reproduction and affect male viability, resulting in overrepresentation of males with the least deleterious of the two maternal X chromosomes. This hypothesis results in a testable prediction: X chromosome transmission bias will increase with time spent in the asexual phase and should therefore be most extreme in the least sexual aphid life cycle class. Here we test this prediction in Myzus persicae. We used multiple heterozygous X-linked microsatellite markers to screen 1085 males from 95 lines of known life cycle. We found significant deviations from equal representation of X chromosomes in 15 lines; however, these lines included representatives of all life cycles. Our results are inconsistent with the hypothesis that deviations from randomness are attributable to mutation accumulation.
Sex-ratio meiotic drive and interspecific competition
4159Unckless, RLC, A. G., Journal of Evolutionary Biology, 27:1513-1521. 2014-01-17 00:00:00.
It has long been known that processes occurring within a species may impact the interactions between species. For example, as competitive ability is sensitive to parameters including reproductive rate, carrying capacity and competition efficiency, the outcome of interspecific competition may be influenced by any process that alters these attributes. Although several such scenarios have been discussed, the influence of selfish genetic elements within one species on competition between species has not received theoretical treatment. We show that, with strong competition, sex-ratio meiotic drive systems can result in a significant shift in community composition because the effective birth rate in the population may be increased by a female-biased sex ratio. Using empirical data, we attempt to estimate the magnitude of this effect in several Drosophila species. We infer that meiotic drive elements, selfish genetic elements within species, can provide a substantial competitive advantage to that species within a community.
Male eyespan size is associated with meiotic drive in wild stalk-eyed flies (Teleopsis dalmanni)
4138Cotton, AJF, M.; Cotton, S.; Pomiankowski, A., Heredity, 112:363-369. 2014-01-16 00:00:00.
This study provides the first direct evidence from wild populations of stalk-eyed flies to support the hypothesis that male eyespan is a signal of meiotic drive. Several stalk-eyed fly species are known to exhibit X-linked meiotic drive. A recent quantitative trait locus analysis in Teleopsis dalmanni found a potential link between variation in male eyespan, a sexually selected ornamental trait, and the presence of meiotic drive. This was based on laboratory populations subject to artificial selection for male eyespan. In this study, we examined the association between microsatellite markers and levels of sex ratio bias (meiotic drive) in 12 wild T. dalmanni populations. We collected two data sets: (a) brood sex ratios of wild-caught males mated to standard laboratory females and (b) variation in a range of phenotypic traits associated with reproductive success of wild-caught males and females. In each case, we typed individuals for eight X-linked microsatellite markers, including several that previously were shown to be associated with male eyespan and meiotic drive. We found that one microsatellite marker was very strongly associated with meiotic drive, whereas a second showed a weaker association. We also found that, using both independent data sets, meiotic drive was strongly associated with male eyespan, with smaller eyespan males being associated with more female-biased broods. These results suggest that mate preference for exaggerated male eyespan allows females to avoid mating with males carrying the meiotic drive gene and is thus a potential mechanism for the maintenance and evolution of female mate preference.
Meiotic drive impacts expression and evolution of X-linked genes in stalk-eyed flies
4155Reinhardt, JAB, C. L.; Paczolt, K. A.; Johns, P. M.; Baker, R. H.; Wilkinson, G. S., PLOS Genetics, 10:e1004362. 2014-01-13 00:00:00.
Although sex chromosome meiotic drive has been observed in a variety of species for over 50 years, the genes causing drive are only known in a few cases, and none of these cases cause distorted sex-ratios in nature. In stalk-eyed flies (Teleopsis dalmanni), driving X chromosomes are commonly found at frequencies approaching 30% in the wild, but the genetic basis of drive has remained elusive due to reduced recombination between driving and non-driving X chromosomes. Here, we used RNAseq to identify transcripts that are differentially expressed between males carrying either a driving X (X-SR) or a standard X chromosome (X-ST), and found hundreds of these, the majority of which are X-linked. Drive-associated transcripts show increased levels of sequence divergence (dN/dS) compared to a control set, and are predominantly expressed either in testes or in the gonads of both sexes. Finally, we confirmed that X-SR and X-ST are highly divergent by estimating sequence differentiation between the RNAseq pools. We found that X-linked transcripts were often strongly differentiated (whereas most autosomal transcripts were not), supporting the presence of a relatively large region of recombination suppression on X-SR presumably caused by one or more inversions. We have identified a group of genes that are good candidates for further study into the causes and consequences of sex-chromosome drive, and demonstrated that meiotic drive has had a profound effect on sequence evolution and gene expression of X-linked genes in this species.
Evolution and biology of supernumerary B chromosomes
4144Houben, AB-M, A. M.; Klemme, S.; Timmis, J. N., Cellular and Molecular Life Sciences, 71:467-478. 2014-01-02 00:00:00.
B chromosomes (Bs) are dispensable components of the genome exhibiting non-Mendelian inheritance and have been widely reported on over several thousand eukaryotes, but still remain an evolutionary mystery ever since their first discovery over a century ago [1]. Recent advances in genome analysis have significantly improved our knowledge on the origin and composition of Bs in the last few years. In contrast to the prevalent view that Bs do not harbor genes, recent analysis revealed that Bs of sequenced species are rich in gene-derived sequences. We summarize the latest findings on supernumerary chromosomes with a special focus on the origin, DNA composition, and the non-Mendelian accumulation mechanism of Bs.
Elimination of Y chromosome-bearing spermatids during spermiogenesis in an autosomal sex-ratio mutant of Drosophila simulans
4183Yasuno, YI, Y. H.; Yamamoto, M. T., Genes & Genetic Systems, 88:113-126. 2013-01-21 00:00:00.
Sex ratio distortion, which is commonly abbreviated as sex-ratio, has been studied in many Drosophila species, but the mechanism remains largely unknown. Here, we report on the sex-ratio mutant of D. simulans named excess of females (exf). The third chromosomal recessive mutation results in a sex ratio of approximately 0.2 or less (males/total). Cytological observation demonstrated that meiosis appeared to be completed normally, but that most Y chromosome-bearing nuclei failed to elongate during spermiogenesis, as revealed by fluorescence in situ hybridization using sex chromosome-specific probes. These aberrant nuclei contained membranous inclusions as revealed by electron microscopic analysis. Most of the aberrant exf spermatids failed to individualize and mature, suggesting that a later stage of spermiogenesis is involved in prevention of production of sperm with abnormal morphology. On the one hand, in exf seminal vesicles, sperm nuclei with a length of 5-8.5 mu m were occasionally observed, in addition to those with wild-type sperm dimensions, that is, a length of approximately 10 mu m. Thus, spermatids with less severe nuclear defects can escape elimination and be released into the seminal vesicles as mature sperm. Furthermore, we constructed His2AvD-GFP and ProtamineB-eGFP transgenic lines in D. simulans, and examined the processes involved in replacement of chromatin proteins over a time course, according to nuclear morphology. We found that both normal and abnormal sperm heads demonstrated equal chromatin replacement during late spermiogenesis. Our results suggest that exf belongs to a unique class of meiotic drive systems in that (1) intranuclear membranous inclusions cause failure of nuclear shaping of Y-bearing spermatids without affecting the histone-protamine transition, and (2) a portion of the aberrant spermatids differentiate into mature sperm; these are transferred to and stored by females.
Natural variation of the Y chromosome suppresses sex ratio distortion and modulates testis-specific gene expression in Drosophila simulans
4167Branco, ATT, Y.; Hartl, D. L.; Lemos, B., Heredity, 111:8-15. 2013-01-05 00:00:00.
