Keywords: Chromosomal drive
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.
Autosomally-encoded segregation distortion of sex chromosomes
35096Greenberg Naomi L., Patten Manus M. and Schenkel Martijn A., Proc. R. Soc. B., 292. 2025-09-01 19:48:42.
Some selfish genetic elements drive at meiosis to achieve transmission distortion, breaking the rules of Mendelian segregation to enhance their own evolutionary success. It has been shown that enhancers of drive must act in cis in order to gain the selfish benefit of drive and that suppressors of drive will be selected at unlinked loci. Here, we model the evolution of an autosomal trans-acting gene (Distorter) that causes the Y chromosome (or even 0 chromosome) to drive without driving itself, a phenomenon we call ‘remote-control meiotic drive’. We show that such a gene may spread in the population when linked to a second locus, Assister, whose alleles are transmitted at different frequencies through sperm as compared to eggs, for which we consider various scenarios, such as sexually antagonistic selection or sex-limited drive. Depending on the mechanistic details of sex-chromosome drive, Distorter’s spread can additionally facilitate transitions between XY and X0 sex determination. Our results provide a proof of principle that stretches the current understanding of segregation modifier and sex allocation theory. Moreover, we identify alternative evolutionary trajectories that could also lead to remote control drive and discuss its potential applications in developing synthetic sex-ratio-distorting elements for use in pest management, for example.
The B Chromosome of Pseudococcus viburni: A Selfish Chromosome that Exploits Whole-Genome Meiotic Drive
34939Isabelle M Vea, Andrés G de la Filia, Kamil S Jaron, Scott E J Barlow, Marion Herbette, Andrew J Mongue, Ross Nelson, Francisco J Ruiz-Ruano, Laura Ross, Genome Biology and Evolution, 17. 2025-06-24 08:56:02.
Meiosis is generally a fair process: each chromosome has a 50% chance of being included into each gamete. However, meiosis can become aberrant with some chromosomes having a higher chance of making it into gametes than others. Yet, why and how such systems evolve remains unclear. Here, we study the unusual reproductive genetics of mealybugs, where only maternal-origin chromosomes are included in gametes during male meiosis, while paternal chromosomes are eliminated. One species—Pseudococcus viburni—has a segregating B chromosome that drives by escaping paternal genome elimination. We present whole genome and gene expression data from lines with and without B chromosomes. We identify B-linked sequences including 204 protein-coding genes and a satellite repeat that makes up a significant proportion of the chromosome. The few paralogs between the B and the core genome are distributed throughout the genome, arguing against a simple, or at least recent, chromosomal duplication of one of the autosomes to create the B. We do, however, find one 373 kb region containing 146 genes that appears to be a recent translocation. Finally, we show that while many B-linked genes are expressed during meiosis, most of these are encoded on the recently translocated region. Only a small number of B-exclusive genes are expressed during meiosis. Of these, only one was overexpressed during male meiosis, which is when the drive occurs: an acetyltransferase involved in H3K56Ac, which has a putative role in meiosis and is, therefore, a promising candidate for further studies.
Conflicting Kinesin-14s in a single chromosomal drive haplotype
34899Meghan J Brady, Anjali Gupta, Jonathan I Gent, et al., Genetics, 2025-06-06 08:37:38.
In maize, there are 2 meiotic drive systems that target large heterochromatic knobs composed of tandem repeats known as knob180 and TR-1. The first meiotic drive haplotype, abnormal chromosome 10 (Ab10) confers strong meiotic drive (∼75% transmission as a heterozygote) and encodes 2 kinesins: KINDR, which associates with knob180 repeats, and TRKIN, which associates with TR-1 repeats. Prior data show that meiotic drive is conferred primarily by the KINDR/knob180 system while the TRKIN/TR-1 system seems to have little or no role, making it unclear why Trkin has been maintained in Ab10 haplotypes. The second meiotic drive haplotype, K10L2, confers a low level of meiotic drive (∼51–52%) and only encodes the TRKIN/TR-1 system. Here, we used long-read sequencing to assemble the K10L2 haplotype and showed that it has strong homology to an internal portion of the Ab10 haplotype. We also carried out CRISPR mutagenesis to test the role of Trkin on Ab10 and K10L2. The data indicate that the Trkin gene on Ab10 does not improve drive or fitness but instead has a weak deleterious effect when paired with a normal chromosome 10. The deleterious effect is more severe when Ab10 is paired with K10L2: in this context, functional Trkin on either chromosome nearly abolishes Ab10 drive. Mathematical modeling based on the empirical data suggests that Trkin is unlikely to persist on Ab10. We conclude that Trkin either confers an advantage to Ab10 in untested circumstances or that it is in the process of being purged from the Ab10 population.
Intrinsically weak sex chromosome drive through sequential asymmetric meiosis
34889Xuefeng Meng, Yukiko M. Yamashita, Science Advances, 11. 2025-06-02 19:06:59.
Meiotic drivers are selfish genetic elements that bias their own transmission, violating Mendel’s Law of Equal Segregation. It has long been recognized that sex chromosome–linked drivers present a paradox: Their success in transmission can severely distort populations’ sex ratio and lead to extinction. This paradox is typically solved by the presence of suppressors or fitness costs associated with the driver, limiting the propagation of the driver. Here, we show that Stellate (Ste) in Drosophila melanogaster represents a novel class of X chromosome–linked driver that operates with an inherent mechanism that weakens its drive strength. Ste protein asymmetrically segregates into Y-bearing cells during meiosis I, subsequently causing their death. Unexpectedly, Ste segregates asymmetrically again during meiosis II, sparing half of the Y-bearing spermatids from Ste-induced defects, thereby weakening the drive strength. Our findings reveal a mechanism by which sex chromosome drivers avoid suicidal success.
A male-transmitted B chromosome undergoes strong meiotic drag in females
34850Patrick M Ferree, Jayla Cummings, Emma Garman, et al, bioRxiv, 2025-05-26 20:12:34.
Many organisms carry extra, non-essential chromosomes known as B chromosomes (Bs), which are selfishly transmitted at super-Mendelian levels to offspring. This heightened transmission, termed drive, occurs during gametogenesis, usually in one of the two parents. In some cases, Bs can experience an opposing process, drag, which reduces their transmission. If these processes occur together in the same organism, one in each parental sex, then they may facilitate the spread of Bs while countering their accumulation in the genome to harmful levels. While previous studies have elucidated mechanistic aspects of B drive, little is known about drag or other factors that govern the inheritance of these selfish genetic elements. Here we examined the inheritance of Paternal Sex Ratio (PSR), a single-copy B in the jewel wasp, Nasonia vitripennis, which is transmitted paternally to offspring. PSR drives by converting female-destined embryos into PSR-transmitting males. Using genetic manipulation, we produced exceptional PSR-carrying females, which were used to assess B transmission potential. We found that females transmit PSR at unexpectedly low levels compared to univalent chromosomes in other organisms. This reduced transmission stems from remarkable loss of PSR from the egg's nucleus upon entry into meiosis, an effect that may be caused by an absence of microtubule-based spindle fibers in meiosis I-arrested wasp eggs. We also found that PSR is strictly limited to a single copy per genome, suggesting that two PSR copies are lethal during development. Our findings reveal the successful inheritance of this selfish B chromosome involves a restriction to a single copy and hidden female meiotic drag in addition to its strong paternal drive.
Mechanisms, Machinery, and Dynamics of Chromosome Segregation in Zea mays
34244Duffy, Marissa E., Michael Ngaw, Shayna E. Polsky, et al., Genes, 15. 2025-01-12 21:25:37.
Zea mays (maize) is both an agronomically important crop and a powerful genetic model system with an extensive molecular toolkit and genomic resources. With these tools, maize is an optimal system for cytogenetic study, particularly in the investigation of chromosome segregation. Here, we review the advances made in maize chromosome segregation, specifically in the regulation and dynamic assembly of the mitotic and meiotic spindle, the inheritance and mechanisms of the abnormal chromosome variant Ab10, the regulation of chromosome–spindle interactions via the spindle assembly checkpoint, and the function of kinetochore proteins that bridge chromosomes and spindles. In this review, we discuss these processes in a species-specific context including features that are both conserved and unique to Z. mays. Additionally, we highlight new protein structure prediction tools and make use of these tools to identify several novel kinetochore and spindle assembly checkpoint proteins in Z. mays.
The Meiotic Drive: Intragenomic Competition and Selection
33610I. A. Zakharov, Russian Journal of Genetics, 60:1311-1318. 2024-11-12 12:02:24.
The article considers the distribution and mechanisms of the meiotic drive as a phenomenon manifested in unequal transmission of gene alleles and/or homologous chromosomes into gametes during meiosis. The meiotic drive has been studied in the most detail in Drosophila, mice, corn, and ascomycete fungi of the genera Neurospora and Podospora. The consequence of the meiotic drive is a shift in the frequencies of alleles in the gene pool and the maintenance of nonadaptive traits in the population.
Genetic study solves the mystery of ‘selfish’ B chromosomes in rye
33606Leibniz Institute of Plant Genetics and Crop Plant Research, Phys.org, 2024-11-12 09:04:08.
Some chromosomes, such as B chromosomes, can increase their inheritance rate to their own advantage. These extra chromosomes are found in many plants, animals, and fungi and rely upon various strategies to avoid being eliminated over time, as most organisms tend to remove non-essential genetic elements. However, the genetic mechanisms by which B chromosomes avoid elimination are poorly understood. An international research team led by IPK Leibniz Institute identified genes on the rye B chromosome that are likely responsible for regulating this process. The results were published in Nature Communications. Supernumerary B chromosomes, unlike A (standard) chromosomes, are not required for the normal growth and development of organisms and as of 2024, B chromosomes have been discovered in almost 3,000 species from all eukaryotic phyla. Most B chromosomes confer no detectable selective consequences at low numbers, but increased numbers can result in phenotypic aberrations and reduced fertility. To avoid elimination, many B chromosomes influence cell division in their favor and increase their copy number in the process. This phenomenon is called chromosome drive. The "selfish" B chromosomes, therefore, only become active when their existence is at stake and not for the benefit of the plant.
PSRs: Selfish chromosomes that manipulate reproductive development
28974Xinmi Zhang, Patrick M. Ferree, Seminars in Cell and Developmental Biology, 159-160:66-73. 2024-03-05 14:28:18.
B chromosomes are intriguing “selfish” genetic elements, many of which exhibit higher-than-Mendelian transmission. This perspective highlights a group of B chromosomes known as Paternal Sex Ratio chromosomes (PSRs), which are found in several insects with haplo-diploid reproduction. PSRs harshly alter the organism’s reproduction to facilitate their own inheritance. A manifestation of this effect is the conversion of female destined individuals into males. Key to this conversion is the mysterious ability of PSRs to cause elimination of the sperm-inherited half of the genome during zygote formation. Here we discuss how PSRs were discovered, what is known about how they alter paternal chromatin dynamics to cause sex conversion, and how PSR-induced genome elimination is different from other forms of programmed genome elimination in different insects. PSRs also stand out because their DNA sequence compositions differ in remarkable ways from their insect’s essential chromosomes, a characteristic suggestive of interspecies origins. Broadly, we also highlight poorly understood aspects of PSR dynamics that need to be investigated.
Meeting Abstracts from the 5th B Chromosome Conference
28643Anonymous, BMC Proceedings, 17. 2023-12-05 09:57:11.
During preparations for the 5th B-Chromosome Conference (5BCC), rapid and significant progress was observed in the germ-restricted chromosomes (GRCs) study. GRCs are additional chromosomes found only in germ cells and are eliminated from somatic cells in many avian and dipteran species. Despite their specificity, GRCs share many features of B chromosomes. In view of this intriguing connection, we have decided to extend a special invitation to researchers dedicated to this fascinating topic and to encourage them to participate in the 5BCC. The 5th B Chromosome Conference will be held in Petnica, Serbia, from 14th to 17th October 2023. The conference will take place in five sessions: Constitution, gene composition, Dynamics in populations, B chromosome effects, Segregation behaviour, Origin and evolution. The conference will unite the most prominent chromosome biologists for a stimulating exchange of ideas and insights. We hope that the conference will help establish better collaboration and enable progress in B chromosome research.
Meiotic segregation and post-meiotic drive of the Festuca pratensis B chromosome
27749R. Ebrahimzadegan, J. Fuchs, J. Chen, V. Schubert, A. Meister, A. Houben and G. Mirzaghaderi, Chromosome Research, 31:26. 2023-09-02 07:34:43.
In many species, the transmission of B chromosomes (Bs) does not follow the Mendelian laws of equal segregation and independent assortment. This deviation results in transmission rates of Bs higher than 0.5, a process known as “chromosome drive”. Here, we studied the behavior of the 103 Mbp-large B chromosome of Festuca pratensis during all meiotic and mitotic stages of microsporogenesis. Mostly, the B chromosome of F. pratensis segregates during meiosis like standard A chromosomes (As). In some cases, the B passes through meiosis in a non-Mendelian segregation leading to their accumulation already in meiosis. However, a true drive of the B happens during the first pollen mitosis, by which the B preferentially migrates to the generative nucleus. During second pollen mitosis, B divides equally between the two sperms. Despite some differences in the frequency of drive between individuals with different numbers of Bs, at least 82% of drive was observed. Flow cytometry-based quantification of B-containing sperm nuclei agrees with the FISH data.
Ability of a selfish B chromosome to evade genome elimination in the jewel wasp, Nasonia vitripennis
27561H. Lee, P. Seo, S. Teklay, E. Yuguchi, E. D. Benetta, J. H. Werren and P. M. Ferree, Heredity, 2023-07-31 07:06:11.
B chromosomes are non-essential, extra chromosomes that can exhibit transmission-enhancing behaviors, including meiotic drive, mitotic drive, and induction of genome elimination, in plants and animals. A fundamental but poorly understood question is what characteristics allow B chromosomes to exhibit these extraordinary behaviors. The jewel wasp, Nasonia vitripennis, harbors a heterochromatic, paternally transmitted B chromosome known as paternal sex ratio (PSR), which causes complete elimination of the sperm-contributed half of the genome during the first mitotic division of fertilized embryos. This genome elimination event may result from specific, previously observed alterations of the paternal chromatin. Due to the haplo-diploid reproduction of the wasp, genome elimination by PSR causes female-destined embryos to develop as haploid males that transmit PSR. PSR does not undergo self-elimination despite its presence with the paternal chromatin until the elimination event. Here we performed fluorescence microscopic analyses aimed at understanding this unexplained property. Our results show that PSR, like the rest of the genome, participates in the histone-to-protamine transition, arguing that PSR does not avoid this transition to escape self-elimination. In addition, PSR partially escapes the chromatin-altering activity of the intracellular bacterium, Wolbachia, demonstrating that this ability to evade chromatin alteration is not limited to PSR's own activity. Finally, we observed that the rDNA locus and other unidentified heterochromatic regions of the wasp's genome also seem to evade chromatin disruption by PSR, suggesting that PSR's genome-eliminating activity does not affect heterochromatin. Thus, PSR may target an aspect of euchromatin to cause genome elimination.
Teosinte Pollen Drive guides maize domestication and evolution by RNAi
27414B. Berube , E. Ernst, J. Cahn, B. Roche, C. d. S. Alves, A. Scheben, A. Siepel, J. Ross-Ibarra, J. Kermicle and R. Martienssen, bioRxiv, 2023.07.12.548689. 2023-07-13 08:27:36.
Meiotic drivers subvert Mendelian expectations by manipulating reproductive development to bias their own transmission. Chromosomal drive typically functions in asymmetric female meiosis, while gene drive is normally postmeiotic and typically found in males. Using single molecule and single-pollen genome sequencing, we describe Teosinte Pollen Drive, an instance of gene drive in hybrids between maize (Zea mays ssp. mays) and teosinte mexicana (Zea mays ssp. mexicana), that depends on RNA interference (RNAi). 22nt small RNAs from a non-coding RNA hairpin in mexicana depend on Dicer-Like 2 (Dcl2) and target Teosinte Drive Responder 1 (Tdr1), which encodes a lipase required for pollen viability. Dcl2, Tdr1, and the hairpin are in tight pseudolinkage on chromosome 5, but only when transmitted through the male. Introgression of mexicana into early cultivated maize is thought to have been critical to its geographical dispersal throughout the Americas, and a tightly linked inversion in mexicana spans a major domestication sweep in modern maize. A survey of maize landraces and sympatric populations of teosinte mexicana reveals correlated patterns of admixture among unlinked genes required for RNAi on at least 3 chromosomes that are also subject to gene drive in pollen from synthetic hybrids. Teosinte Pollen Drive likely played a major role in maize domestication and evolution, and offers an explanation for the widespread abundance of "self" small RNAs in the germlines of plants and animals.Competing Interest StatementThe authors have declared no competing interest.
Cell biology: Selfish B chromosomes unleashed by a dysfunctional chromosome segregation system
25563P. Ferree, Current Biology, 33:R431-R434. 2023-06-05 09:18:59.
A study in the fruit fly Drosophila melanogaster shows that a defective chromosome segregation system allows non-essential B chromosomes to transmit at higher-than-Mendelian frequencies.
B chromosomes reveal a female meiotic drive suppression system in Drosophila melanogaster
25112S. L. Hanlon and R. S. Hawley, Current Biology, 2023-04-27 10:03:45.
Selfish genetic elements use a myriad of mechanisms to drive their inheritance and ensure their survival into the next generation, often at a fitness cost to its host.(1)(,)(2) Although the catalog of selfish genetic elements is rapidly growing, our understanding of host drive suppression systems that counteract self-seeking behavior is lacking. Here, we demonstrate that the biased transmission of the non-essential, non-driving B chromosomes in Drosophila melanogaster can be achieved in a specific genetic background. Combining a null mutant of matrimony, a gene that encodes a female-specific meiotic regulator of Polo kinase,(3)(,)(4) with the TM3 balancer chromosome creates a driving genotype that is permissive for the biased transmission of the B chromosomes. This drive is female-specific, and both genetic components are necessary, but not individually sufficient, for permitting a strong drive of the B chromosomes. Examination of metaphase I oocytes reveals that B chromosome localization within the DNA mass is mostly abnormal when drive is the strongest, indicating a failure of the mechanism(s) responsible for the proper distribution of B chromosomes. We propose that some proteins important for proper chromosome segregation during meiosis, like Matrimony, may have an essential role as part of a meiotic drive suppression system that modulates chromosome segregation to prevent genetic elements from exploiting the inherent asymmetry of female meiosis.
Mitotic exchange in female germline stem cells is the major source of Sex Ratio chromosome recombination in Drosophila pseudoobscura
23719S. Koury, G3 Genes|Genomes|Genetics, 2022-10-04 06:48:20.
Sex Ratio chromosomes in Drosophila pseudoobscura are selfish X chromosome variants associated with three non-overlapping inversions. In the male germline, Sex Ratio chromosomes distort segregation of X and Y chromosomes (99:1), thereby skewing progeny sex ratio. In the female germline, segregation of Sex Ratio chromosomes is mendelian (50:50), but non-overlapping inversions strongly suppress recombination establishing a 26 Megabase haplotype (constituting ∼20% of the haploid genome). Rare crossover events located between non-overlapping inversions can disrupt this haplotype, and recombinants have sometimes been found in natural populations. We recently reported on the first lab-generated Sex Ratio recombinants occurring at a rate of 0.0012 crossovers per female meiosis. An improved experimental design presented here reveals these recombination events were at least 4-fold more frequent than previously estimated. Furthermore, recombination events were strongly clustered, indicating the majority arose from mitotic exchange in female germline stem cells and not from meiotic crossing-over in primary oocytes. Finally, asymmetric recovery of complementary recombinants was consistent with unequal exchange causing the recombination-induced viability defects. Incorporating these experimental results into population models for Sex Ratio chromosome evolution provided a substantially better fit to natural population frequencies and allowed maintenance of the highly differentiated 26 Megabase Sex Ratio haplotype without invoking strong epistatic selection. This study provides the first estimate of spontaneous mitotic exchange for naturally-occurring chromosomes in Drosophila female germline stem cells, reveals a much higher Sex Ratio chromosome recombination rate, and develops a mathematical model that accurately predicts the rarity of recombinant Sex Ratio chromosomes in natural populations.
B Chromosomes in Psalidodon scabripinnis (Characiformes, Characidae) Species Complex
23552D. Silva, J. P. Castro, C. A. G. Goes, R. Utsunomia, M. R. Vidal, C. N. Nascimento, L. F. Lasmar, F. G. Paim, L. B. Soares, C. Oliveira, F. Porto-Foresti, R. F. Artoni and F. Foresti, Animals (Basel), 12. 2022-08-25 06:15:39.
B chromosomes are extra-genomic components of cells found in individuals and in populations of some eukaryotic organisms. They have been described since the first observations of chromosomes, but several aspects of their biology remain enigmatic. Despite being present in hundreds of fungi, plants, and animal species, only a small number of B chromosomes have been investigated through high-throughput analyses, revealing the remarkable mechanisms employed by these elements to ensure their maintenance. Populations of the Psalidodon scabripinnis species complex exhibit great B chromosome diversity, making them a useful material for various analyses. In recent years, important aspects of their biology have been revealed. Here, we review these studies presenting a comprehensive view of the B chromosomes in the P. scabripinnis complex and a new hypothesis regarding the role of the B chromosome in the speciation process.
Non-Mendelian transmission of accessory chromosomes in fungi
23305J. Komluski, E. H. Stukenbrock and M. Habig, Chromosome Research, 2022-07-26 07:44:07.
Non-Mendelian transmission has been reported for various genetic elements, ranging from small transposons to entire chromosomes. One prime example of such a transmission pattern are B chromosomes in plants and animals. Accessory chromosomes in fungi are similar to B chromosomes in showing presence/absence polymorphism and being non-essential. How these chromosomes are transmitted during meiosis is however poorly understood-despite their often high impact on the fitness of the host. For several fungal organisms, a non-Mendelian transmission or a mechanistically unique meiotic drive of accessory chromosomes have been reported. In this review, we provide an overview of the possible mechanisms that can cause the non-Mendelian transmission or meiotic drives of fungal accessory chromosomes. We compare processes responsible for the non-Mendelian transmission of accessory chromosomes for different fungal eukaryotes and discuss the structural traits of fungal accessory chromosomes affecting their meiotic transmission. We conclude that research on fungal accessory chromosomes, due to their small size, ease of sequencing, and epigenetic profiling, can complement the study of B chromosomes in deciphering factors that influence and regulate the non-Mendelian transmission of entire chromosomes.
Selfish centromeres and the wastefulness of human reproduction
23129L. D. Hurst, PLOS Biology, 20:e3001671. 2022-07-05 10:34:00.
Many human embryos die in utero owing to an excess or deficit of chromosomes, a phenomenon known as aneuploidy; this is largely a consequence of nondisjunction during maternal meiosis I. Asymmetries of this division render it vulnerable to selfish centromeres that promote their own transmission, these being thought to somehow underpin aneuploidy. In this essay, I suggest that these vulnerabilities provide only half the solution to the enigma. In mammals, as in utero and postnatal provisioning is continuous, the costs of early death are mitigated. With such reproductive compensation, selection can favour a centromere because it induces lethal aneuploidy: if, when taken towards the polar body, it instead kills the embryo via aneuploidy, it gains. The model is consistent with the observation that reduced dosage of a murine drive suppressor induces aneuploidy and with the fact that high aneuploidy rates in vertebrates are seen exclusively in mammals. I propose further tests of this idea. The wastefulness of human reproduction may be a price we pay for nurturing our offspring.
Rye B chromosomes differently influence the expression of A chromosome-encoded genes depending on the host species
23119A. Boudichevskaia, A. Fiebig, K. Kumke, A. Himmelbach and A. Houben, Chromosome Research, 2022-07-04 08:55:15.
The B chromosome (B) is a dispensable component of the genome in many species. To evaluate the impact of Bs on the transcriptome of the standard A chromosomes (A), comparative RNA-seq analyses of rye and wheat anthers with and without additional rye Bs were conducted. In both species, 5-6% of the A-derived transcripts across the entire genomes were differentially expressed in the presence of 2Bs. The GO term enrichment analysis revealed that Bs influence A chromosome encoded processes like "gene silencing"; "DNA methylation or demethylation"; "chromatin silencing"; "negative regulation of gene expression, epigenetic"; "post-embryonic development"; and "chromosome organization." 244 B chromosome responsive A-located genes in + 2B rye and + B wheat shared the same biological function. Positively correlated with the number of Bs, 939 and 1391 B-specific transcripts were identified in + 2B and + 4B wheat samples, respectively. 85% of B-transcripts in + 2B were also found in + 4B transcriptomes. 297 B-specific transcripts were identified in + 2B rye, and 27% were common to the B-derived transcripts identified in + B wheat. Bs encode mobile elements and housekeeping genes, but most B-transcripts were without detectable similarity to known genes. Some of these genes are involved in cell division-related functions like Nuf2 and might indicate their importance in maintaining Bs. The transcriptome analysis provides new insights into the complex interrelationship between standard A chromosomes and supernumerary B chromosomes.
Centromere drive: model systems and experimental progress
23148D. Dudka and M. A. Lampson, Chromosome Research, 2022-06-22 09:28:53.
Centromeres connect chromosomes and spindle microtubules to ensure faithful chromosome segregation. Paradoxically, despite this conserved function, centromeric DNA evolves rapidly and centromeric proteins show signatures of positive selection. The centromere drive hypothesis proposes that centromeric DNA can act like a selfish genetic element and drive non-Mendelian segregation during asymmetric female meiosis. Resulting fitness costs lead to genetic conflict with the rest of the genome and impose a selective pressure for centromeric proteins to adapt by suppressing the costs. Here, we describe experimental model systems for centromere drive in yellow monkeyflowers and mice, summarize key findings demonstrating centromere drive, and explain molecular mechanisms. We further discuss efforts to test if centromeric proteins are involved in suppressing drive-associated fitness costs, highlight a model for centromere drive and suppression in mice, and put forth outstanding questions for future research.
Meiotic behavior, transmission and active genes of B chromosomes in the cichlid Astatotilapia latifasciata: new clues about nature, evolution and maintenance of accessory elements
22951A. L. Cardoso, N. B. Venturelli, I. da Cruz, F. M. de Sá Patroni, D. de Moraes, R. A. de Oliveira, R. Benavente and C. Martins, Molecular Genetics and Genomics, 2022-06-15 06:37:12.
Supernumerary B chromosomes (Bs) are dispensable genetic elements widespread in eukaryotes and are poorly understood mainly in relation to mechanisms of maintenance and transmission. The cichlid Astatotilapia latifasciata can harbor Bs in a range of 0 (named B -) and 1-2 (named B +). The B in A. latifasciata is rich in several classes of repetitive DNA sequences, contains protein coding genes, and affects hosts in diverse ways, including sex-biased effects. To advance in the knowledge about the mechanisms of maintenance and transmission of B chromosomes in A. latifasciata, here, we studied the meiotic behavior in males and transmission rates of A. latifasciata B chromosome. We also analyzed structurally and functionally the predicted B chromosome copies of the cell cycle genes separin-like, tubb1-like and kif11-like. We identified in the meiotic structure relative to the B chromosome the presence of proteins associated with Synaptonemal Complex organization (SMC3, SYCP1 and SYCP3) and found that the B performs self-pairing. These data suggest that isochromosome formation was a step during B chromosome evolution and this element is in a stage of diversification of the two arms keeping the self-pairing behavior to protect the A chromosome complement of negative effects of recombination. Moreover, we observed no occurrence of B-drive and confirmed the presence of cell cycle genes copies in the B chromosome and their transcription in encephalon, muscle and gonads, which can indicates beneficial effects to hosts and contribute to B maintenance.