X-linked sex-ratio distorters that disrupt spermatogenesis can cause a deficiency in functional Y-bearing sperm and a female-biased sex ratio. Y-linked modifiers that restore a normal sex ratio might be abundant and favored when a X-linked distorter is present. Here we investigated natural variation of Y-linked suppressors of sex-ratio in the Winters systems and the ability of these chromosomes to modulate gene expression in Drosophila simulans. Seventy-eight Y chromosomes of worldwide origin were assayed for their resistance to the X-linked sex-ratio distorter gene Dox. Y chromosome diversity caused males to sire similar to 63% to similar to 98% female progeny. Genome-wide gene expression analysis revealed hundreds of genes differentially expressed between isogenic males with sensitive (high sex ratio) and resistant (low sex ratio) Y chromosomes from the same population. Although the expression of about 75% of all testis-specific genes remained unchanged across Y chromosomes, a subset of post-meiotic genes was upregulated by resistant Y chromosomes. Conversely, a set of accessory gland-specific genes and mitochondrial genes were downregulated in males with resistant Y chromosomes. The D. simulans Y chromosome also modulated gene expression in XXY females in which the Y-linked protein-coding genes are not transcribed. The data suggest that the Y chromosome might exert its regulatory functions through epigenetic mechanisms that do not require the expression of protein-coding genes. The gene network that modulates sex ratio distortion by the Y chromosome is poorly understood, other than that it might include interactions with mitochondria and enriched for genes expressed in post-meiotic stages of spermatogenesis.
Local dynamics of a fast-evolving sex-ratio system in Drosophila simulans
4166Bastide, HG, P. R.; Ogereau, D.; Cazemajor, M.; Montchamp-Moreau, C., Molecular Ecology, 22:5352-5367. 2013-01-04 00:00:00.
By distorting Mendelian transmission to their own advantage, X-linked meiotic drive elements can rapidly spread in natural populations, generating a sex-ratio bias. One expected consequence is the triggering of a co-evolutionary arms race between the sex chromosome that carries the distorter and suppressors counteracting its effect. Such an arms race has been theoretically and experimentally established and can have many evolutionary consequences. However, its dynamics in contemporary populations is still poorly documented. Here, we investigate the fate of the young X-linked Paris driver in Drosophila simulans from sub-Saharan Africa to the Middle East. We provide the first example of the early dynamics of distorters and suppressors: we find consistent evidence that the driving chromosomes have been rising in the Middle East during the last decade. In addition, identical haplotypes are at high frequencies around the two co-evolving drive loci in remote populations, implying that the driving X chromosomes share a recent common ancestor and suggesting that East Africa could be the cradle of the Paris driver. The segmental duplication associated with drive presents an unusual structure in West Africa, which could reflect a secondary state of the driver. Together with our previous demonstration of driver decline in the Indian Ocean where suppression is complete, these data provide a unique picture of the complex dynamics of a co-evolutionary arms race currently taking place in natural populations of D.simulans.
Local selection underlies the geographic distribution of sex-ratio drive in Drosophila neotestacea
4186Dyer, KA, Evolution, 66:973-984. 2012-01-04 00:00:00.
Selfish genetic elements promote their own transmission to the next generation, often at a cost to the host individual. A sex-ratio (SR) driving X chromosome prevents the maturation of Y-bearing sperm, and as a result is transmitted to 100% of the offspring, all of which are female. Because the spread of a SR chromosome can result in a female-biased population sex ratio, the ecological and evolutionary consequences of harboring this selfish element can be severe. In this study, we show that the prevalence of SR drive in Drosophila neotestacea varies between 0% and 30% among populations, and is common in the south whereas rare in the north. The prevalence of SR is not associated with the presence of suppressors of drive, geographic distance, or genetic distance based on autosomal microsatellite loci. Instead, our results indicate that ecological selection on SR drive varies among populations, as the prevalence of SR is highly correlated with climatic factors, with the severity of winter the best determinant of SR frequency. Thus, ecological and demographic factors may have significant consequences for the short and long term evolutionary dynamics of selfish elements and the manner with which they coevolve with the rest of the genome.
Scrambling eggs: Meiotic drive and the evolution of female recombination rates
4185Brandvain, YC, G., Genetics, 190:709-723. 2012-01-03 00:00:00.
Theories to explain the prevalence of sex and recombination have long been a central theme of evolutionary biology. Yet despite decades of attention dedicated to the evolution of sex and recombination, the widespread pattern of sex differences in the recombination rate is not well understood and has received relatively little theoretical attention. Here, we argue that female meiotic drivers-alleles that increase in frequency by exploiting the asymmetric cell division of oogenesis-present a potent selective pressure favoring the modification of the female recombination rate. Because recombination plays a central role in shaping patterns of variation within and among dyads, modifiers of the female recombination rate can function as potent suppressors or enhancers of female meiotic drive. We show that when female recombination modifiers are unlinked to female drivers, recombination modifiers that suppress harmful female drive can spread. By contrast, a recombination modifier tightly linked to a driver can increase in frequency by enhancing female drive. Our results predict that rapidly evolving female recombination rates, particularly around centromeres, should be a common outcome of meiotic drive. We discuss how selection to modify the efficacy of meiotic drive may contribute to commonly observed patterns of sex differences in recombination.
Reduced polymorphism associated with X chromosome meiotic drive in the stalk-eyed fly Teleopsis dalmanni
4202Christianson, SJB, C. L.; Wilkinson, G. S., PLOS One, 6:e27254. 2011-01-20 00:00:00.
Sex chromosome meiotic drive has been suggested as a cause of several evolutionary genetic phenomena, including genomic conflicts that give rise to reproductive isolation between new species. In this paper we present a population genetic analysis of X chromosome drive in the stalk-eyed fly, Teleopsis dalmanni, to determine how this natural polymorphism influences genetic diversity. We analyzed patterns of DNA sequence variation at two X-linked regions (comprising 1325 bp) approximately 50 cM apart and one autosomal region (comprising 921 bp) for 50 males, half of which were collected in the field from one of two allopatric locations and the other half were derived from lab-reared individuals with known brood sex ratios. These two populations are recently diverged but exhibit partial postzygotic reproductive isolation, i.e. crosses produce sterile hybrid males and fertile females. We find no nucleotide or microsatellite variation on the drive X chromosome, whereas the same individuals show levels of variation at autosomal regions that are similar to field-collected flies. Furthermore, one field-caught individual collected 10 years previously had a nearly identical X haplotype to the drive X, and is over 2% divergent from other haplotypes sampled from the field. These results are consistent with a selective sweep that has removed genetic variation from much of the drive X chromosome. We discuss how this finding may relate to the rapid evolution of postzygotic reproductive isolation that has been documented for these flies.
Rapid rise and fall of selfish sex-ratio X Chromosomes in Drosophila simulans: Spatiotemporal analysis of phenotypic and molecular data
4199Bastide, HC, M.; Ogereau, D.; Derome, N.; Hospital, F.; Montchamp-Moreau, C., Molecular Biology and Evolution, 28:2461-2470. 2011-01-17 00:00:00.
Sex-ratio drive, which has been documented in several Drosophila species, is induced by X-linked segregation distorters. Contrary to Mendel's law of independent assortment, the sex-ratio chromosome (X(SR)) is inherited by more than half the offspring of carrier males, resulting in a female-biased sex ratio. This segregation advantage allows X(SR) to spread in populations, even if it is not beneficial for the carriers. In the cosmopolitan species D. simulans, the Paris sex-ratio is caused by recently emerged selfish X(SR) chromosomes. These chromosomes have triggered an intragenomic conflict, and their propagation has been halted over a large area by the evolution of complete drive suppression. Previous molecular population genetics analyses revealed a selective sweep indicating that the invasion of X(SR) chromosomes was very recent in Madagascar (likely less than 100 years ago). Here, we show that X(SR) chromosomes are now declining at this location as well as in Mayotte and Kenya. Drive suppression is complete in the three populations, which display little genetic differentiation and share swept haplotypes, attesting to a common and very recent ancestry of the X(SR) chromosomes. Patterns of DNA sequence variation also indicate a fitness cost of the segmental duplication involved in drive. The data suggest that X(SR) chromosomes started declining first on the African continent, then in Mayotte, and finally in Madagascar and strongly support a scenario of rapid cycling of X chromosomes. Once drive suppression has evolved, standard X(ST) chromosomes locally replace costly X(SR) chromosomes in a few decades.
Segregation distortion and the evolution of sex-determining mechanisms
4216Kozielska, MW, F. J.; Beukeboom, L. W.; Pen, I., Heredity, 104:100-112. 2010-01-14 00:00:00.
Segregation distorters are alleles that distort normal segregation in their own favour. Sex chromosomal distorters lead to biased sex ratios, and the presence of such distorters, therefore, may induce selection for a change in the mechanism of sex determination. The evolutionary dynamics of distorter-induced changes in sex determination has only been studied in some specific systems. Here, we present a generic model for this process. We consider three scenarios: a driving X chromosome, a driving Y chromosome and a driving autosome with a male-determining factor. We investigate how the invasion prospects of a new sex-determining factor are affected by the strength of distortion and the fitness effect of the distorting allele. Our models show that in many cases, segregation distortion does create selection pressure, allowing novel sex-determining alleles to spread. When distortion leads to female-biased sex ratios, a new masculinizing gene can invade, leading to a new male heterogametic system. When distortion leads to male-biased sex ratios, a feminizing factor can invade and cause a switch to female heterogamety. In many cases, the distorter-induced change in the sex-determining system eventually leads to loss of the distorter from the population. Hence, the presence of sex chromosomal distorters will often only be transient, and the distorters may remain unnoticed. The role of segregation distortion in the evolution of sex determination may, therefore, be underestimated. Heredity (2010) 104, 100-112; doi:10.1038/hdy.2009.104; published online 12 August 2009
Segregation analysis of a sex ratio distortion locus in congenic mice
4214Casellas, JF, C. R.; Verdugo, R. A.; Medrano, J. F., Journal of Heredity, 101:351-359. 2010-01-12 00:00:00.
The congenic HG.CAST-(D17Mit196-D17Mit190) (HQ17(hg/hg)) mouse strain showed a significant departure on the expected 50%/50% offspring sex ratio in more than 2400 progeny (55.7% females). The entire pedigree file included data from 13 nonoverlapping purebred generations and an F-2 cross with the C57BL/6J inbred strain. Offspring sex ratio data were analyzed on the basis of 40 purebred HQ17(hg/hg) sires and 29 F-1 HQ17(hg/hg) x B6 sires under a Bayesian Binomial segregation model accounting for 4 different autosomal inheritance models of gene action (i.e., additive, dominance, recessive, and overdominance) and X-linked and Y-linked loci. For each model, the segregation effect was evaluated as a single regression coefficient for all sires or assuming 2 independent regression coefficients accounting for offspring sex ratio departures in purebred and F-1 sires, respectively. The deviance information criterion clearly favored the autosomal dominance model with different regression coefficients for the 2 groups of sires. Under this model, the dominance effect increased the percentage of female offspring by 4.3% (HQ17(hg/hg) purebred sires) and 8.2% (F-1 sires) with the highest posterior density regions ranging from 0.5% to 10.6% and from 1.3% to 14.4%, respectively. This article provides significant evidence of genetic determinism for sex ratio distortion in the HQ17(hg/hg) strain and develops new analytical tools to perform segregation studies on dichotomous traits.
Segregation distortion in chicken and the evolutionary consequences of female meiotic drive in birds
4213Axelsson, EA, A.; Van, A. P.; Li, L.; Megens, H. J.; Vereijken, A. L. J.; Crooijmans, Rpma; Groenen, M. A. M.; Ellegren, H.; Willerslev, E.; Nielsen, R., Heredity, 105:290-298. 2010-01-11 00:00:00.
As all four meiotic products give rise to sperm in males, female meiosis result in a single egg in most eukaryotes. Any genetic element with the potential to influence chromosome segregation, so that it is preferentially included in the egg, should therefore gain a transmission advantage; a process termed female meiotic drive. We are aware of two chromosomal components, centromeres and telomeres, which share the potential to influence chromosome movement during meioses and make the following predictions based on the presence of female meiotic drive: (1) centromere-binding proteins should experience rapid evolution as a result of a conflict between driving centromeres and the rest of the genome; and (2) segregation patterns should be skewed near centromeres and telomeres. To test these predictions, we first analyze the molecular evolution of seven centromere-binding proteins in nine divergent bird species. We find strong evidence for positive selection in two genes, lending support to the genomic conflict hypothesis. Then, to directly test for the presence of segregation distortion, we also investigate the transmission of B9000 single-nucleotide polymorphisms in 197 chicken families. By simulating fair Mendelian meioses, we locate chromosomal regions with statistically significant transmission ratio distortion. One region is located near the centromere on chromosome 1 and a second region is located near the telomere on the p-arm of chromosome 1. Although these observations do not provide conclusive evidence in favour of the meiotic drive/genome conflict hypothesis, they do lend support to the hypothesis that centromeres and telomeres drive during female meioses in chicken. Heredity (2010) 105, 290-298; doi:10.1038/hdy.2009.193; published online 27 January 2010
Sex ratio distorter reduces sperm competitive ability in an insect
4243Price, TARB, A. J.; Avent, T. D.; Snook, R. R.; Hurst, G. D. D.; Wedell, N., Evolution, 62:1644-1652. 2008-01-21 00:00:00.
Selfish genetic elements (SGEs) are ubiquitous in animals and often associated with low male fertility due to reduced sperm number in male carriers. In the fruit fly Drosophila pseudoobscura, the meiotic driving X chromosome "sex ratio" kills Y-bearing sperm in carrier males (SR males), resulting in female only broods. We competed SR males against the ejaculates of noncarrying standard males (ST males), and quantified the number of sperm transferred by SR and ST males to females. We show that SR males are very poor sperm competitors, which is partly related to transfer of fewer sperm during mating. However, sperm numbers alone cannot explain the observed paternity reduction, indicating SR males' sperm may be of reduced quality, possibly due to damage during the killing of the noncarrying Y-sperm. The reduction in sperm competitive ability due to SR is large enough to potentially stabilize the spread of sex ratio drive through populations. The poor sperm competitive ability of SR males coupled with their low fitness as mates could favor increased remating by females to reduce paternity by SR males. Given the generally poor performance of SGE-carrying males in sperm competition, this may generate strong selective pressure favoring polyandry in many species.
X chromosome drive
4239Jaenike, J, Current Biology, 18:R508-R511. 2008-01-17 00:00:00.
In the past 10 years, the world record for the men's 100 meter dash has declined from 9.79 to 9.74 seconds, the detection of such small differences being made possible by sophisticated electronic timing devices. If someone were to run the 100 meters in 9.73999 seconds in the 2008 Olympics, would the timers be sensitive enough to show him to be the world's fastest human? Natural selection could, as differences in fitness of that magnitude (10?6) can be detected in species with large effective population sizes. In this way, natural selection can bring about the evolution of exquisitely well-adapted creatures.; Getting back to the race, imagine that one of the runners somehow manages to get a 50-meter head start. With such an advantage, even an overweight, out of shape, or injured athlete could win the gold. An analogous situation applies in evolution. Mendelian segregation, in which the two alleles carried by a heterozygous individual are passed to equal numbers of gametes, ensures that alleles compete fairly and that they succeed (or fail) on the basis of their effects on survival and fertility. Meiotic drive — the process by which alleles are not represented equally in an individual's gametes — subverts the entire process. In the best documented examples of drive, one allele may be passed on to ?100% of an organism's gametes, equivalent to a runner getting a 50-meter head start in the 100 meter dash. The overrepresentation of such alleles in gametes can more than make up for any associated deficiencies in survival and fertility. As a result, these alleles — and those closely linked to them — can rapidly spread through a population, and thus actually cause a decline in the adaptation of a species to its environment. If such an allele drives all the way to fixation, the species may end up a little less well adapted, but there would be no evidence that meiotic drive was the cause, as drive would no longer be expressed.
Chromosome-wide linkage disequilibrium as a consequence of meiotic drive
4249Dyer, KAC, B.; Jaenike, J., Proceedings of the National Academy of Sciences of the United States of America, 104:1587-1592. 2007-01-07 00:00:00.