Supergene potential of a selfish centromere
22893F. Finseth, K. Brown, A. Demaree and L. Fishman, Philos Trans R Soc Lond B Biol Sci, 377:20210208. 2022-06-13 06:07:46.
Selfishly evolving centromeres bias their transmission by exploiting the asymmetry of female meiosis and preferentially segregating to the egg. Such female meiotic drive systems have the potential to be supergenes, with multiple linked loci contributing to drive costs or enhancement. Here, we explore the supergene potential of a selfish centromere (D) in Mimulus guttatus, which was discovered in the Iron Mountain (IM) Oregon population. In the nearby Cone Peak population, D is still a large, non-recombining and costly haplotype that recently swept, but shorter haplotypes and mutational variation suggest a distinct population history. We detected D in five additional populations spanning more than 200 km; together, these findings suggest that selfish centromere dynamics are widespread in M. guttatus. Transcriptome comparisons reveal elevated differences in expression between driving and non-driving haplotypes within, but not outside, the drive region, suggesting large-scale cis effects of D's spread on gene expression. We use the expression data to refine linked candidates that may interact with drive, including Nuclear Autoantigenic Sperm Protein (NASP(SIM3)), which chaperones the centromere-defining histone CenH3 known to modify Mimulus drive. Together, our results show that selfishly evolving centromeres may exhibit supergene behaviour and lay the foundation for future genetic dissection of drive and its costs. This article is part of the theme issue 'Genomic architecture of supergenes: causes and evolutionary consequences'.
Non-Mendelian segregation and transmission drive of B chromosomes
22725J. P. M. Camacho, Chromosome Research, 2022-06-03 07:58:27.
Selfish genetic elements (SGE) get a transmission advantage (drive) thanks to their non-Mendelian inheritance. Here I identify eight steps during the reproductive cycle that can be subverted by SGEs to thrive in natural populations. Even though only three steps occur during meiosis, most cases of segregation distortion are considered “meiotic drive sensu lato.” As this is a source of unnecessary contradictions, I suggest always using the term “transmission ratio distortion” (TRD). Chromosomal SGEs (e.g., B chromosomes) exhibit almost all types of TRD. In plants, the best-studied type of TRD for B chromosomes occurs post-meiotically during male gametophyte maturation. However, in animals, the two main types are pre-meiotic and meiotic TRDs, in all cases associated with gonotaxis (i.e., a preference of B chromosomes for germ cells). Frequently, TRD drivers in genic SGEs (e.g., t-alleles and segregation distorters in Drosophila) are paralogous copies of genes from the standard genome, whereas their targets can be other genes or satellite DNA (satDNA). As B chromosomes are often rich in satDNA and contain paralogous copies of A chromosome genes, perhaps their drive mechanisms are similar to those of genic SGEs. So far, the only association between a B chromosome gene and TRD is the gene haplodizer in Nasonia vitripennis. The discovery of B-genes controlling B-drive in other species does not appear to be far off, but experimental crosses will be needed to simultaneously test the TRD of a given B chromosome and the expression of its genes.
The maize abnormal chromosome 10 meiotic drive haplotype: a review
22690R. K. Dawe, Chromosome Research, 2022-06-02 14:35:18.
The maize abnormal chromosome 10 (Ab10) haplotype encodes a meiotic drive system that converts heterochromatic knobs into centromere-like bodies that are preferentially segregated through female meiosis. Ab10 was first described in the 1940s and has been intensively studied. Here I provide a comprehensive review of the literature, starting from the discovery of knobs and Ab10, preceding through the classic literature, and finishing with molecular structure and mechanisms. The defining features of the Ab10 haplotype are its two specialized kinesins, Kinesin driver and TR-1 kinesin, that activate neocentromeres at knobs containing different classes of the tandem repeat. In most Ab10 haplotypes, the two kinesin/knob systems cooperate to promote maximum meiotic drive. However, recent interpretations suggest that each kinesin/knob system can function as an independent meiotic driver and that in some cases they compete with each other. Ab10 is present at low frequencies throughout the genus Zea and has significantly expanded genome size by promoting the formation of knobs throughout the genome.
Mendelian nightmares: the germline-restricted chromosome of songbirds
21724P. Borodin, A. Chen, W. Forstmeier, S. Fouché, L. Malinovskaya, Y. Pei, R. Reifová, F. J. Ruiz-Ruano, S. A. Schlebusch, M. Sotelo-Muñoz, A. Torgasheva, N. Vontzou and A. Suh, Chromosome Res, 2022-04-13 09:33:56.
Germline-restricted chromosomes (GRCs) are accessory chromosomes that occur only in germ cells. They are eliminated from somatic cells through programmed DNA elimination during embryo development. GRCs have been observed in several unrelated animal taxa and show peculiar modes of non-Mendelian inheritance and within-individual elimination. Recent cytogenetic and phylogenomic evidence suggests that a GRC is present across the species-rich songbirds, but absent in non-passerine birds, implying that over half of all 10,500 bird species have extensive germline/soma genome differences. Here, we review recent insights gained from genomic, transcriptomic, and cytogenetic approaches with regard to the genetic content, phylogenetic distribution, and inheritance of the songbird GRC. While many questions remain unsolved in terms of GRC inheritance, elimination, and function, we discuss plausible scenarios and future directions for understanding this widespread form of programmed DNA elimination.
The non-Mendelian behavior of plant B chromosomes
21722J. Chen, J. A. Birchler and A. Houben, Chromosome Res, 2022-04-12 09:30:19.
B chromosomes, also known as supernumerary chromosomes, are dispensable elements in the genome of many plants, animals, and fungi. Many B chromosomes have evolved one or more drive mechanisms to transmit themselves at a higher frequency than predicted by Mendelian genetics, and these mechanisms counteract the tendency of non-essential genetic elements to be lost over time. The frequency of Bs in a population results from a balance between their effect on host fitness and their transmission rate. Here, we will summarize the findings of the drive process of plant B chromosomes, focusing on maize and rye.
B-A Chromosome Translocations Possessing an A Centromere Partly Overcome the Root-Restricted Process of Chromosome Elimination in Aegilops speltoides
21726D. Li, A. Ruban, J. Fuchs, H. Kang and A. Houben, Frontiers in Cell and Developmental Biology, 10. 2022-03-28 09:37:44.
Some eukaryotes exhibit dramatic genome size differences between cells of different organs, resulting from the programmed elimination of chromosomes. Aegilops speltoides is an annual diploid species from the Poaceae family, with a maximum number of eight B chromosomes (Bs) in addition to its inherent seven pairs of standard A chromosomes (As). The Bs of this species undergo precise elimination in roots early in embryo development. In areal parts of the plant, the number of Bs is stable. To affect the root restricted process of B chromosome elimination, we employed X-ray mutagenesis, and different types of restructured Bs were identified. Standard Bs were observed in all analyzed shoots of mutagenized plants, while B-A translocations were only observed in 35.7% of F1 plants. In total 40 different B variants inconsistently escaped the elimination process in roots. As a result, mosaicism of B chromosome variants was found in roots. Only a small B chromosome fragment fused to an A chromosome was stably maintained in roots and shoots across F1 to F3 generations. The absence of B-A translocation chromosomes possessing a derived B centromere in root cells implies that the centromere of the B is a key component of the chromosome elimination process.
Studying the active role of the maize B chromosome in the modulation of gene expression
21018University of Missouri, Phys Org, 2022-03-16 06:42:22.
A team of University of Missouri biologists has made a new discovery that provides novel insights into the function and properties of the maize B chromosome. The study was led by Dr. Xiaowen Shi and Dr. Hua Yang, postdoctoral fellows in the Birchler lab, and reported in the January 2022 issue of the Plant Journal. Apart from the 20 normal A chromosomes, maize contains a nonessential B chromosome that can be present or absent from some individuals within a population. This chromosome has properties that drive its transmission in populations despite being nonessential. The maize B chromosome has wide applications in various genetic studies, including experimental mapping, chromosomal dosage studies, chromosomal behavior, and genetic engineering of minichromosomes. Although the maize B chromosome was discovered nearly a century ago, it has been thought to be an essentially inert chromosome. However, little was known about the function and properties of the maize B chromosome.
Effect of aneuploidy of a nonessential chromosome on gene expression in maize
19950X. Shi, H. Yang, C. Chen, J. Hou, T. Ji, J. Cheng and J. A. Birchler, Plant Journal, 2022-01-07 09:41:30.
The nonessential supernumerary maize B chromosome (B) has recently been shown to contain active genes and be capable of impacting gene expression of the A chromosomes. However, the effect of the B chromosome on gene expression is still unclear. In addition, it is unknown whether the accumulation of the B chromosome would have a cumulative effect on gene expression. To examine these questions, the global expression of genes, microRNAs (miRNAs), and transposable elements (TEs) of leaf tissue of W22 maize (Zea mays) plants with 0 to 7 copies of the B chromosome was studied. All experimental genotypes with B chromosomes displayed a trend of upregulated gene expression for a subset of A-located genes compared to the control. Over three thousand A-located genes are significantly differentially expressed in all experimental genotypes with the B chromosome relative to the control. Modulations of these genes are largely determined by the presence rather than the copy number of the B chromosome. By contrast, the expression of most B-located genes is positively correlated with B copy number, showing a proportional gene dosage effect. The B chromosome also causes increased expression of A-located miRNAs. Differentially expressed miRNAs potentially regulate their targets in a cascade of effects. Furthermore, the varied copy number of the B chromosome leads to the differential expression of A-located and B-located TEs. The findings provide novel insights into the function and properties of the B chromosome.
Exploiting a Y chromosome-linked Cas9 for sex selection and gene drive
19623S. Gamez, D. Chaverra-Rodriguez, A. Buchman, N. P. Kandul, S. C. Mendez-Sanchez, J. B. Bennett, C. H. Sánchez, T. Yang, I. Antoshechkin, J. E. Duque, P. A. Papathanos, J. M. Marshall and O. S. Akbari, Nature Communications, 7202. 2021-12-10 21:34:38.
CRISPR-based genetic engineering tools aimed to bias sex ratios, or drive effector genes into animal populations, often integrate the transgenes into autosomal chromosomes. However, in species with heterogametic sex chromsomes (e.g. XY, ZW), sex linkage of endonucleases could be beneficial to drive the expression in a sex-specific manner to produce genetic sexing systems, sex ratio distorters, or even sex-specific gene drives, for example. To explore this possibility, here we develop a transgenic line of Drosophila melanogaster expressing Cas9 from the Y chromosome. We functionally characterize the utility of this strain for both sex selection and gene drive finding it to be quite effective. To explore its utility for population control, we built mathematical models illustrating its dynamics as compared to other state-of-the-art systems designed for both population modification and suppression. Taken together, our results contribute to the development of current CRISPR genetic control tools and demonstrate the utility of using sex-linked Cas9 strains for genetic control of animals.
Evolution of B Chromosomes: From Dispensable Parasitic Chromosomes to Essential Genomic Players
19731M. Johnson Pokorná and R. Reifová, Frontiers in Genetics, 12:727570. 2021-12-09 12:58:40.
B chromosomes represent additional chromosomes found in many eukaryotic organisms. Their origin is not completely understood but recent genomic studies suggest that they mostly arise through rearrangements and duplications from standard chromosomes. They can occur in single or multiple copies in a cell and are usually present only in a subset of individuals in the population. Because B chromosomes frequently show unstable inheritance, their maintenance in a population is often associated with meiotic drive or other mechanisms that increase the probability of their transmission to the next generation. For all these reasons, B chromosomes have been commonly considered to be nonessential, selfish, parasitic elements. Although it was originally believed that B chromosomes had little or no effect on an organism's biology and fitness, a growing number of studies have shown that B chromosomes can play a significant role in processes such as sex determination, pathogenicity and resistance to pathogens. In some cases, B chromosomes became an essential part of the genome, turning into new sex chromosomes or germline-restricted chromosomes with important roles in the organism's fertility. Here, we review such cases of "cellular domestication" of B chromosomes and show that B chromosomes can be important genomic players with significant evolutionary impact.
Synthetic sex ratio distorters based on CRISPR for the control of harmful insect populations
19369Fasulo, B., Meccariello, A., Papathanos, P. A., and Windbichler, N., AREA-WIDE INTEGRATED PEST MANAGEMENT: Development and Field Application, 2021-11-29 16:55:38.
Since the overall reproductive output of a population is typically determined by the fertility of its females, which are rate-limiting in gamete production, a successful way to genetically control a population should involve artificially biasing the sex ratio towards males. In male heterogametic species, this could be achieved by the expression of a transgene-encoded endonuclease during spermatogenesis that would target and "shred" the X chromosome at several loci. This would prevent the transmission of X chromosome bearing gametes to the progeny, generating only males. Recent developments in molecular and synthetic biology have provided genome editing tools with great potential to engineer the genome of different species. Given the targeting flexibility of CRISPR-based endonucleases, it may now be possible to test whether X chromosome shredding has the potential to become a universal strategy to genetically control a wide variety of insect pests, of both agricultural and public health relevance.
The supernumerary B chromosome of maize: drive and genomic conflict
19136J. A. Birchler and H. Yang, Open Biol, 11:210197. 2021-11-04 14:47:43.
The supernumerary B chromosome of maize is dispensable, containing no vital genes, and thus is variable in number and presence in lines of maize. In order to be maintained in populations, it has a drive mechanism consisting of nondisjunction at the pollen mitosis that produces the two sperm cells, and then the sperm with the two B chromosomes has a preference for fertilizing the egg as opposed to the central cell in the process of double fertilization. The sequence of the B chromosome coupled with B chromosomal aberrations has localized features involved with nondisjunction and preferential fertilization, which are present at the centromeric region. The predicted genes from the sequence have paralogues dispersed across all A chromosomes and have widely different divergence times suggesting that they have transposed to the B chromosome over evolutionary time followed by degradation or have been co-opted for the selfish functions of the supernumerary chromosome.
Meiotic self-pairing of the Psalidodon (Characiformes, Characidae) iso-B chromosome: A successful perpetuation mechanism
18905D. Silva, C. Araya-Jaime, M. Yamashita, M. R. Vidal, C. Oliveira, F. Porto-Foresti, R. F. Artoni and F. Foresti, Genetics and Molecular Biology, 44:e20210084. 2021-10-08 14:29:15.
B chromosomes are non-essential additional genomic elements present in several animal and plant species. In fishes, species of the genus Psalidodon (Characiformes, Characidae) harbor great karyotype diversity, and multiple populations carry different types of non-essential B chromosomes. This study analyzed how the dispensable supernumerary B chromosome of Psalidodon paranae behaves during meiosis to overcome checkpoints and express its own meiosis-specific genes. We visualized the synaptonemal complexes of P. paranae individuals with zero, one, or two B chromosomes using immunodetection with anti-medaka SYCP3 antibody and fluorescence in situ hybridization with a (CA)15 microsatellite probe. Our results showed that B chromosomes self-pair in cells containing only one B chromosome. In cells with two identical B chromosomes, these elements remain as separate synaptonemal complexes or close self-paired elements in the nucleus territory. Overall, we reveal that B chromosomes can escape meiotic silencing of unsynapsed chromatin through a self-pairing process, allowing expression of their own genes to facilitate regular meiosis resulting in fertile individuals. This behavior, also seen in other congeneric species, might be related to their maintenance throughout the evolutionary history of Psalidodon.
A supernumerary “B-sex” chromosome drives male sex determination in the Pachón cavefish, Astyanax mexicanus
18508B. Imarazene, K. Du, S. Beille, E. Jouanno, R. Feron, Q. Pan, J. Torres-Paz, C. Lopez-Roques, A. Castinel, L. Gil, C. Kuchly, C. Donnadieu, H. Parrinello, L. Journot, C. Cabau, M. Zahm, C. Klopp, T. Pavlica, A. Al-Rikabi, T. Liehr, S. A. Simanovsky, J. Bo, Current Biology, 2021-09-09 14:28:39.
Sex chromosomes are generally derived from a pair of classical type-A chromosomes, and relatively few alternative models have been proposed up to now.(1)(,)(2) B chromosomes (Bs) are supernumerary and dispensable chromosomes with non-Mendelian inheritance found in many plant and animal species(3)(,)(4) that have often been considered as selfish genetic elements that behave as genome parasites.(5)(,)(6) The observation that in some species Bs can be either restricted or predominant in one sex(7-14) raised the interesting hypothesis that Bs could play a role in sex determination.(15) The characterization of putative B master sex-determining (MSD) genes, however, has not yet been provided to support this hypothesis. Here, in Astyanax mexicanus cavefish originating from Pachón cave, we show that Bs are strongly male predominant. Based on a high-quality genome assembly of a B-carrying male, we characterized the Pachón cavefish B sequence and found that it contains two duplicated loci of the putative MSD gene growth differentiation factor 6b (gdf6b). Supporting its role as an MSD gene, we found that the Pachón cavefish gdf6b gene is expressed specifically in differentiating male gonads, and that its knockout induces male-to-female sex reversal in B-carrying males. This demonstrates that gdf6b is necessary for triggering male sex determination in Pachón cavefish. Altogether these results bring multiple and independent lines of evidence supporting the conclusion that the Pachón cavefish B is a "B-sex" chromosome that contains duplicated copies of the gdf6b gene, which can promote male sex determination in this species.
Haldane’s duel: intragenomic conflict, selfish Y chromosomes and speciation
17509S. W. Roy, Trends in Genetics, 2021-06-21 14:14:05.
Haldane?s rule, which states that the heterogametic sex (XY or ZW females) fares more poorly in interspecific hybrids, is generally attributed to absence of one of the two species' X/Z chromosomes. However, Haldane?s rule is also observed in mouse placentas despite paternal X silencing. This pattern could reflect Y chromosomes having evolved to promote growth due to maternal?paternal conflict. If so, balanced sex investment arises from a complex intra- and intergenomic duel.
Sequence of the supernumerary B chromosome of maize provides insight into its drive mechanism and evolution
17202N. Blavet, H. Yang, H. Su, P. Solanský, R. N. Douglas, M. Karafiátová, L. Šimková, J. Zhang, Y. Liu, J. Hou, X. Shi, C. Chen, M. El-Walid, M. E. McCaw, P. S. Albert, Z. Gao, C. Zhao, G. Ben-Zvi, L. Glick, G. Kol, J. Shi, J. Vrána, H. Šimková, J. C. Lamb,, Proceedings of the National Academy of Sciences, 118:e2104254118. 2021-06-05 15:32:03.
B chromosomes are nonvital chromosomes found in thousands of plants and animals that persist through various drive mechanisms. The drive mechanism of the maize B chromosome consists of mitotic nondisjunction at the second pollen division to produce two unequal sperm and then the sperm with the B chromosomes preferentially fertilizes the egg in double fertilization. A high-quality sequence of the maize B chromosome together with genetic analysis reveals the cis factor for nondisjunction is a B chromosome-specific repeat interspersed in and around the centromere. The gene and transposable element content of the B chromosome and relaxed purifying selection of transposed protein-encoding genes suggest that the chromosome has been present in the evolutionary lineage for millions of years.B chromosomes are enigmatic elements in thousands of plant and animal genomes that persist in populations despite being nonessential. They circumvent the laws of Mendelian inheritance but the molecular mechanisms underlying this behavior remain unknown. Here we present the sequence, annotation, and analysis of the maize B chromosome providing insight into its drive mechanism. The sequence assembly reveals detailed locations of the elements involved with the cis and trans functions of its drive mechanism, consisting of nondisjunction at the second pollen mitosis and preferential fertilization of the egg by the B-containing sperm. We identified 758 protein-coding genes in 125.9 Mb of B chromosome sequence, of which at least 88 are expressed. Our results demonstrate that transposable elements in the B chromosome are shared with the standard A chromosome set but multiple lines of evidence fail to detect a syntenic genic region in the A chromosomes, suggesting a distant origin. The current gene content is a result of continuous transfer from the A chromosomal complement over an extended evolutionary time with subsequent degradation but with selection for maintenance of this nonvital chromosome.Raw data used for sequence assembly of B73 line possessing B chromosome(s) and additional RNA-seq data used for B chromosome annotation are available in NCBI-SRA as BioProject PRJNA633287. Sequence reads for B-deficiency-carrying lines of maize are available in NCBI-SRA as BioProject PRJNA634743. Input-seq data are available in NCBI-GEO under accession GSE152074. The final B chromosome sequence and its annotation are available at MaizeGDB (https://www.maizegdb.org/) under the name Zm-B73_B-CHROMOSOME-MBSC-1.0 with the identifier Zm00044a.
Nondisjunction and unequal spindle organization accompany the drive of Aegilops speltoides B chromosomes
17169D. Wu, A. Ruban, J. Fuchs, J. Macas, P. Novák, M. Vaio, Y. Zhou and A. Houben, New Phytologist, 223:1340-1352. 2021-06-04 19:45:39.
B chromosomes (Bs) are supernumerary chromosomes, which are often preferentially inherited. When transmission rates of chromosomes are higher than 0.5, not obeying the Mendelian law of equal segregation, the resulting transmission advantage is collectively referred to as ?chromosome drive?. Here we analysed the drive mechanism of Aegilops speltoides Bs. The repeat AesTR-183 of A. speltoides Bs, which also can be detected on the Bs of Aegilops mutica and rye, was used to track Bs during pollen development. Nondisjunction of CENH3-positive, tubulin interacting B sister chromatids and an asymmetric spindle during first pollen grain mitosis are key for the accumulation process. A quantitative flow cytometric approach revealed that, independent of the number of Bs present in the mother plant, Bs accumulate in the generative nuclei to > 93%. Nine out of 11 tested (peri)centromeric repeats were shared by A and B chromosomes. Our findings provide new insights into the process of chromosome drive. Quantitative flow cytometry is a useful and reliable method to study the drive frequency of Bs. Nondisjunction and unequal spindle organization accompany during first pollen mitosis the drive of A. speltoides Bs. The prerequisites for the drive process seems to be common in Poaceae.
Researchers report reference genome for maize B chromosome
17172Chinese Academy of Sciences, Phys Org, 2021-05-31 19:46:00.
Three groups recently reported a reference sequence for the supernumerary B chromosome in maize in a study published online in PNAS. Dr. James Birchler's group from University of Missouri, Dr. Jan Barto's group from Institute of Experimental Botany of the Czech Academy of Sciences and Dr. Han Fangpu's group from the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences worked collaboratively on the study. Supernumerary B chromosomes persist in thousands of plant and animal genomes despite being nonessential. They are maintained in populations by mechanisms of "drive" that make them inherited at higher than typical Mendelian rates. Key properties such as its origin, evolution, and the molecular mechanism for its accumulation in maize have remained unclear even though such chromosomes have been a potent tool for studying maize genetics.
Mechanistically comparing reproductive manipulations caused by selfish chromosomes and bacterial symbionts
16958E. Dalla Benetta, O. S. Akbari and P. M. Ferree, Heredity, 126:707-716. 2021-05-01 14:52:23.
Insects naturally harbor a broad range of selfish agents that can manipulate their reproduction and development, often leading to host sex ratio distortion. Such effects directly benefit the spread of the selfish agents. These agents include two broad groups: bacterial symbionts and selfish chromosomes. Recent studies have made steady progress in uncovering the cellular targets of these agents and their effector genes. Here we highlight what is known about the targeted developmental processes, developmental timing, and effector genes expressed by several selfish agents. It is now becoming apparent that: (1) the genetic toolkits used by these agents to induce a given reproductive manipulation are simple, (2) these agents target sex-specific cellular processes very early in development, and (3) in some cases, similar processes are targeted. Knowledge of the molecular underpinnings of these systems will help to solve long-standing puzzles and provide new tools for controlling insect pests.
Selfish chromosomal drive shapes recent centromeric histone evolution in monkeyflowers
16833F. R. Finseth, T. C. Nelson and L. Fishman, PLOS Genetics, 17:e1009418. 2021-04-22 14:16:46.
Centromeres must mediate faithful chromosomal transmission during cell division and sexual reproduction, but both the DNA and protein components of centromeres diverge rapidly across species. The selfish centromere model argues that this paradoxical diversity results from a genetic conflict between centromeric DNA variants driving through female meiosis to gain over-transmission and kinetochore proteins co-evolving to re-establish Mendelian segregation. We use whole genome sequencing and genetic crossing experiments to demonstrate active evolutionary interactions between a selfish centromere and a key kinetochore protein (CenH3A) in the wildflower Mimulus guttatus. We show that both inter-specific and intra-population differences in CenH3A affect centromeric drive in hybrids, and that adaptive evolution of CenH3A has followed the recent and costly spread of the driver in a wild population. This work provides novel empirical support for the proposed antagonistic co-evolution of the DNA and protein components of centromeres, with important consequences for understanding cellular function, individual fitness, and species divergence.
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'.
Chromosome drives via CRISPR-Cas9 in yeast
15025H. Xu, M. Han, S. Zhou, B.-Z. Li, Y. Wu and Y.-J. Yuan, Nature Communications, 11:4344. 2020-08-28 15:22:50.
Our results show that the entire Saccharomyces cerevisiae chromosome can be eliminated efficiently through only one double-strand break around the centromere via CRISPR-Cas9. As a proof-of-concept experiment of this CRISPR-Cas9 chromosome drive system, the synthetic yeast chromosome X is completely eliminated, and the counterpart wild-type chromosome X harboring a green fluorescent protein gene or the components of a synthetic violacein pathway are duplicated by sexual reproduction. We also demonstrate the use of chromosome drive to preferentially transmit complex genetic traits in yeast.
Extensive Recombination Suppression and Epistatic Selection Causes Chromosome-Wide Differentiation of a Selfish Sex Chromosome in Drosophila pseudoobscura
14296Z. L. Fuller, S. A. Koury, C. J. Leonard, R. E. Young, K. Ikegami, J. Westlake, S. Richards, S. W. Schaeffer and N. Phadnis, Genetics, 216:205. 2020-07-30 14:22:50.
Here, we conduct a multifaceted study of the multiply inverted Drosophila pseudoobscura SR chromosome to understand the evolutionary history, genetic architecture, and present-day dynamics that shape this enigmatic selfish chromosome.
A Protamine Knockdown Mimics the Function of Sd in Drosophila melanogaster
12536L. F. Gingell and J. R. McLean, G3-Genes Genomes Genetics, 10:2111-2115. 2020-06-01 15:29:52.