Adaptation by natural selection proceeds most efficiently when alleles compete solely on the basis of their effects on the survival and reproduction of their carriers. A major condition for this is equal Mendelian segregation, but meiotic drive can short-circuit this process. The evolution of drive often involves multiple, interacting genetic components, together with enhancers and suppressors of drive. Chromosomal inversions that suppress crossing over are also frequently associated with drive systems. This study investigates the effects of these processes on patterns of molecular evolution in the fly Drosophila recens, which is polymorphic for a driving X chromosome (X-D). Whereas standard wild-type chromosomes exhibit high levels of polymorphism at multiple loci, all of the X-D chromosomes effectively carry a single multilocus haplotype that spans at least 130 cM. The X-D is associated with a complex set of inversions that completely suppresses recombination between the standard wild-type chromosome and X-D in heterozygous females, which maintain nonrandom associations among loci that presumably interact epistatically for the expression of drive. The long-term costs of foregoing recombination may be substantial; in combination with its low equilibrium frequency, this makes the XD chromosome susceptible to the accumulation of deleterious mutations. Consistent with this, X-D chromosomes are apparently fixed for a recessive mutation that causes female sterility. Thus, the X-D in D. recens appears to be in chromosome-wide linkage disequilibrium and in the early stages of mutational degradation.
Fitness effects of X chromosome drive in the stalk-eyed fly, Cyrtodiopsis dalmanni
4271Wilkinson, GSJ, P. M.; Kelleher, E. S.; Muscedere, M. L.; Lorsong, A., Journal of Evolutionary Biology, 19:1851-1860. 2006-01-09 00:00:00.
Sex-ratio (SR) males produce predominantly female progeny because most Y chromosome sperm are rendered nonfunctional. The resulting transmission advantage of X-SR chromosomes should eventually cause population extinction unless segregation distortion is masked by suppressors or balanced by selection. By screening male stalk-eyed flies, Cyrtodiopsis dalmanni, for brood sex ratio we found unique SR alleles at three X-linked microsatellite loci and used them to determine if SR persists as a balanced polymorphism. We found that X-SR/X-ST females produced more offspring than other genotypes and that SR males had lower sperm precedence and exhibited lower fertility when mating eight females in 24 h. Adult survival was independent of SR genotype but positively correlated with eye span. We infer that the SR polymorphism is likely maintained by a combination of weak overdominance for female fecundity and frequency dependent selection acting on male fertility. Our discovery of two SR haplotypes in the same population in a 10-year period further suggests that this SR polymorphism may be evolving rapidly.
Sex-ratio meiotic drive in Drosophila simulans: cellular mechanism, candidate genes and evolution
4265Montchamp-Moreau, C, Biochemical Society Transactions, 34:562-565. 2006-01-03 00:00:00.
The sex-ratio trait, reported in a dozen Drosophila species, is a type of naturally occurring meiotic drive in which the driving elements are located on the X chromosome. Typically, as the result of a shortage of Y bearing spermatozoa, males carrying a sex-ratio X chromosome produce a large excess of female offspring. The presence of sex-ratio chromosomes in a species can have considerable evolutionary consequences, because they can affect individual fitness and trigger extended intragenomic conflict. Here, I present the main results of the study performed in Drosophilosimulans. In this species, the loss of Y-bearing spermatozoa is related to the inability of the Y chromosome sister-chromatids to separate properly during meiosis II. Fine genetic mapping has shown that the primary sex-ratio locus on the X chromosome contains two distorter elements acting synergistically, both of which are required for drive expression. one element has been genetically mapped to a tandem duplication. To infer the natural history of the trait, the pattern of DNA sequence polymorphism in the surrounding chromosomal region is being analysed in natural populations of D. simulans harbouring sex-ratio X chromosomes. Initial results have revealed the recent spread of a distorter allele.
Genetic linkage between a sexually selected trait and X chromosome meiotic drive
4276Johns, PMW, L. L.; Wilkinson, G. S., Proceedings of the Royal Society B-Biological Sciences, 272:2097-2103. 2005-01-14 00:00:00.
Previous studies on the stalk-eyed fly, Cyrtodiopsis dalmanni, have shown that males with long eye-stalks win contests and are preferred by females, and artificial selection on male relative eye span alters brood sex-ratios. Subsequent theory proposes that X-linked meiotic drive can catalyse the evolution of mate preferences when drive is linked to ornament genes. Here we test this prediction by mapping meiotic drive and quantitative trait loci (QTL) for eye span. To map QTL we genotyped 24 microsatellite loci using 1228 F2 flies from two crosses between lines selected for long or short eye span. The crosses differed by presence or absence of a drive X chromosome, X-D, in the parental male. Linkage analysis reveals that X-D dramatically reduces recombination between X and X-D chromosomes. In the X-D cross, half of the F2 males carried the drive haplotype, produced partially elongated spermatids and female-biased broods, and had shorter eye span. The largest QTL mapped 1.3 cM from drive on the X chromosome and explained 36% of the variation in male eye span while another QTL mapped to an autosomal region that suppresses drive. These results indicate that selfish genetic elements that distort the sex-ratio can influence the evolution of exaggerated traits.
Sex chromosome meiotic drive in hybrid males of the common shrew (Sorex araneus)
4273Fedyk, SB, U.; Chetnicki, W., Folia Biologica-Krakow, 53:133-141. 2005-01-11 00:00:00.
Patterns of sex chromosome segregation in six homozygous males of the common Shrew (Sorex araneus LINNAEUS, 1758) belonging to two chromosomal races, as well as in 16 interracial hybrids were studied. I ascd on their karyotypes the hybrids can be subdivided into two groups: (a) complex heterozygotes, which form meiotic quadrivalents in chain and chain + ring configurations, and (b) complex heterozygotes, which Form meiotic pentavalents in chain configurations. Random 1 : 1) segregation of sex chromosomes was found ill homozygous as well as those he erozygous males which form meiotic complexes Of four chromosomes. However, in some, hybrids with meiotic pentavalents we observed a strong preferential segregation in favou of X chromosomes.
Evolution of autosomal suppression of the sex-ratio trait in Drosophila
4302Vaz, SCC, A. B., Genetics, 166:265-277. 2004-01-20 00:00:00.
The sex-ratio trait is the production of female-biased progenies due to X-linked meiotic drive in males of several Drosophila species. The driving X chromosome (called SR) is not fixed due to at least two stabilizing factors: natural selection (favoring ST, the nondriving standard X) and drive suppression by either Y-linked or autosomal genes. The evolution of autosomal suppression is explained by Fisher's principle, a mechanism of natural selectin that leads to equal proportion of males and females in a sexually reproducing population. In fact, sex-ratio expression is partially suppressed by autosomal genes in at least three Drosophila species. The population genetics of this system is not completely understood. In this article we develop a mathematical model for the evolution of autosomal suppressors of SR (sup alleles) and show that: (i) an autosomal suppressor cannot invade when SR is very deleterious in males (c < 1/3, where c is the fitness of SR/Y males); (ii) "SR/ST, sup/+" polymorphisms occur when SR is partially deleterious (similar to0.3 < c < 1); while (iii) SR neutrality (c = 1) results in sup fixation and thus in total abolishment of drive. So, surprisingly, as long as there is any selection against SR/Y males, neutral autosomal suppressors will not be fixed. In that case, when a polymorphic equilibrium exists, the average female proportion in SR/Y males' progeny is given approximately by (ac + 1 - a + roota(9) ( c + 1 )(2) + 1 - 4ac)/4ac, where a is the fitness of SR/ST females.
Meiotic drive and sex chromosome cycling
4291Hall, DW, Evolution, 58:925-931. 2004-01-09 00:00:00.
Sex-linked meiotic drive is found in a broad variety of taxa, including insects, birds, and mammals. In populations of some species, we see four types of sex chromosomes segregating: normal and driving X chromosomes and susceptible and resistant Y chromosomes. A theoretical analysis shows that a stable four-chromosome equilibria is a more common outcome in these systems than previously recognized. Cycling of sex chromosome frequencies and associated changes in the sex ratio are other predicted outcomes. The absence of cycling in nature may be due to migration among populations.