Segregation Distorter (SD) is an autosomal meiotic drive system found worldwide in natural populations of Drosophila melanogaster. This gene complex induces the preferential and nearly exclusive transmission of the SD chromosome in SD/SD+ males. This selfish propagation occurs through the interplay of the Sd locus, its enhancers and the Rsp(s) locus during spermatid development. The key distorter locus, Sd, encodes a truncated but enzymatically active RanGAP (RanGTPase-activating protein), a key nuclear transport factor in the Ran signaling pathway. When encoded by Sd, RanGAP is mislocalized to the nucleus interior, which then traps Ran inside the nucleus and disrupts nuclear import. As a result of this aberrant nuclear transport, a process known as the histone-to-protamine transition that is required for proper spermatid condensation fails to occur in SD/SD+ males. In this process, sperm-specific protamine proteins enter the spermatid nucleus and replace the formerly chromatin-complexed histones. Previously, we have shown that mutations affecting nuclear import and export can enhance distortion in an SD background, thus verifying that a defect in nuclear transport is responsible for the unequal transmission of chromosomes. Herein, we show that specifically reducing protamines induces distortion in an SD background, verifying that protamines are transported via the RanGAP/GEF pathway and indicating that E(SD) plays a significant and unique role in the process of distortion
Recessive Z-linked lethals and the retention of haplotype diversity in a captive butterfly population
11972I. J. Saccheri, S. Whiteford, C. J. Yung and A. E. van't Hof, Heredity, 2020-05-13 18:21:56.
Sex chromosomes are predicted to harbour elevated levels of sexually antagonistic variation due to asymmetries in the heritability of recessive traits in the homogametic versus heterogametic sex.
Interpopulation spread of a parasitic B chromosome is unlikely through males in the grasshopper Eyprepocnemis plorans
6166M. I. Manrique-Poyato, J. Cabrero, M. D. López-León, F. Perfectti, R. Gómez and J. P. M. Camacho, Heredity, 124:197-206. 2019-07-08 18:29:07.
The near-neutral model of B chromosome evolution predicts that population invasion is quite fast. To test this prediction, in 1994, we introduced males of the grasshopper Eyprepocnemis plorans from a B-carrying population into a B-lacking population and monitored the evolution of B-chromosome frequency up to 2013. We observed fluctuating very low B frequency across years but, remarkably, the B chromosome introduced (the B2 variant) was found up to 1996 only, whereas the B1 variant was present from 1996 onwards, presumably introduced by fishermen using E. plorans males as bait. Effective introgression of genetic material from the donor population was evidenced by the presence of a satellite DNA on autosome 9 (up to 1999) and the presence of one individual in 2006 showing an ISSR marker profile being highly similar to that found in the donor population. This indicated that the males introduced by us effectively mated with resident females, but donor genes rapidly decreased in frequency after this non-recurrent migration event. Taken together, our results indicated: (i) that the non-recurrent migration event had a slight, transient genetic effect on the recipient population, which was diluted in only a few generations; and (ii) that even with recurrent migration (forced by fishermen) the B chromosome failed to increase in frequency. Bearing in mind that B chromosomes in this species drive through females only, we hypothesize that B chromosomes most likely failed invasion in both migration events because the migrating sex shows no B-drive.
B chromosome first—mechanisms behind the drive of B chromosomes uncovered
17167Leibniz Institute of Plant Genetics and Crop Plant Research, Phy Org, 2019-06-04 19:27:50.
The specific number of chromosomes is one of the defining characteristics of a species. Whilst the common fruit fly carries 8 chromosomes, the genome of bread wheat counts 42 chromosomes. In comparison, the human genome is made out of a total of 46 chromosomes. However, about 15% of all eukaryotic species additionally carry supernumerary chromosomes referred to as "B chromosomes". Other than the essential chromosomes of the genome, B chromosomes are expendable and often preferentially inherited. This leads to a transmission advantage for B chromosomes called "chromosome drive". To date, little knowledge exists about the mechanisms behind this phenomenon. Researchers from the Leibniz Institute of Plant Genetics and Crop Plant Research (IPK) in Gatersleben have now been able to decipher the mechanisms behind the drive of B chromosomes in the goatgrass Aegilops speltoides. The novel insights in the workings of chromosome drive were recently published in New Phytologist.
CRISPR in Parasitology: Not Exactly Cut and Dried!
3894Bryant, JMB, S.; Glover, L.; Hutchinson, S.; Rachidi, N., Trends in Parasitology, 35:409-422. 2019-01-12 00:00:00.
CRISPR/Cas9 technology has been developing rapidly in the field of parasitology, allowing for the dissection of molecular processes with unprecedented efficiency. Optimization and implementation of a new technology like CRISPR, especially in nonmodel organisms, requires communication and collaboration throughout the field. Recently, a 'CRISPR in Parasitology' symposium was held at the Institut Pasteur Paris, bringing together scientists studying Leishmania, Plasmodium, Trypanosoma, and Anopheles. Here we share technological advances and challenges in using CRISPR/Cas9 in the parasite and vector systems that were discussed. As CRISPR/Cas9 continues to be applied to diverse parasite systems, the community should now focus on improvement and standardization of the technique as well as expanding the CRISPR toolkit to include Cas9 alternatives/derivatives for more advanced applications like genome-wide functional screens.
Origin, composition, and structure of the supernumerary B chromosome of Drosophila melanogaster
3978Hanlon, SLM, Danny E.; Eche, Salam; Hawley, R. Scott, Genetics, 210:1197. 2018-01-16 00:00:00.
The number of chromosomes carried by an individual species is one of its defining characteristics. Some species, however, can also carry supernumerary chromosomes referred to as B chromosomes. B chromosomes were recently identified in a laboratory stock of Drosophila melanogaster—an established model organism with a wealth of genetic and genomic resources—enabling us to subject them to extensive molecular analysis. We isolated the B chromosomes by pulsed-field gel electrophoresis and determined their composition through next-generation sequencing. Although these B chromosomes carry no known euchromatic sequence, they are rich in transposable elements and long arrays of short nucleotide repeats, the most abundant being the uncharacterized AAGAT satellite repeat. Fluorescent in situ hybridization on metaphase chromosome spreads revealed this repeat is located on chromosome 4, strongly suggesting the origin of the B chromosomes is chromosome 4. Cytological and quantitative comparisons of signal intensity between chromosome 4 and the B chromosomes supports the hypothesis that the structure of the B chromosome is an isochromosome. We also report the identification of a new B chromosome variant in a related laboratory stock. This B chromosome has a similar repeat signature as the original but is smaller and much less prevalent. We examined additional stocks with similar genotypes and did not find B chromosomes, but did find these stocks lacked the AAGAT satellite repeat. Our molecular characterization of D. melanogaster B chromosomes is the first step toward understanding how supernumerary chromosomes arise from essential chromosomes and what may be necessary for their stable inheritance.
Genetics and genomics of an unusual selfish sex ratio distortion in an insect
3976Hamilton, PTH, C. N.; Curtis, C. I.; Perlman, S. J., Current Biology, 28:3864-3870. 2018-01-14 00:00:00.
Diverse selfish genetic elements have evolved the ability to manipulate reproduction to increase their transmission, and this can result in highly distorted sex ratios [1]. Indeed, one of the major explanations for why sex determination systems are so dynamic is because they are shaped by ongoing coevolutionary arms races between sex-ratio-distorting elements and the rest of the genome [2]. Here, we use genetic crosses and genome analysis to describe an unusual sex ratio distortion with striking consequences on genome organization in a booklouse species, Liposcelis sp. (Insecta: Psocodea), in which two types of females coexist. Distorter females never produce sons but must mate with males (the sons of nondistorting females) to reproduce [3]. Although they are diploid and express the genes inherited from their fathers in somatic tissues, distorter females only ever transmit genes inherited from their mothers. As a result, distorter females have unusual chimeric genomes, with distorter-restricted chromosomes diverging from their nondistorting counterparts and exhibiting features of a giant non-recombining sex chromosome. The distorter-restricted genome has also acquired a gene from the bacterium Wolbachia, a well-known insect reproductive manipulator; we found that this gene has independently colonized the genomes of two other insect species with unusual reproductive systems, suggesting possible roles in sex ratio distortion in this remarkable genetic system.
Satellite DNAs unveil clues about the ancestry and composition of B chromosomes in three grasshopper species
3995Milani, DB, Vanessa; Ferretti, Ana; Palacios-Gimenez, Octavio; Melo, Adriana; Moura, Rita; Loreto, Vilma; Song, Hojun; Cabral-de-Mello, Diogo, Genes, 9:e523. 2018-01-13 00:00:00.
Supernumerary (B) chromosomes are dispensable genomic elements occurring frequently among grasshoppers. Most B chromosomes are enriched with repetitive DNAs, including satellite DNAs (satDNAs) that could be implicated in their evolution. Although studied in some species, the specific ancestry of B chromosomes is difficult to ascertain and it was determined in only a few examples. Here we used bioinformatics and cytogenetics to characterize the composition and putative ancestry of B chromosomes in three grasshopper species, Rhammatocerus brasiliensis, Schistocerca rubiginosa, and Xyleus discoideus angulatus. Using the RepeatExplorer pipeline we searched for the most abundant satDNAs in Illumina sequenced reads, and then we generated probes used in fluorescent in situ hybridization (FISH) to determine chromosomal position. We used this information to infer ancestry and the events that likely occurred at the origin of B chromosomes. We found twelve, nine, and eighteen satDNA families in the genomes of R. brasiliensis, S. rubiginosa, and X. d. angulatus, respectively. Some satDNAs revealed clustered organization on A and B chromosomes varying in number of sites and position along chromosomes. We did not find specific satDNA occurring in the B chromosome. The satDNAs shared among A and B chromosomes support the idea of putative intraspecific ancestry from small autosomes in the three species, i.e., pair S11 in R. brasiliensis, pair S9 in S. rubiginosa, and pair S10 in X. d. angulatus. The possibility of involvement of other chromosomal pairs in B chromosome origin is also hypothesized. Finally, we discussed particular aspects in composition, origin, and evolution of the B chromosome for each species.
Production of YY Supermale and XY Physiological Female Common Carp for Potential Eradication of this Invasive Species
11508M. Y. Jiang, X. X. Wu, K. X. Chen, H. R. Luo, W. Yu, S. T. Jia, Y. M. Li, Y. F. Wang, P. H. Yang, Z. Y. Zhu and W. Hu, Journal of the World Aquaculture Society, 49:315-327. 2018-01-03 15:22:53.
The common carp, Cyprinus carpio, is the third most cultivated freshwater species worldwide, but is also considered an invasive species. The Trojan Y chromosome strategy is one of the most promising methods to eradicate this invasive species. However, obtaining fertile YY supermale (MYY) and YY physiological female (FYY) common carp was thought to be very difficult. The present study focused on the production of androgenetic MYY and XY physiological female (FXY) common carp. We first optimized the conditions for artificially induced androgenesis. The results indicated that the optimum ultraviolet (UV) exposure time was 4min at an irradiation distance of 26cm and the optimum initiation time with heat shock (40 +/- 0.5C for 2min) was 30min after fertilization. Then, we produced androgenetic MYY Yellow River carp with only paternal inheritance, which were viable and identified by paternity testing and test crossing. Finally, we successfully produced FXY Yellow River carp by feeding 60-d-old MXY with commercially available feed mixed with 17-estradiol (200mg/kg) and Flutamide (200mg/kg) for 3mo. In conclusion, by combining artificially induced androgenesis with an artificially induced sex reversal technique, we could cost-effectively produce MYY and FXY common carp.
B Chromosomes in populations of mammals revisited
4025Vujoševi?, MR, Marija; Blagojevi?, Jelena, Genes, 9:487. 2018-01-03 00:00:00.
The study of B chromosomes (Bs) started more than a century ago, while their presence in mammals dates since 1965. As the past two decades have seen huge progress in application of molecular techniques, we decided to throw a glance on new data on Bs in mammals and to review them. We listed 85 mammals with Bs that make 1.94% of karyotypically studied species. Contrary to general view, a typical B chromosome in mammals appears both as sub- or metacentric that is the same size as small chromosomes of standard complement. Both karyotypically stable and unstable species possess Bs. The presence of Bs in certain species influences the cell division, the degree of recombination, the development, a number of quantitative characteristics, the host-parasite interactions and their behaviour. There is at least some data on molecular structure of Bs recorded in nearly a quarter of species. Nevertheless, a more detailed molecular composition of Bs presently known for six mammalian species, confirms the presence of protein coding genes, and the transcriptional activity for some of them. Therefore, the idea that Bs are inert is outdated, but the role of Bs is yet to be determined. The maintenance of Bs is obviously not the same for all species, so the current models must be adapted while bearing in mind that Bs are not inactive as it was once thought.
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.
Transmission and drive involving parasitic B chromosomes
3984Jones, RN, Genes, 9:e388. 2018-01-02 00:00:00.
B chromosomes (Bs) are enigmatic additional elements in the genomes of thousands of species of plants, animals, and fungi. How do these non-essential, harmful, and parasitic chromosomes maintain their presence in their hosts, making demands on all the essential functions of their host genomes? The answer seems to be that they have mechanisms of drive which enable them to enhance their transmission rates by various processes of non-mendelian inheritance. It is also becoming increasingly clear that the host genomes are developing their own mechanisms to resist the impact of the harmful effects of the Bs.
B chromosome in Plantago lagopus Linnaeus, 1753 shows preferential transmission and accumulation through unusual processes
4037Dhar, MKK, G.; Kaul, S., Comparative Cytogenetics, 11:375-391. 2017-01-15 00:00:00.