To what extent do different types of sex ratio distorters interfere?
4288Engelstadter, JM, H.; Hurst, G. D. D., Evolution, 58:2382-2386. 2004-01-06 00:00:00.
Within the Diptera, two different selfish genetic elements are known to cause the production of female-biased sex ratios: maternally inherited bacteria that kill male zygotes (male-killers), and X chromosomes causing the degeneration of Y-bearing sperm in males (meiotic drive). We here develop a mathematical model for the dynamics of these two sex-ratio distorters where they co-occur. We show that X chromosome meiotic drive elements can be expected to substantially lower the equilibrium frequency of male-killers and can even lead to their extinction. Conversely, male-killers can also decrease the equilibrium frequency of X drivers and cause their extinction. Thus, we predict that there will be some complementarity in the incidence of X chromosome meiotic drive and male-killing in natural populations, with a lower than expected number of species bearing both elements.
Sperm competition and the dynamics of X chromosome drive: Stability and extinction
4326Taylor, JEJ, J., Genetics, 160:1721-1731. 2002-01-04 00:00:00.
Several empirical studies of sperm competition in populations polymorphic for a driving X chromosome have revealed that Sex-ratio males (those carrying a driving X) are at a disadvantage relative to Standard males. Because the frequency of the driving X chromosome determines the population-level sex ratio and thus alters male and female mating rates, the evolutionary consequences of sperm competition for sex chromosome meiotic drive are subtle. As the SR allele increases in frequency, the ratio of females to males also increases, causing an increase in the male mating rate and a decrease in the female mating rate. While the former change may exacerbate the disadvantage of Sex-ratio males during sperm competition, the latter change decreases the incidence of sperm competition within the population. We analyze a model of the effects of sperm competition on a driving X chromosome and show that these opposing trends in male and female mating rates call result in two coexisting locally stable equlibria, one corresponding to a balanced polymorphism of the SR and ST alleles and the second to fixation of the ST allele. Stochastic fluctuations of either the Population sex ratio or the SR frequency call then drive the population away from the balanced polymorphism and into the basin of attraction for the second equilibrium, resulting in fixation of the SR allele and extinction of the population.
Sex-ratio drive in Drosophila simulans: Variation in segregation ratio of X chromosomes from a natural population
4325Montchamp-Moreau, CC, M., Genetics, 162:1221-1231. 2002-01-03 00:00:00.
The sex-ratio trait that exists in a dozen Drosophila species is a case of naturally occurring X chromosome drive that causes males to produce female-biased progeny. Autosomal and Y polymorphism for suppressors are known to cause variation in drive expression, but the X chromosome polymorphism has never been thoroughly investigated. We characterized 41 X chromosomes from a natural population of Drosophila simulans that had been transferred to a suppressor-free genetic background. We found two clear-cut groups of chromosomes, sex-ratio and standard. The sex-ratio X chromosomes differed in their segregation ratio (81-96% females in the progeny), the less powerful drivers being less stable in their expression. A sib analysis, using a moderate driver, indicated that within-X variation in drive expression depended on genetic (autosomal) or epigenetic factors and that the age of the males also affected the trait. The other X chromosomes produced equal or roughly equal sex ratios, but again with significant variation. The continuous pattern of variation observed within both groups suggested that, in addition to a major sex-ratio gene, many X-linked loci of small effect modify the segregation ratio of this chromosome and are maintained in a polymorphic state. This was also supported by the frequency distribution of sex ratios produced by recombinant X chromosomes.
Sperm development, age and sex chromosome meiotic drive in the stalk-eyed fly, Cyrtodiopsis whitei
4337Wilkinson, GSS, M. I., Heredity, 87:17-24. 2001-01-15 00:00:00.
The cytological basis of X chromosome meiotic drive or sex ratio (SR) has been reported for several species of Drosophila but not for other species. Here we describe how sperm development in the stalk-eyed fly, Cyrtodiopsis whitei, influences progeny sex proportion, in order to determine if a common developmental mechanism could cause meiotic drive in these distantly related taxa. Because age has been found to affect the degree of segregation distortion in some Drosophila, we tested flies from six to 26 weeks of age. We find that spermatocyst bundles in SR males frequently contain incompletely elongated spermatid nuclei independently of male age. Older males have, however, more spermatocyst bundles in their testes than younger males. Abnormal spermatid elongation affects male fertility since SR males produce 74% as many progeny per week as ST males. The proportion of spermatocyst bundles with improperly elongated spermatid nuclei explains 71% of the variation in progeny sex proportion. After reviewing the literature on sperm development and meiotic drive, we conclude that the cytological basis of meiotic drive in diopsids closely resembles Drosophila. Across species in both groups, the production of fertile males is associated with less than half of all spermatids not elongating normally in a spermatocyst bundle. We discuss the possibility that frequency-dependent selection on male fertility could stabilize the drive polymorphism in these unusual flies.
Sex chromosome meiotic drive
4332Jaenike, J, Annual Review of Ecology and Systematics, 32:25-49. 2001-01-10 00:00:00.
Sex chromosome drive refers to the unequal transmission of X and Y chromosomes from individuals of the heterogametic sex, resulting in biased sex ratios among progeny and within populations, The presence of driving sex chromosomes can reduce mean fitness within a population, bring about intragenomic conflict between the X chromosome, the Y, and the autosomes, and alter the intensity or mode of sexual selection within species. Sex chromosome drive, or its genetic equivalent, is known in plants, mammals. and flies. Many species harboring driving X chromosomes have evolved Y-linked and autosomal suppressors of drive. If a drive polymorphism is not stable, then driving chromosomes may spread to fixation and cause the extinction of a species. Certain characteristics of species, such as population density and female mating rate, may affect the probability of fixation of driving chromosomes. Thus, sex chromosome drive could be an agent of species-level selection.
Sex-ratio meiotic drive in Drosophila simulans is related to equational nondisjunction of the Y chromosome
4338Cazemajor, MJ, D.; Montchamp-Moreau, C., Genetics, 154:229-236. 2000-01-16 00:00:00.
The sex-ratio trait, an example of naturally occurring X-linked meiotic drive, has been reported in a dozen Drosophila species. Males carrying a sex-ratio X chromosome produce an excess of female offspring caused by a deficiency of Y-bearing sperm. In Drosophila simulans, such males produce similar to 70-90% female offspring, and 15-30% of the male offspring are sterile. Here, we investigate the cytological basis of the drive in this species. We show that the sex-ratio trait is associated with nondisjunction of Y chromatids in meiosis II. Fluorescence in situ hybridization (FISH) using sex-chromosome-specific probes provides dir-ect evidence that the drive is caused by the failure of the resulting spermatids to develop into functional sperm. XYY progeny were not observed, indicating that few or no YY spermatids escape failure. The recovery of XO males among the progeny of sex-ratio males shows that some nullo-XY spermatids become functional sperm and likely explains the male sterility. A review of the cytological data. in other species shows that aberrant behavior of the Y chromosome may be a common basis of sex-ratio meiotic drive in Drosophila and the signal that triggers differential spermiogenesis failure.
Nonrandom segregation of the mouse univalent X chromosome: Evidence of spindle-mediated meiotic drive
4344LeMaire-Adkins, RH, P. A., Genetics, 156:775-783. 2000-01-02 00:00:00.
A fundamental principle of Mendelian inheritance is random segregation of alleles to progeny; however, examples of distorted transmission either of specific alleles or of whole chromosomes have been described in a variety of species. In humans and mice, a distortion in chromosome transmission is often associated with a chromosome abnormality. One such example is the fertile XO female mouse. A transmission distortion effect that results in an excess of XX over XO daughters among the progeny of XO females has been recognized for nearly four decades. Utilizing contemporary methodology that combines immuno-fluorescence, FISH, and three-dimensional confocal microscopy, we have readdressed the meiotic segregation behavior of the single X chromosome in oocytes from XO females produced on two different inbred backgrounds. Our studies demonstrate that segregation of the univalent X chromosome at the first meiotic division is nonrandom, with preferential retention of the X chromosome in the oocyte in similar to 60% of cells. We propose that this deviation from Mendelian expectations is facilitated by a spindle-mediated mechanism. This mechanism, which appears to be a general feature of the female meiotic process, has implications the frequency of nondisjunction in our species.
Models of sex-ratio meiotic drive and sexual selection in stalk-eyed flies
4358Lande, RW, G. S., Genetics Research, 74:245-253. 1999-01-16 00:00:00.
Hypertrophied sexually dimorphic eye stalks have evolved independently in several families of Diptera, with the eyespan of males exceeding their total body length in some species. These structures function in intermale contests for territories and in mate attraction, the classical mechanisms of sexual selection. In the family Diopsidae, species with extremely exaggerated eye stalks and marked sexual dimorphism in relative eyespan also usually have strongly female-biased sex ratios in nature caused by X-linked meiotic drive, whereas species with relatively small eye stalks have little or no sexual dimorphism, often lack meiotic drive and have even sex ratios. We investigate the possible connection between sexual selection and sex-ratio meiotic drive by analysing a three-locus model for the evolution of female choice for a male character associated with meiotic drive. Both meiotic drive and the male character are X-linked and the female preference is autosomal. Our model shows that suppressed recombination between meiotic drive and the male character, e.g. by inversion of the X chromosome, is necessary for sex-ratio selection to promote the origin of female mating preferences and exaggerated secondary sexual characters. With complete suppression of recombination, sexual selection reduces the frequency of meiotic drive, and may eliminate it. Very rare recombination, gene conversion or mutation, at rates characteristic of chromosome inversions in Drosophila, restores the meiotic drive polymorphism to its original equilibrium. Sex-ratio meiotic drive may thus act as a catalyst accelerating the origin of female mating preference and exaggerated male traits.
How common are meiotically driving sex chromosomes in insects?
4357Jiggins, FMH, G. D. D.; Majerus, M. E. N., American Naturalist, 154:481-483. 1999-01-15 00:00:00.
In summary, we argue that the hypothesis that sex chromosome; meiotic drive is common within the insects is in; fact not proved. We feel that, although it is unlikely that; it will be found exclusively in the Diptera, there is a case; to be made that the Diptera are a hot spot for the occurrence; of sex chromosome meiotic drive. Further research; is clearly necessary to clarify this point. If the Diptera; are found to be different, then we must ask why this; is so. Meiotic drive often involves the destruction of gametes; that do not bear the drive allele causing a reduction; in the number of functional gametes. This may mean that; meiotically driving sex chromosomes are unlikely to spread; in female heterogametic taxa such as the Lepidoptera because; a reduction in the number of eggs is more costly; than a reduction in the number of sperm. Among male; heterogametic taxa, it is unclear what could make Dipteran; meiosis special.
Suppression of sex-ratio meiotic drive and the maintenance of Y-chromosome polymorphism in Drosophila
4356Jaenike, J, Evolution, 53:164-174. 1999-01-14 00:00:00.
Like several other species of Drosophila, D. quinaria is polymorphic for X-chromosome meiotic drive; matings involving males that carry a "sex-ratio" X chromosome (X(SR)) result in the production of strongly female-biased offspring sex ratios (Jaenike 1996). A survey of isofemale lines of D, quinaria from several populations reveals that there is genetic variation for partial suppression of this meiotic drive. Crossing experiments show that there is Y-linked. and probably autosomal, variation for suppression of drive. Y-linked suppressors of X-chromosome drive have now been described in several species of Diptera. I develop a simple model for the maintenance of Y-chromosome polymorphism in species polymorphic for X-linked meiotic drive. One interesting feature of this model is that, if there is a stable Y-chromosome polymorphism, then the equilibrium frequency of the standard and sex-ratio X chromosomes is determined solely by Y-chromosome parameters, not by the fitness effects of the different X chromosomes on their carriers. This model suggests that Y-chromosome polymorphism may be easier to maintain than previously thought, and I hypothesize that karyotypic variation in Y chromosomes will be found to be associated with suppression of sex-ratio meiotic drive in other species of Drosophila.
Are Drosophila SR drive chromosomes always balanced?
4351Carvalho, ABV, S. C., Heredity, 83:221-228. 1999-01-09 00:00:00.
SR chromosomes are the best-known case of sex chromosome meiotic drive. These X chromosomes cause the production of female-biased progenies in several Drosophila species; Due to their meiotic drive advantage, they are expected to spread and become fixed, resulting in population extinction due to the lack of males. However, this apparently does not occur: SR chromosomes are maintained in balanced polymorphisms, resulting from the equilibrium between their meiotic drive advantage and deleterious fitness effects. In this paper we review the current explanations for their deleterious effects and we argue that it is highly improbable that all newly emerged SR are sufficiently deleterious to avoid fixation. Unbalanced SR almost certainly arise and go unnoticed because of three possible outcomes: (i) fixation followed by extinction of the population or species; (ii) fixation followed by the emergence and fixation of drive suppressors, restoring the normal 1:1 sexual proportion; or (iii) transformation into balanced SR due to partial suppression. If these outcomes really occur, then extant cases of sex-chromosome meiotic drive such as SR, causing small deviations on the population sexual proportion are only the tip of the iceberg and strong sexual proportion shifts (possibly followed by extinction) are a more common feature of species evolution than is usually assumed.
Evolution of driving X chromosomes and resistance factors in experimental populations of Drosophila simulans
4350Capillon, CA, A., Evolution, 53:506-517. 1999-01-08 00:00:00.
Sex-ratio drive is a particular case of meiotic drive, described in several Drosophila species, that causes males bearing driving X chromosome to produce a large excess of females in their progeny. In Drosophila simulans, driving X chromosomes and resistance factors located on the Y chromosome and on the autosomes have been previously reported. In this paper, we report the study of the dynamics of sex-ratio factors in experimental populations. We followed the evolution in frequency of driving X chromosomes in the absence of resistance factors and the evolution of resistance factors in the presence of driving X chromosomes. The driving X chromosome was lost, contrarily to theoretical expectations that predict its rapid invasion. Autosomal resistances increased in frequency, and resistant Y chromosomes invaded the population very quickly, as predicted by theoretical models. Fitness measurements showed that the loss of the driving X chromosome was due to a strong deleterious effect that was expressed only when distorting males were in competition with standard males. However, the spread of autosomal resistances reduced this deleterious effect. Implications for the maintenance of polymorphism in natural populations are discussed.
Male eye span in stalk-eyed flies indicates genetic quality by meiotic drive suppression
4379Wilkinson, GSP, D. C.; Crymes, L., Nature, 391:276-279. 1998-01-17 00:00:00.
In some species, females choose mates possessing ornaments that predict offspring survival(1-5). However, sexual selection by female preference for male genetic quality(6-8) remains controversial because conventional genetic mechanisms maintain insufficient variation in male quality to account for costly preference and ornament evolution(9,10). Here we show that females prefer ornaments that indicate genetic quality generated by transmission conflict between the sex chromosomes, By comparing sex-ratio distributions in stalk-eyed fly (Cyrtodiopsis) progeny we found that female-biased sex ratios occur in species exhibiting eye-stalk sexual dimorphism(11,12) and female preferences for long eye span(13,14). Female-biased sex ratios result from meiotic drive(15), the preferential transmission of a 'selfish' X-chromosome. Artificial selection for 22 generations on male eye-stalk length in sexually dimorphic C. dalmanni produced longer eye-stalks and male-biased progeny sex ratios in replicate lines. Because male-biased progeny sex ratios occur when a drive-resistant Y chromosome pairs with a driving X chromosome(15), long eye span is genetically linked to meiotic drive: suppression. Male eye span therefore signals genetic quality by influencing the reproductive value of offspring(16).
Polymorphism for Y-linked suppressors of sex-ratio in two natural populations of Drosophila mediopunctata
4383Carvalho, ABV, S. C.; Klaczko, L. B., Genetics, 146:891-902. 1997-01-21 00:00:00.