Plantago lagopus is a diploid (2n = 2x = 12) weed belonging to family Plantaginaceae. We reported a novel B chromosome in this species composed of 5S and 45S ribosomal DNA and other repetitive elements. In the present work, presence of B chromosome(s) was confirmed through FISH on root tip and pollen mother cells. Several experiments were done to determine the transmission of B chromosome through male and female sex tracks. Progenies derived from the reciprocal crosses between plants with (1B) and without (0B) B chromosomes were studied. The frequency of B chromosome bearing plants was significantly higher than expected, in the progeny of 1B female x 0B male. Thus, the B chromosome seems to have preferential transmission through the female sex track, which may be due to meiotic drive. One of the most intriguing aspects of the present study was the recovery of plants having more chromosomes than the standard complement of 12 chromosomes. Such plants were isolated from the progenies of B chromosome carrying plants. The origin of these plants can be explained on the basis of a two step process; formation of unreduced gametes in 1B plants and fusion of unreduced gametes with the normal gametes or other unreduced gametes. Several molecular techniques were used which unequivocally confirmed similar genetic constitution of 1B (parent) and plants with higher number of chromosomes.
Preferential accumulation of sex and Bs chromosomes in biarmed karyotypes by meiotic drive and rates of chromosomal changes in fishes
4147Molina, WFM, P. A.; Bertollo, L. A. C.; Bidau, C. J., Anais Da Academia Brasileira De Ciencias, 86:1801-1812. 2014-01-05 00:00:00.
Mechanisms of accumulation based on typical centromeric drive or of chromosomes carrying pericentric inversions are adjusted to the general karyotype differentiation in the principal Actinopterygii orders. Here, we show that meiotic drive in fish is also supported by preferential establishment of sex chromosome systems and B chromosomes in orders with predominantly bi-brachial chromosomes. The mosaic of trends acting at an infra-familiar level in fish could be explained as the interaction of the directional process of meiotic drive as background, modulated on a smaller scale by adaptive factors or specific karyotypic properties of each group, as proposed for the orthoselection model.
Transmission rate variation among three B chromosome variants in the fish Prochilodus lineatus (Characiformes, Prochilodontidae)
4178Penitente, MV, T. A.; Senhorini, J. A.; Bortolozzi, J.; Foresti, F.; Porto-Foresti, F., Anais Da Academia Brasileira De Ciencias, 85:1371-1377. 2013-01-16 00:00:00.
Cytogenetic studies were developed in Prochilodus lineatus (Valenciennes 1836), describing an interesting system of small supernumerary chromosomes. The purpose of this work is to study the frequency and morphology of B chromosomes in individuals from the parental line and the inheritance patterns of these elements in individuals obtained from controlled crosses in the species P. lineatus. The transmission rate of B chromosomes, revealed a k(B)=0.388 for the acrocentric type, a kB=0.507 for the metacentric type and a k(B)=0.526 for the submetacentric type. The obtained results raise hypothesis that B-acrocentric chromosomes are involved in an extinction process in this species, while the metacentric and submetacentric supernumerary elements comprises a neutral mechanism and follows a Mendelian transmission rate.
Spread of a new parasitic B chromosome variant is facilitated by high gene flow
4176Manrique-Poyato, MIL-L, M. D.; Cabrero, J.; Perfectti, F.; Camacho, J. P. M., PLOS One, 8:e83712. 2013-01-14 00:00:00.
The B-24 chromosome variant emerged several decades ago in a Spanish population of the grasshopper Eyprepocnemis plorans and is currently reaching adjacent populations. Here we report, for the first time, how a parasitic B chromosome (a strictly vertically transmitted parasite) expands its geographical range aided by high gene flow in the host species. For six years we analyzed B frequency in several populations to the east and west of the original population and found extensive spatial variation, but only a slight temporal trend. The highest B-24 frequency was found in its original population (Torrox) and it decreased closer to both the eastern and the western populations. The analysis of Inter Simple Sequence Repeat (ISSR) markers showed the existence of a low but significant degree of population subdivision, as well as significant isolation by distance (IBD). Pairwise N(e)m estimates suggested the existence of high gene flow between the four populations located in the Torrox area, with higher values towards the east. No significant barriers to gene flow were found among these four populations, and we conclude that high gene flow is facilitating B-24 diffusion both eastward and westward, with minor role for B-24 drive due to the arrival of drive suppressor genes which are also frequent in the donor population.
B chromosomes in plants
4188Jones, N, Plant Biosystems, 146:727-737. 2012-01-06 00:00:00.
B chromosomes (Bs) can be described as "selfish chromosomes", a term that has been used for the repetitive DNA which comprises the bulk of the genome in large genome species, except that Bs have a life of their own as independent chromosomes. They can accumulate in number by various processes of mitotic or meiotic drive, especially in the gametophyte phase of the life cycle of flowering plants. This parasitic property of drive ensures their survival and spread in natural populations, even against a gradient of harmful effects on the host plant phenotype. B chromosomes are inhabitants of the nucleus and they are subject to control by "genes" in the A chromosome (As) complement. This interaction with the As, together with the balance between drive and harmful effects makes a dynamic system in the life of a Bs. In this review, we concentrate mainly on recent developments in the Bs of rye and maize, two of the species currently receiving most attention. We focus on their population dynamics and on the molecular basis of their structural organisation and mechanisms of drive, as well as on their mode of origin and potential applications in plant biotechnology.
Nondisjunction in favor of a Chromosome: The mechanism of rye B chromosome drive during pollen mitosis
4184Banaei-Moghaddam, AMS, V.; Kumke, K.; Weiss, O.; Klemme, S.; Nagaki, K.; Macas, J.; Gonzalez-Sanchez, M.; Heredia, V.; Gomez-Revilla, D.; Gonzalez-Garcia, M.; Vega, J. M.; Puertas, M. J.; Houben, A., Plant Cell, 24:4124-4134. 2012-01-02 00:00:00.
B chromosomes (Bs) are supernumerary components of the genome and do not confer any advantages on the organisms that harbor them. The maintenance of Bs in natural populations is possible by their transmission at higher than Mendelian frequencies. Although drive is the key for understanding B chromosomes, the mechanism is largely unknown. We provide direct insights into the cellular mechanism of B chromosome drive in the male gametophyte of rye (Secale cereale). We found that nondisjunction of Bs is accompanied by centromere activity and is likely caused by extended cohesion of the B sister chromatids. The B centromere originated from an A centromere, which accumulated B-specific repeats and rearrangements. Because of unequal spindle formation at the first pollen mitosis, nondisjoined B chromatids preferentially become located toward the generative pole. The failure to resolve pericentromeric cohesion is under the control of the B-specific nondisjunction control region. Hence, a combination of nondisjunction and unequal spindle formation at first pollen mitosis results in the accumulation of Bs in the generative nucleus and therefore ensures their transmission at a higher than expected rate to the next generation.
B Chromosomes and Sex in Animals
4200Camacho, JPMS, M.; Cabrero, J., Sexual Development, 5:155-166. 2011-01-18 00:00:00.
Supernumerary (B) chromosomes are dispensable elements found in many eukaryote genomes in addition to standard (A) chromosomes. In many respects, B chromosomes resemble sex chromosomes, so that a common ancestry for them has frequently been suggested. For instance, B chromosomes in grasshoppers, and other insects, show a pycnotic cycle of condensation-decondensation during meiosis remarkably similar to that of the X chromosome. In some cases, B chromosome size is even very similar to that of the X chromosome. These resemblances have led to suggest the X as the B ancestor in many cases. In addition, sex chromosome origin from B chromosomes has also been suggested. In this article, we review the existing evidence for both evolutionary pathways, as well as sex differences for B frequency at adult and embryo progeny levels, B chromosome effects or B chromosome transmission. In addition, we review cases found in the literature showing sex-ratio distortion associated with B chromosome presence, the most extreme case being the paternal sex ratio (PSR) chromosomes in some Hymenoptera. We finally analyse the possibility of B chromosome regularisation within the host genome and, as a consequence of it, whether B chromosomes can become regular members of the host genome. Copyright (C) 2011 S. Karger AG, Basel
B chromosomes and genome size in flowering plants
4301Trivers, RB, A.; Palestis, B. G., Genome, 47:1-8. 2004-01-19 00:00:00.
B chromosomes are extra chromosomes found in some, but not all, individuals within a species, often maintained by giving themselves an advantage in transmission, i.e. they drive. Here we show that the presence of B chromosomes correlates to and varies strongly and positively with total genome size (excluding the Bs and corrected for ploidy) both at a global level and via a comparison of independent taxonomic contrasts. B chromosomes are largely absent from species with small genomes; however, species with large genomes are studied more frequently than species with small genomes and Bs are more likely to be reported in well-studied species. We controlled for intensity of study using logistic regression. This regression analysis also included effects of degree of outbreeding, which is positively associated with Bs and genome size, and chromosome number, which is negatively associated with Bs and genome size, as well as variable ploidy (more than one ploidy level in a species). Genome size, breeding system and chromosome number all contribute independently to the distribution of B chromosomes, while variable ploidy does not have a significant effect. The genome size correlates are consistent with reduced selection against extra DNA in species with large genomes and with increased generation of B sequences from large A genomes.
Rapid suppression of drive for a parasitic B chromosome
4298Perfectti, FC, J. M.; Mesa, J. A.; Cabrero, J.; Bakkali, M.; Lopez-Leon, M. D.; Camacho, J. P. M., Cytogenetic and Genome Research, 106:338-343. 2004-01-16 00:00:00.
The persistence of parasitic B chromosomes in natural populations depends on both B ability to drive and host response to counteracting it. In the grasshopper Eyprepocnemis plorans, the B-24 chromosome is the most widespread B chromosome variant in the Torrox area ( Malaga, Spain). Its evolutionary success, replacing its ancestral neutralized B variant, B-2, was based on meiotic drive in females, as we showed in a sample caught in 1992. In females collected six years later, mean B-24 transmission ratio (k(B)) was 0.523, implying a very rapid decrease from the 0.696 observed in 1992. This shows that B-24 neutralization is running very fast and suggests that it might most likely be based on a single gene of major effect. Copyright (C) 2004 S. Karger AG, Basel.
The distribution of B chromosomes across species
4297Palestis, BGT, R.; Burt, A.; Jones, R. N., Cytogenetic and Genome Research, 106:151-158. 2004-01-15 00:00:00.
In this review we look at the broad picture of how B chromosomes are distributed across a wide range of species. We review recent studies of the factors associated with the presence of Bs across species, and provide new analyses with updated data and additional variables. The major obstacle facing comparative studies of B chromosome distribution is variation among species in the intensity of cytogenetic study. Because Bs are, by definition, not present in all individuals of a species, they may often be overlooked in species that are rarely studied. We give examples of corrections for differences in study effort, and show that after a variety of such corrections, strong correlations remain. Several major biological factors are associated with the presence of B chromosomes. Among flowering plants, Bs are more likely to occur in outcrossing than in inbred species, and their presence is also positively correlated with genome size and negatively with chromosome number. They are no more frequent in polyploids than in diploids, nor in species with multiple ploidies. Among mammals, Bs are more likely to occur in species with karyotypes consisting of mostly acrocentric chromosomes. We find no evidence for an association with chromosome number or genome size in mammals, although the sample for genome size is small. The associations with breeding system and acrocentric chromosomes were both predicted in advance, but those with genome size and chromosome number were discovered empirically and we can offer only tentative explanations for the very strong associations we have uncovered. Our understanding of why B chromosomes are present in some species and absent in others is still in its infancy, and we suggest several potential avenues for future research. Copyright (C) 2004 S. Karger AG, Basel.
B chromosomes are more frequent in mammals with acrocentric karyotypes: support for the theory of centromeric drive
4296Palestis, BGB, A.; Jones, R. N.; Trivers, R., Proceedings of the Royal Society B-Biological Sciences, 271:S22-S24. 2004-01-14 00:00:00.
The chromosomes of mammals tend to be either mostly acrocentric (having one long arm) or mostly bi-armed, with few species having intermediate karyotypes. The theory of centromeric drive suggests that this observation reflects a bias during female meiosis, favouring either more centromeres or fewer, and that the direction of this bias changes frequently over evolutionary time. B chromosomes are selfish genetic elements found in some individuals within some species. B chromosomes are often harmful, but persist because they drive (i.e. they are transmitted more frequently than expected). We predicted that species with mainly acrocentric chromosomes would be more likely to harbour B chromosomes than those with mainly bi-armed chromosomes, because female meiosis would favour more centromeres over fewer in species with one-armed chromosomes. Our results show that B chromosomes are indeed more common in species with acrocentric chromosomes, across all mammals, among rodents, among non-rodents and in a test of independent taxonomic contrasts. These results provide independent evidence supporting the theory of centromeric drive and also help to explain the distribution of selfish DNA across species. In addition, we demonstrate an association between the shape of the B chromosomes and the shape of the typical ('A') chromosomes.
Mapping of paternal-sex-ratio deletion chromosomes localizes multiple regions involved in expression and transmission
4294McAllister, BFB, L. W.; Werren, J. H., Heredity, 92:5-13. 2004-01-12 00:00:00.
The paternal-sex-ratio (PSR) chromosome in the parasitic wasp Nasonia vitripennis is a submetacentric supernumerary (B chromosome). Males transmit PSR, but after fertilization it causes the loss of the paternal autosomes. Paternal genome loss caused by PSR results in the conversion of a female (diploid) zygote into a male (haploid) under haplodiploid sex determination. In this study, site-specific markers were developed to assay deletion derivatives of PSR. Both polymerase chain reaction and Southern hybridization were used to detect the presence/absence of 16 single-site markers on a set of 20 functional and nine nonfunctional deletion chromosomes. Based on the pattern of marker loss on the deletion chromosomes, the basic organization of PSR was revealed. Two sets of markers were deleted independently, apparently representing the two arms of the submetacentric chromosome. The presence or absence of specific regions was examined in relation to phenotypic characteristics of the deletion chromosomes; ability to cause paternal genome loss, and stability in mitotic cell divisions. Rather than identifying a single region on PSR as being responsible for PSR function, the results suggest that the retention of one of two chromosomal regions is sufficient for causing paternal genome loss. Furthermore, a region was identified that is tightly correlated with mitotic stability, as measured from chromosomal transmission rates. Functional chromosomes with short-arm deletions had high (similar to100%) transmission rates, whereas functional chromosomes with long-arm deletions had low (similar to85%) transmission rates.
B chromosomes in Sternorrhyncha (Hemiptera, Insecta)
4293Maryanska-Nadachowska, A, Cytogenetic and Genome Research, 106:210-214. 2004-01-11 00:00:00.
In the hemipteroid insects of the suborder Sternorrhyncha, B chromosomes are relatively common in comparison with other suborders of Hemiptera. However, the occurrence of supernumerary chromosomes is restricted, in most cases, to several genera or closely related species. At least in some species of Psylloidea with the XY sex determination system, a mitotically stable B chromosome integrated into an achiasmatic segregation system with the X, and became fixed as a Y chromosome. In some Aphidoidea with a multiple X system of sex determination, B chromosomes appear to be in fact non-functional X chromosomes. Supernumerary chromosomes thus probably play an important role in the evolution of sex determination systems in Sternorrhyncha. Copyright (C) 2004 S. Karger AG, Basel.
The B chromosome polymorphism of the grasshopper Eyprepocnemis plorans in North Africa. IV. Transmission of rare B chromosome variants
4284Bakkali, MC, J. P. M., Cytogenetic and Genome Research, 106:332-337. 2004-01-02 00:00:00.
In addition to the principal B chromosome (B-1) in Moroccan populations of the grasshopper Eyprepocnemis plorans, nine B chromosome variants appeared at low frequency. The transmission of five of these rare B chromosome variants through females was analysed in three natural populations. Sixteen controlled crosses provided useful information on the transmission of B-M2, B-M6 and B-M7 in Smir, B-M3 and B-M6 in SO. DE. A. (Societe de Developpement Agricole lands near Ksar-el-Kebir city), and B-M2 and BM10 in Mechra, all located in Morocco. Since six female parents carried two different B variants, a total of 22 progeny analyses could be studied. Intraindividual variation in B transmission rate (k(B)) was observed among the successive egg pods in 26.7% of the females, but this variation did not show a consistent temporal pattern. Only the B-M2 and B-M6 variants in Smir showed net drive, although variation was high among crosses, especially for B-M2. These two variants are thus good candidates for future regenerations ( the replacement of a neutralized B, B-1 in this case, by a new driving variant, B-M2 or B-M6) in Smir, the northern population where the B polymorphism is presumably older. The analysis of all crosses performed in the three populations, including those reported previously for the analysis of B-1 transmission, showed that the largest variance in k(B) among crosses stands at the individual level, and not at population or type of B levels. The implications of these findings for the occurrence of possible regeneration processes in Moroccan populations are discussed. Copyright (C) 2004 S. Karger AG, Basel.
B chromosomes in plants: escapees from the A chromosome genome?
4312Jones, NH, A., Trends in Plant Science, 8:417-423. 2003-01-10 00:00:00.
B chromosomes are dispensable elements that do not recombine with the A chromosomes of the regular complement and that follow their own evolutionary track. In some cases, they are known to be nuclear parasites with autonomous modes of inheritance, exploiting 'drive' to ensure their survival in populations. Their I selfishness' brings them into conflict with their host nuclear genome and generates a host-parasite relationship, with anti-B-chromosome genes working to ameliorate the worst of their excesses in depriving their hosts of genetic resources. Molecular studies are homing in on their sequence organization to give us an insight into the origin and evolution of these enigmatic chromosomes, which are, with rare exceptions, without active genes.
Localization of the genes controlling B chromosome transmission rate in maize (Zea mays ssp. mays, Poaceae)
4367Chiavarino, AMR, M.; Rosi, P.; Poggio, L.; Naranjo, C. A., American Journal of Botany, 85:1581-1585. 1998-01-05 00:00:00.
In previous papers we found that the frequency of B chromosomes in native races of maize varies considerably in different populations. Moreover, we found genotypes that control high and low transmission rates (TR) of B chromosomes in the Pisingallo race. In the present work crosses were made to determine whether the genes controlling B-TR are located on the normal chromosome set (As) or on the B chromosomes (Bs). We made female f.0B X male m.2B crosses between and within high (H) and low (L) B-TR groups. The Bs were transmitted on the male side in all cases. The mean B-TR from the progeny of f.0B (H) X m.2B (H) and f.0B (H) X m.2B (L) crosses was significantly higher than that from f.0B (L) X m.2B (L) and f.0B (L) X m.2B (H) crosses. The results show that the B-TR of the crosses corresponds to the H or L B-TR of the 0B female parents irrespective of the Bs of the male parent. This indicates that B-TR is genetically controlled by the 0B female parent and that these genes are located on the A chromosomes.
Selfish DNA and breeding system in flowering plants
4366Burt, AT, R., Proceedings of the Royal Society B-Biological Sciences, 265:141-146. 1998-01-04 00:00:00.
In many species, some individuals carry one or more B chromosomes: extra, or supernumerary chromosomes not part of the normal complement. In most well-studied cases, B's lower the fitness of their carrier and persist in populations only because of accumulation mechanisms analogous to meiotic drive. It has been suggested that such genomic parasites are expected to persist only in outcrossed sexual species, in which uninfected lines of descent can be continuously reinfected; in inbred or asexual species, all selection is between lines of descent, and the genomic parasites are either lost or must evolve into commensals or mutualists. Here we present a simple population genetic model of the effect of outcrossing rate on the frequency of B chromosomes, and find that outcrossing facilitates the spread of parasitic B's, but inhibits the spread of mutualists. Data compiled from the literature on breeding system and B chromosomes of British plants indicate that B's are much more likely to be reported from obligately outcrossed species than inbred species. These results support the ideas that most B chromosomes are parasitic, and that breeding systems play a central role in the biology of selfish genes.
Genetic control of B chromosome transmission rate in Zea mays ssp mays (Poaceae)
4401Rosato, MC, A. M.; Naranjo, C. A.; Puertas, M. J.; Poggio, L., American Journal of Botany, 83:1107-1112. 1996-01-19 00:00:00.
We selected genotypes of high and low B chromosome transmission rate (TR) in a native race of maize (Pisingallo) from northwest Argentina. We made 20 female 0B x male 1B and 20 f.1B x m.0B crosses. The former (G0m) showed a large variation of B TR, with a mean of TR +/- SE = 0.52 +/- 0.06, ranging from 0.17 to 0.98. In the latter (G0f) the mean was TR = 0.47 +/- 0.02 ranging from 0.31 to 0.58. Plants showing the highest and the lowest TR were selected to constitute the progenitors of the G1 generations. We made 19 f.0B x m.2B crosses, studying 24-30 plants per cross. The TR of the high (H) and low (L) lines in G1m (G1mH and G1mL) significantly differed (TRH = 0.65 +/- 0.03, TRL = 0.40 +/- 0.01), indicating that the H and L lines are different groups. The large variation in male TR suggests that preferential fertilization of gametes carrying B chromosomes does not always occur. We also selected plants showing high and low TR in the progenies of f.1B x m.0B crosses (G0f), and made 24 f.1B x m.0B crosses, studying 23-30 plants per cross. The TRs of the H and L lines in G1f (G1fH and G1fL) were significantly different (TRH = 0.48 +/- 0.025, TRL = 0.40 +/- 0.02). The TRs in G0f and G1fL were significantly different (TR = 0.47 +/- 0.02 and 0.40 +/- 0.02, respectively), while this was not the case between G0f and G1fH. Our results demonstrate the existence of genotypes controlling B TR in this native population of maize.
The effect of B chromosomes on mating success of the grasshopper Eyprepocnemis plorans
4397Martin, SA, P.; HenriquesGil, N., Genetica, 97:197-203. 1996-01-15 00:00:00.
The mating ability of E. plorans was tested in laboratory conditions in six experimental units composed of ten males and fifteen females during 31 days. When significant differences were found (three from the six cages, and in totals) they involved a decrease of matings involving males with B chromosomes. The same tendency seems to exist in females, but to a lesser extent, so that a significant effect is only detected when the totals are considered. Accessory chromosomes also delay, in both sexes, the occurrence of the first mating. No mating preferences depending on the number of Bs were detected.
Evidence for B chromosome drive suppression in the grasshopper Eyprepocnemis plorans
4393Herrera, JAL, M. D.; Cabrero, J.; Shaw, M. W.; Camacho, J. P. M., Heredity, 76:633-639. 1996-01-11 00:00:00.
The grasshopper Eyprepocnemis plorans is polymorphic for both a B chromosome and a heterochromatic segment of chromatin on the smallest autosome. Females transmit these to their offspring more frequently after copulating with a male from a population without Bs than after copulating with a male from their own population. Paternity analyses using the heterochromatic segment as a marker showed that the effect of male on female transmission does not depend on fertilization because it occurs even when all the eggs are fertilized by sperm from another mating. The possible mechanisms include behavioural differences in mating and transfer of substances affecting female meiosis in male ejaculate. The data support the idea that the B chromosome is initially subject to meiotic drive in populations in which it has not previously existed, and that genes which suppress this drive are then selected.
The inheritance of B chromosomes in Allium schoenoprasum L
4391Bougourd, SMP, A. B., Chromosome Research, 4:151-158. 1996-01-09 00:00:00.
The inheritance of B chromosomes has been investigated in Allium schoenoprasum from the River Wye, Powys; controlled crosses between plants of known B chromosome constitution were carried out, and the numbers of Bs present in the progenies scored. There was considerable heterogeneity in B chromosome transmission rates across the range of B chromosome numbers, through both the male and female lines, and in crosses involving odd and even numbers of Bs. The mean B number of the progeny was lower than the Mendelian expectation in 41 of the 57 crosses, significantly so in about a third of cases. No progeny had a mean B number significantly higher than expected. The mean transmission rate per B was not significantly different between parents with different numbers of Bs, in either the male or female line, or between plants carrying odd or even numbers of Bs. Transmission through the male line (0.43) was higher than through the female line (0.35), but not significantly so. The overall mean transmission rate per B (0.4), estimated from all crosses, was significantly lower than the expected rate of 0.5. This study provides conclusive evidence that B chromosomes in A. schoenoprasum from the River Wye lack an effective accumulation mechanism and, furthermore, show significant losses during transmission to the progeny of controlled crosses.
Bewildering Bs – An impression of the 1st B-chromosome conference
4408Beukeboom, LW, Heredity, 73:328-336. 1994-01-06 00:00:00.
Ever since their first discovery B chromosomes have attracted attention. Why are they so appealing? The standard chromosomes of an organism are A chromosomes; B chromosomes are extra to this normal complement. In the B chromosome 'bible' (Jones & Rees, 1982) Bs are defined as dispensable supernumerary chromosomes that are not homologous and do not pair with A chromosomes. They have been further characterized as (1) morphologically different from As (usually smaller), (2) being inherited in a nonMendelian fashion, (3) not (or only rarely) having nucleolus organisers, (4) often displaying nondisjunction at anaphase of mitosis resulting in frequencies varying between organs in the same individual, (5) reducing fertility and growth when present in high numbers, and (6) carrying no genes with major effects. These features of Bs were recently discussed at an international conference and the main ideas presented by the participants are reported here. From 21 to 25 September 19
Deletion analysis of the selfish B-chromosome, Paternal Sex-Ratio (PSR), in the parasitic wasp Nasonia vitripennis
4420Beukeboom, LWW, J. H., Genetics, 133:637-648. 1993-01-18 00:00:00.
Paternal Sex Ratio (PSR) is a ''selfish'' B chromosome in the parasitoid wasp Nasonia vitripennis. It is transmitted via sperm, but causes supercondensation and destruction of the paternal chromosomes in early fertilized eggs. Because this wasp has haplodiploid sex determination, the effect of PSR is to convert diploid (female) eggs into haploid (male) eggs that carry PSR. Characterizing its genetic structure is a first step toward understanding mechanisms of PSR action. The chromosome is largely heterochromatic and contains several tandemly repeated DNA sequences that are not present on the autosomes. A deletion analysis of PSR was performed to investigate organization of repeats and location of functional domains causing paternal chromosome destruction. Deletion profiles using probes to PSR-specific repetitive DNA indicate that most repeats are organized in blocks on the chromosome. This study shows that the functional domains of PSR can be deleted, resulting in nonfunctional PSR chromosomes that are transmitted to daughters. A functional domain may be linked with the psr22 repeat, but function may also depend on abundance of PSR-specific repeats on the chromosome. It is hypothesized that the repeats act as a ''sink'' for a product required for proper paternal chromosome processing. Almost all deletion chromosomes remained either functional of nonfunctional in subsequent generations following their creation. One chromosome was exceptional in that it reverted from nonfunctionality to functionality in one lineage. Transmission rates of nonfunctional deletion chromosomes were high through haploid males, but low through diploid females.
Transmission and expression of the parasitic Paternal Sex-Ratio (PSR) chromosome
4419Beukeboom, LWW, J. H., Heredity, 70:437-443. 1993-01-17 00:00:00.