In several Drosophila species there is a trait known as ''sex-ratio'': males carrying certain X chromosomes (called ''SR'') produce female biased progenies due to X-Y meiotic drive. In Drosophila mediopunctata this trait has a variable expression due to Y-linked suppressors of sex-ratio expression, among other factors. There are two types of Y chromosomes (suppressor and nonsuppressor) and two types of SR chromosomes (suppressible and unsuppressible). Sex-ratio expression is suppressed in males with the SRsuppressible/Y-suppressor genotype, whereas the remaining three genotypes produce female biased progenies. Now we have found that similar to 10-20% of the Y chromosomes from two natural populations 1500 km apart are suppressors of sex-ratio expression. Preliminary estimates indicate that Y-suppressor has a meiotic drive advantage of 6% over Y-nonsuppressor. This Y polymorphism for a nonneutral trait is unexpected under current population genetics theory. We propose that this polymorphism is stabilized by an equilibrium between meiotic drive and natural selection, resulting from interactions in the population dynamics of X and Y alleles. Numerical simulations showed that this mechanism may stabilize nonneutral Y polymorphisms such as we have found in D. mediopunctata.
The sex-ratio trait in Drosophila simulans: Geographical distribution of distortion and resistance
4381Atlan, AM, H.; Landre, C.; Montchamp-Moreau, C., Evolution, 51:1886-1895. 1997-01-19 00:00:00.
The sex-ratio trait we describe here in Drosophila simulans results from X-linked meiotic drive. Males bearing a driving X chromosome can produce a large excess of females (about 90%) in their progeny. This is, however, rarely the case in the wild, where resistance factors, including autosomal suppressors and insensitive Y chromosomes, prevent the expression of the driver. In this study, we searched for drive and resistance factors in strains of Drosophila simulans collected all over the world. Driving X chromosomes were found in all populations whenever a good sample size was available. Their frequency may reach up to 60%. However, the presence of driving X chromosomes never results in an excess of females, due to the systematic co-occurrence of resistance factors. The highest frequencies of driving X chromosomes were observed in islands, while populations from East and Central Africa (the supposed center of origin of the species) showed the highest level of resistance. The geographical pattern of drive and resistance factors, as well as the results of crosses between strains from different geographical areas, suggest that the sex-ratio system described here has a unique and ancient origin in the species.
Sex chromosome meiotic drive in stalk-eyed flies
4388Presgraves, DCS, E.; Wilkinson, G. S., Genetics, 147:1169-1180. 1997-01-06 00:00:00.
Meiotically driven sex chromosomes can quickly spread to fixation and cause population extinction unless balanced by selection or suppressed by genetic modifiers. We report results of genetic analyses that demonstrate that extreme female-biased sex ratios in two sister species of stalk-eyed flies, Cyrtodiopsis dalmanni and C. whitei, are due to a meiotic drive element on the X chromosome (X-d). Relatively high frequencies of X-d in C. dalmanni and C. whitei (13-17% and 29%, respectively) cause female-biased sex ratios in natural populations of both species. Sex ratio distortion is associated with spermatid degeneration in male carriers of X-d. Variation in sex ratios is caused by Y-linked and autosomal factors that decrease the intensity of meiotic drive. Y-linked polymorphism for resistance to drive exists in C. dalmanni in which a resistant Y chromosome reduces the intensity and reverses the direction of meiotic drive. When paired with X-d, modifying Y chromosomes (Y-m) cause the transmission of predominantly Y-bearing sperm, and on average, production of 63% male progeny. The absence of sex ratio distortion in closely related monomorphic outgroup species suggests that this meiotic drive system may predate the origin of C. whitei and C. dalmanni. We discuss factors likely to be involved in the persistence of these sex linked polymorphisms and consider the impact of X-d on the operational sex ratio and the intensity of sexual selection in these extremely sexually dimorphic flies.
Sex-ratio distortion in Drosophila simulans – cooccurrence of a meiotic drive and a suppressor of drive
4406Mercot, HA, A.; Jacques, M.; Montchampmoreau, C., Journal of Evolutionary Biology, 8:283-300. 1995-01-04 00:00:00.
A sex-ratio distortion factor was found at high frequency in D. simulans strains from Seychelles and New Caledonia. This factor is poorly or not expressed within those strains which are resistant to it. Its presence was detected by crossing females from New Caledonia or the Seychelles with males from a different geographic origin. Most of the Fl males obtained produced an excess of females (up to 99%) in their progeny. The two strains are infected with Wolbachia, but these micro-organismms are not involved in the sex-ratio distortion. The sex-ratio factor is shown to be an X-linked meiotic driver; nuclear resistance factor(s) act by suppressing the drive. It is likely that the same X-located driver invaded the two populations, which subsequently developed resistance factor(s) against it.
Y-linked suppressors of the sex-ratio trait in Drosophila mediopunctata
4412Decarvalho, ABK, L. B., Heredity, 73:573-579. 1994-01-10 00:00:00.
X-linked meiotic drive causing female-biased progenies is known to occur in nine Drosophila species and is called 'sex-ratio'. In D. mediopunctata this trait is associated with the X:21 chromosome inversion and has variable expression. We describe here a powerful Y-linked suppressor system of sex-ratio expression in this species. There are two types of Y chromosomes (suppressor and nonsuppressor) and two types of X:21 chromosomes (suppressible and unsuppressible). Sex-ratio expression is suppressed in males with the 21 (suppressible)/Y-suppressor genotype, whereas the remaining three genotypes produce female-biased progenies.
Autosomal suppressors of sex-ratio in Drosophila mediopunctata
4422Decarvalho, ABK, L. B., Heredity, 71:546-551. 1993-01-20 00:00:00.
The sex-ratio trait has been described as the production of progenies with excess of females due to X-linked meiotic drive in the parental males. This trait has a variable expression in Drosophila mediopunctata. We describe here the existence and chromosomal localization of autosomal suppressors of sex-ratio in this species. There are at least four such genes (one on each major autosome) and the strongest effect.is localized on chromosome IV. These genes possibly result from the operation of 'Fisher's Principle'; a mechanism of Natural Selection leading to a 1:1 sex ratio.
The evolution of unusual chromosomal systems in coccoids: Extraordinary sex-ratios revisited
4424Haig, D, Journal of Evolutionary Biology, 6:69-77. 1993-01-02 00:00:00.
Coccoids (scale insects) exhibit a wide variety of chromosomal systems. In many species, paternal chromosomes are eliminated from the male germline such that all of a male's sperm transmit an identical set of maternal chromosomes. In such species, an offspring's sex is determined by whether or not paternal chromosomes are inactivated in the egg's cytoplasm after fertilization. This paper presents a model of the evolution of paternal genome loss in coccoids from an ancestral system of XX-XO sex determination. The model is based on Hamilton's (1967) theory that different genetic elements within the genome have different unbeatable sex ratios. In this model (1) meiotic drive by the X chromosome in XO males causes female-biased sex ratios; (2) the maternal set of autosomes in males evolves effective sex linkage to exploit X-drive; and (3) genes expressed in mothers are selected to convert some of their XX daughters into sons. A similar model may explain the evolution of haplodiploidy.
A comparative approach to the population genetics theory of segregation distortion
4440Feldman, MWO, Sarah P., American Naturalist, 137:443-456. 1991-01-18 00:00:00.
Mathematical models of four well-known naturally occurring systems of segregation distortion are compared. These include the sex-ratio chromosome of Drosophila pseudoobscura, the Segregation Distorter (SD) complex of D. melanogaster, the t locus in Mus musculus, and the sex-ratio system in Aedes aegypti. Dynamics of these models are compared with the classical one-locus multiple-allele viability system. For the SD complex and the sex-ratio model of A. aegypti, the role of recombination is reviewed. Departures from Mendelian segregation cause fascinating irregularities in the relationship between linkage and linkage disequilibrium, as well as in predictions for the evolution of recombination itself.