B-chromosomes are often considered genomic parasites. They are extra to the normal chromosomal complement, are unnecessary for survival of an individual, and are often inherited at higher than Mendelian rates. Paternal Sex Ratio (PSR) is an extreme example of a parasitic B-chromosome in the wasp Nasonia vitripennis. It is transmitted via sperm but then destroys the other paternal chromosomes in the early fertilized egg. PSR disrupts the normal haplodiploid sex determination system of this wasp by converting diploid (female) eggs into haploid eggs that develop into PSR-bearing males. Transmission and expression of PSR was measured in single pair crosses between carrier males and standard females. Presence of the chromosome was detected by probing offspring with PSR-specific repetitive DNA. Most (equal to or more than 90 per cent) PSR males produced all-male offspring. Overall transmission rates of PSR to fertilized eggs varied beween 0.94 and 1.0. Some males (up to 10 per cent) produced daughters at varying frequencies. Of 226 daughters tested, only one carried PSR (and this may have been a laboratory error) indicating that daughters result from failure of transmission rather than loss of expression. Transmission of PSR to males in families that included female offspring varied from 0 to 94 per cent. Incomplete transmission is most likely the result of loss of PSR in some spermatogonial cell lineages and indicates some mitotic instability. Implications of the results to the aetiology and population genetics of PSR are discussed.
Meiotic behavior of B- chromosomes in the grasshopper Omocestus burri: A case of drive in females
4427Santos, JLD, A. L.; Fernandez, A.; Diez, M., Hereditas, 118:139-143. 1993-01-05 00:00:00.
The meiotic behaviour of an iso-B chromosome of the grasshopper Omocestus burri has been studied in a natural population located at Villar del Cobo (Teruel, Spain) by means of cytological observations of male and female meiosis. Whereas B chromosomes do not accumulate in the male germ line, they are included in the secondary oocyte at an increased frequency over mendelian expectations. Therefore, an accumulation mechanism of the accessory chromosome based on meiotic drive takes place in the females of this species.
Male and female segregation distortion for heterochromatic supernumerary segments on the s(8) chromosome of the grasshopper Chorthippus jacobsi
4432Lopezleon, MDC, J.; Camacho, J. P. M., Chromosoma, 101:511-516. 1992-01-10 00:00:00.
The mode of inheritance of supernumerary segments located on three different chromosome pairs was investigated in controlled crosses with specimens of the grasshopper Chorthippus jacobsi. While extra segments located on chromosomes M5 and M6 showed Mendelian inheritance, that on S8 did not. Thus, the two supernumerary heterochromatic chromosome segments located distally on the S8 chromosome accumulated through non-Mendelian transmission through both sexes. The observed transmission patterns may be explained by gametic selection for spermatozoa carrying segmented S8 chromosomes, in addition to meiotic drive for segmented S8 chromosomes in heterozygous females. The significance of these findings for the maintenance of these polymorphisms in natural populations is discussed.
Population genetics of a parasitic chromosome – Experimental analysis of PSR in subdivided populaltions
4431Beukeboom, LWW, J. H., Evolution, 46:1257-1268. 1992-01-09 00:00:00.
Nasonia vitripennis is a parasitoid wasp that harbors several non-Mendelian sex-ratio distorters. These include MSR (Maternal Sex Ratio), a cytoplasmic element that causes nearly all-female families, and PSR (Paternal Sex Ratio), a supernumerary chromosome that causes all-male families. As in other hymenoptera, N. vitripennis has haplodiploid sex determination. Normally, unfertilized (haploid) eggs develop into males and fertilized (diploid) eggs develop into females. The PSR chromosome violates this normal pattern; it is inherited through sperm, but then causes destruction of the paternal chromosomes (except itself, thus converting diploid fertilized eggs (normally females) into haploid eggs that develop into PSR-bearing males. PSR is an extreme example of "parasitic" or "selfish" DNA. Because N. vitripennis has a highly subdivided population structure in nature, population-level selection may be important in determining the dynamics of PSR in natural populations. A theoretical analysis shows that subdivided population structure reduces PSR frequency, whereas high fertilization proportion (such as produced by the MSR element) increases PSR frequency. Population experiments using two deme sizes (3- and 12-foundress groups) and strains producing two fertilization proportions [wild-type (LabII)-57-67% female, and MSR (MI)-90-93% female] confirm these predictions. PSR achieved frequencies over 0.90 in 12-foundress group MSR populations in contrast to 0.20-0.40 in wild-type 12-foundress populations. PSR was selected against in wild-type populations composed of three-foundress groups. In MSR populations with three-foundress groups, presence of PSR selected against the MSR cytoplasmic element, eventually leading to low frequencies of both PSR and MSR. Complicated dynamics may occur when these two sex-ratio distorters are both present in highly subdivided populations. The existence of PSR in natural populations may depend on the presence of MSR. Results indicate that population subdivision could be important in determining the frequency of sex ratio distorters in N. vitripennis.
Effects of deletions on mitotic stability of the Paternal Sex-Ratio (PSR) chromosome from Nasonia
4430Beukeboom, LWR, K. M.; Werren, J. H., Chromosoma, 102:20-26. 1992-01-08 00:00:00.
Paternal-Sex-Ratio (PSR) is a B chromosome that causes all-male offspring in the parasitoid wasp Nasonia vitripennis. It is only transmitted via sperm of carrier males and destroys the other paternal chromosomes during the first mitotic division of the fertilized egg. Because of haplodiploidy, the effect of PSR is to convert diploid (female) eggs into haploid eggs that develop into PSR-bearing males. The PSR chromosome was previously found to contain several families of repetitive DNA, which appear to be present in local blocks. PSR chromosomes with irradiation-induced deletions have decreased rates of transmission and increased variation in transmission. This study investigates whether these differences in transmission of deletion chromosomes are due to mitotic instability. Two deletion chromosomes (E306 and F316) and the wild-type PSR chromosome were examined. A cytogenetic assay of testes revealed that wild-type PSR males contained the chromosome in 98%-100% of their spermatocytes. Similar counts from carriers of two deletion chromosomes were lower and varied between individuals from 50%-100%. One F316 male did not contain the chromosome in any of its spermatocytes although the chromosome was present in somatic tissues based on hybridization to PSR-specific repetitive DNA. A molecular analysis of males found the wild-type PSR chromosome to be present in all somatic tissues. Tissue specific differences in the presence or PSR were found in several males from the two deletion lines. The results show that deletions can result in mosaicism due to increased mitotic instability of PSR. Such individuals sometimes partially or completely fail to transmit the chromosome. Patterns of mosaicism of 13 chromosomes in other organisms are discussed.
The paternal-sex-ratio chromosome of Nasonia
4457Werren, JH, American Naturalist, 137:392-402. 1991-01-15 00:00:00.
Paternal sex ratio (PSR) is a supernumerary chromosome that is transmitted through sperm to fertilized eggs and then gains a transmission advantage by causing supercondensation of the paternal chromosomes (except itself). Because of haplodiploidy, this converts diploid females into haploid (PSR) males. PSR gains a transmission advantage because the unpaired chromosome has a higher frequency of transmission through male (mitotic) gametngenesis than through female (meiotic) gametogenesis. PSR increases in panmictic populations that produce more than 50% fertilized eggs. PSR frequency is typically reduced by population subdivision because of increased local competition among PSR males and reduced availability of females resulting from PSR action. Genetic and molecular studies of PSR have recently begun. PSR is highly heterochromatic and contains large tandem arrays of repetitive DNA unique to it. PSR apparently acts by "imprinting" the set of paternal chromosomes, resulting in their supercondensation during mitosis. Deletion studies are under way to characterize the genetic basis of PSR action and "protection" from its own action.
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) .
Meiotic drive against an autosomal supernumerary segment promoted by the presence of a B-chromosome in females of the grasshopper Eyprepocnemis plorans
4448Lopezleon, MDC, J.; Camacho, J. P. M., Chromosoma, 100:282-287. 1991-01-06 00:00:00.
Twenty-seven out of 50 progeny analyses performed with specimens of the grasshopper Eyprepocnemis plorans were informative about the transmission of a supernumerary heterochromatic chromosome segment. The simultaneous presence of a B chromosome in some of the parents involved in the crosses permitted us to test the relationship between both types of supernumerary heterochromatin with respect to their transmission. The results demonstrated that the supernumerary segment is partly eliminated through females possessing B chromosomes. The implications of this in relation to the occurrence of the extra segment in natural populations are discussed.
B-chromosome drive
4445Jones, RN, American Naturalist, 137:430-442. 1991-01-03 00:00:00.
The view of B-chromosome polymorphisms that is coming into favor resembles the so-called "parasitic" model, which was first advanced 45 yr ago. Since that time, repeated and ongoing efforts have been made to ascribe an adaptive role to B's (e.g., in terms of phenotypic advantage, effects of variability, etc.), but success in this direction has been singularly lacking. Consistently and repeatedly, the data indicate that harmful effects, or at best neutral or undetectable effects, are associated with B's and provide evidence of the potency of accumulation mechanisms based on drive. Modeling exercises have proved useful in examining these polymorphisms, and of the ones that have been published to date, all have favored drive as the main force generating B-chromosome polymorphisms.
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
Control of meiotic drive of B-chromosomes in the mealybug, Pseudococcus affinis (obscurus)
4467Nur, UB, B. L. H., Genetics, 115:499-510. 1987-01-05 00:00:00.
Isofemale lines of Pseudococcus affznis (MASKELL) differ in their ability to maintain B chromosomes (Bs) due to the presence of genotypes that affect the rate of transmission (k) of the Bs. The nature of these genotypes was analyzed by comparing ks of males carrying the same B and the same paternal genome (which is heterochromatic), but differing in their maternal genome. In males from line L-60, which maintained the B at a frequency of over 4.0 Bs per individual, the mean k varied between 0.7 and 0.95 in different experiments. Over the same period, the mean k of males with a maternal genome from one of two lines in which the B was rapidly lost (L-1 19), increased from 0.5 to 0.9, and that of the other line (L-230) decreased gradually from 0.6 to less than 0.1. The ks appear not to be correlated with the geographical or parental origin of the B. The observed changes in k are attributed at least in part to changes in the frequency of genotypes (alleles) which can drastically reduce the transmission of the B and, when present in high frequency, can lead to its rapid loss. The frequency distribution of the ks of sons of F, females from the cross L-230 X L-60 suggests that the two lines differ at two unlinked loci with additive effects on k. The genome of L-119 also caused the B to undergo nondisjunction in about 10% of the primary spermatocytes. A comparison between the ks of the males tested and those of males from a natural population suggests that in that population the B is “parasitic” and that the frequency of transmission-reducing genotypes is low.
Tthe genetic control of meiotic drive acting on the B-chromosome of Myrmeleotettix maculatus (Orthoptera, Aacrididae)
4473Shaw, MWH, G. M., Heredity, 54:187-194. 1985-01-11 00:00:00.
Crosses between populations with and without B-chromosomes were made, and backcrossed to the non B parent for two generations. No polygenic differences in male or female meiotic transmission were found, but a modifier of meiotic drive segregated in the experiment, drastically reducing female transmission rate. We tentatively interpret this as evidence of a coevolutionary race between the B-chromosome and the A genome.
Polymorphism in the rates of meiotic drive acting on the B-chromosome of Myrmeleotettix maculatus
4472Shaw, MWH, G. M.; Anderson, D. A., Heredity, 55:61-68. 1985-01-10 00:00:00.
A survey of all the available data on meiotic transmission rates in M. maculatus suggests that a polymorphism in female transmission rate exists in most natural populations. Differences in the frequency of the types or in the transmission rates they manifest may exist between populations or over time, but the evidence is not compelling. The data on males are also equivocal, especially because the environment may affect male transmission rate. We suggest that the polymorphism in female rates may demonstrate the selective equilibrium between the B and the genome in which it exists.
Genotypes suppressing meiotic drive of a B-chromosome in the mealybug, Pseudococcus obscurus
4471Nur, UB, B. L. H., Genetics, 110:73-92. 1985-01-09 00:00:00.
The rate of transmission (k) of a supernumerary B chromosome in male mealybugs is shown tq depend strongly on the chromosome set of materpal origin. When both parents came from an isofemale line in which the frequency of the B chromosome increased rapidly and stabilized at a mean of more than 4.0 B chromosomes per individual, was 0,92 and 0.95 in two series of crosses. However, when the female parent came from one of two isofemale lines in which the frequency of the B chromosome decreased from 2.0 to 0 in a few generations, R ranged from 0.53 to 0.78. The high ks, which represent a strong meiotic drive, are apparently responsible for the observed increase in the frequency of the B chromosome in several lines from a mean of about 0.5 to more than 4.0 in about 20 generations. The rapid loss of the B chromosome in other lines is attributed to genetic factors which caused the reduction in the rate of transmission qf the B chromosome.
Meiotic drive for B-chromosomes in primary oocytes of Myrmeleotettix maculatus (Orthoptera-Acrididae)
4489Hewitt, GM, Chromosoma, 56:381-391. 1976-01-07 00:00:00.
Using a modified technique which allowed observation of chromosome orientation in the primary oocyte of grasshoppers at the onset of anaphase, it has been possible to establish that the B-chromosome is distributed preferentially on the egg side of the metaphase plate rather than the polar body side. The frequency of this preferential orientation matches very closely the level of preferential transmission determined from breeding experiments using individuals from the same population. The spindle is asymmetrical in the primary oocyte of this species, and a possible explanation of the meiotic drive is proposed as a result of the conical shape of nucleoplasm surrounding this spindle. The autosomal chiasma frequency of these females is generally lower than comparable males and is increased by the presence of B chromosomes; but the chiasma frequencies of the sexes respond differently to the addition of 1 and 2 B-chromosomes.
Parasitic nature of extra fragment chromosomes
6003Östergren G., Botaniska Notiser, 2:157-163. 1945-12-30 19:52:12.
This paper is intended as a contribution to the discussion concerning the significance of the extra fragments or »accessory chromosomes» as they are called by Håk ansso n(1945), which are not too rarely found in cross-fertilizing populations. I think reasonable support may he given to the view that in many cases these chromosomes have no useful function at all to the species carrying them, but that they often lead an exclusively parasitic existence.
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