X-chromosome segregation distortion in Drosophila
4439Curtsinger, JW, American Naturalist, 137:344-348. 1991-01-17 00:00:00.
The sex-ratio trait exhibits both discrete and continuous variation in Drosophila pseudoobscura. The discrete variation is caused by X-chromosome meiotic drive. The evolutionary forces maintaining the meiotic-drive polymorphism include strong viability selection against homozygous females and virility selection in males. The relative importance of the two factors is unresolved. The evidence for a genetic component in the continuous sex-ratio variation comes from artificial-selection experiments, large-scale estimation of segregation ratios corrected for viability effects, and an examination of the rates of spermatid loss. Segregation ratio can be treated as a typical quantitative character subject to major and minor genetic modifiers.
Male sex-ratio trait in Drosophila pseudoobscura: Frequency of autosomal aneuploid sperm
4437Cobbs, GJ, L.; Gordon, L., Genetics, 127:381-390. 1991-01-15 00:00:00.
Males with the SR X chromosome show the "sex-ratio" (sr) phenotype in which they produce almost entirely daughters. The few sons (about 1%) are invariably sterile X/O males and result entirely from nullo-XY sperm. The "male-sex-ratio" (msr) phenotype is a modified form of sr in which SR/Y males produce a higher frequency of sterile X/O sons. The msr trait is due to the presence of the SR X-chromosome in males which are also homozygous for one or more autosomes from the L116 strain. Here the frequency of nullo-3 and diplo-3 sperm from msr males was measured by crossing to a compound-3 strain and found to be 13.8% and 3.2%, respectively, of the total viable sperm. The sr males produced very low levels of nullo-3 sperm at a frequency not different from control X/Y males and a slightly elevated frequency of diplo-3 sperm over X/Y males. The msr males were found to have only 12% the fecundity of sr males and in matings to cause a high frequency of brown inviable eggs. These results indicate that high rates of autosomal aneuploidy are not restricted to chromosome 3 but also occur for chromosomes 2, 4 and 5. The overall frequency of autosomal aneuploid sperm is estimated to be approximately 50%. Microscopic studies of meiosis in testes from msr males indicates meiotic nondisjunction and meiotic chromosome loss are responsible for the msr phenotype. Last, microscopic studies of sperm cysts from msr males reveal high levels of spermiogenic failure.
Segregation distorters
4451Lyttle, TW, Annual Review of Genetics, 25:511-557. 1991-01-09 00:00:00.
Segregation distorters are genetic elements that exhibit the phenomenon of meiotic drive; that is, the mechanics of the meiotic divisions cause one member of a pair of heterozygous alleles or heteromorphic chromosomes to be transmitted to progeny in excess of the expected Mendelian proportion of 50% ( 1 34 , 1 36). In this review, we refer to these as genic or chromosomal drive, respectively. Genic meiotic drive is initially limited in its impact to the population dynamics of the drive locus itself and those loci fortuitiously in close linkage. Alleles at these latter loci may enjoy indirect drive through genetic hitchhiking, leading eventually to the establishment of drive haplotypes (64). The haplotype may be extended by incorporating chromosome rearrangements that reduce recombination and promote further linkage disequilibrium between the drive locus and more distant modifier loci ( 1 04 , 1 28, 1 64). In the extreme , the haplotype becomes coextensive with the chromosome, leading to a form of chromosomal meiotic drive. For a parent heterozygous for either type of drive system, the statistic k is used to denote the proportion of progeny (and by inference, successful gametes) that carry the allele or chromosome exhibiting segregation distortion. Thus , k can vary from 0 . 5 (Mendelian segregation segregation) to 1 .0 (complete segregation distortion with only one gamete class recovered in the progeny) .
Genetics-driving genes and chromosomes
4463Charlesworth, B, Nature, 332:394-395. 1988-01-21 00:00:00.
Thereare several genetic and chromosomal systems in which Mendel's first law - the equal probability of transmission of maternal and paternal alternative alleles or homologues - is violated. This phenomenon was named 'meiotic drive' in 1957 by Sandler and Novitski, who drew attention to the fact that it operates as an evolutionary force which can cause an increase in the population frequency of the allele or chromosome which is favoured in transmission, even if it confers a disadvantage on its carriers in terms of fitness at the level of the individual
Sex-ration, meiotic drive, and group selectin in Drosophila pseudoobscura
4498Policansky, D, American Naturalist, 108:75-90. 1974-01-16 00:00:00.
Sex ratio (SR) is a widespread genetic condition of the X-chromosome in Drosophila species which causes males to produce progenies consisting almost entirely of females. Results of samples from natural populations of Drosophila pseudoobscura and results of some laboratory experiments indicate that SR males produce only about half as many sperm as normal (ST) males; because of this deficiency, SR males are able to fertilize only about half as many eggs as ST males. These results explain the observed equilibria of frequencies of SR in natural populations, and suggest that, at least in the case of D. pseudoobscura, male fertility may be partly limited by the number of sperm males can produce.
Extraordinary sex ratios
6186W. D. Hamilton, Science, 156:477-488. 1967-04-03 19:10:19.
The two sexes are usually produced in approximately equal numbers. Fisher (1) was the first to explain why, under natural selection, this should be so, irrespective of the particular mechanism of sex determination. His rather tersely expressed argument has been clarified by subsequent writers (2) and seems to be widely accepted. In bare outline, the factor of parental care being ignored, it may be given as follows: 1) Suppose male births are less common than female. 2) A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring. 3) Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them. 4) Therefore the genes for male-producing tendencies spread, and male births become commoner. 5) As the 1:1 sex ratio is approached, the advantage associated with producing males dies away. 6) The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.
Meiotic drive in Drosophila involving chromosome breakage
4506Erickson, J, Genetics, 51:555-571. 1965-01-04 00:00:00.
In ordinary genetic systems the members of a pair of unlike alleles, or of a pair of unlike chromosomes, are recovered in equal numbers among the off spring, barring complications affecting viability. Contrary to this expectation, in a number of studies it has been found that one allele or chromosome is recovered in significantly more than 50 percent of the functional gametes. The time and mode of the action resulting in inequality varies; where these phenomena result from events of the meiotic process they are known as instances of meiotic drive (SANDLER and NOVITSKI 1957). The case described herein was isolated from a chronically irradiated population (NOVITSKI and HANKS 1961), and was referred to as “29G” previously (HANKS 1961; ERICKSON and HANKS 1961). HANKS (1964) has renamed the line “RD”, referring to a disruption in recovery of the Y chromosome. which gives rise to a high-female sex ratio in the off spring of RD males.
Time of temperature sensitivity of meiotic drive in Drosophila melanogaster
4508Erickson, JH, G. D., American Naturalist, 95:247-250. 1961-01-06 00:00:00.
In a line of Drosophila melanogaster demonstrating meiotic drive, it was found that the high recovery rate of the X-chromosome could be nearly nullified by temperature treatment. A series of experiments were carried out to determine at what stage of the life-cycle this treatment is effective. The effect is observed to occur during a part of the process of spermatogenesis, either during the primary spermatocyte stage or extending through this and the meiotic divisions which follow.
A new sex-ratio abnormality in Drosophila obscura
6047S. Gershenson, Genetics, 13:488. 1928-12-30 21:32:10.
1. The sex-ratio in the normal lines of Drosophila obscura is very near to the theoretical 1 : 1. 2. Out of 19 females caught in nature, two were heterozygous for a gene which causes strong deviations in the normal sex distribution. 3. The researches made have shown that this gene is localized in the X-chromosome and is transmitted like an ordinary sex-linked gene. 4. This gene is absolutely sex-limited, as it is not manifested either in heterozygous or homozygous females. 5. The males bearing this gene give in their progeny about 96 percent of females and only about 4 percent of males. 6. This gene has no influence on the development of the zygotes already formed, but acts directly upon the mechanism of sex-distribution. It provokes a sharp preponderance of females by almost totally removing the spermatozoa with the Y-chromosome from the fertilization process, acting thus like a gametic lethal (in the genetic sense of this term).
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