Scholarly Literature

This is a database of scholarly literature that concentrates currently on natural and engineered selfish genetic elements (gene drives).  The latest are shown here.
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Meiotic drive adaptive testes enlargement during early development in the stalk-eyed fly

24087
S. L. Bradshaw, L. Meade, J. Tarlton-Weatherall and A. Pomiankowski,  Biology Letters,  18:20220352. 2022-11-30 09:02:09.
The sex ratio (SR) X-linked meiotic drive system in stalk-eyed flies destroys Y-bearing sperm. Unlike other SR systems, drive males do not suffer fertility loss. They have greatly enlarged testes which compensate for gamete killing. We predicted that enlarged testes arise from extended development with resources re-allocated from the accessory glands, as these tend to be smaller in drive males. To test this, we tracked the growth of the testes and accessory glands of wild-type and drive males over 5-6 weeks post-eclosion before males attained sexual maturity. Neither of the original predictions is supported by these data. Instead, we found that the drive male testes were enlarged at eclosion, reflecting a greater allocation of resources to the testes during pupation. Testes grow at a higher rate during early adult development in drive males, but there was no evidence that this retards the growth of the accessory glands. Further experiments are proposed to investigate whether smaller accessory glands only arise in drive males post-copulation or when flies are subjected to nutritional stress. Our experimental findings support the idea that enlarged testes in drive males arise as an adaptive allocation of resources to traits that enhance male reproductive success.

Does severe hypoxia during irradiation of Aedes aegypti pupae improve sterile male performance?

24069
D. A. Tussey, K. J. Linthicum and D. A. Hahn,  Parasites and Vectors,  15:446. 2022-11-28 10:25:24.
The yellow fever mosquito, Aedes aegypti, vectors several pathogens responsible for human diseases. As a result, this mosquito species is a priority for control by mosquito control districts in Florida. With insecticide resistance development becoming a concern, alternative control strategies are needed for Ae. aegypti. Sterile insect technique (SIT) is an increasingly popular option that is being explored as a practical area-wide control method. However, questions about sterile male performance persist. The objectives of this study were to determine the extent to which hypoxia exposure prior to and during irradiation effects the longevity, activity and mating competitiveness of sterile male Ae. aegypti.

Engineering stringent genetic biocontainment of yeast with a protein stability switch

24034
S. A. Hoffmann and Y. Cai,  bioRxiv,  2022.11.24.517818. 2022-11-24 09:51:15.
Synthetic biology holds immense promise to tackle key problems we are facing, for instance in resource use, environmental health, and human health care. However, comprehensive safety measures are needed to deploy genetically engineered microorganisms in open-environment applications. Here, we describe a genetic biocontainment system based on conditional stability of essential proteins. We used a yeast-adapted destabilizing domain degron, which can be stabilized by estradiol addition (ERdd). Leveraging the yeast GFP collection and lab automation platforms, we ERdd-tagged 775 essential genes and screened for strains with estradiol dependent growth. Three genes, SPC110, DIS3 and RRP46, were found to be particularly suitable. Respective strains showed no growth defect in the presence of estradiol and strong growth inhibition in its absence. Of these, SPC110-ERdd offered the most stringent containment, with an escape frequency of 7.0x10-8, and full growth restoration at 100 nM estradiol. By systematically analysing the containment escapees, we identified the non- essential C-terminal region of SPC110 as target for escape mutations. Its removal decreased the escape frequency with a single ERdd tag further to 4.3x10-9. Combining SPC110-ERdd with a second ERdd tag on either DIS3 or RRP46 resulted in escape frequencies below the detection limit of the used assay (<2x10-10). Being based on conditional protein stability, this approach is mechanistically orthogonal to previously reported intrinsic biocontainment systems. It thus can be readily combined with other systems, for instance ones based on transcriptional or translational control of essential gene expression, to achieve multiplexed, extremely stringent control over the survival of engineered organisms.Competing Interest StatementThe authors have declared no competing interest.

Developing radiation-based sterile insect technique (SIT) for controlling Aedes aegypti: identification of a sterilizing dose

24028
C. Chen, R. L. Aldridge, S. Gibson, J. Kline, V. Aryaprema, W. Qualls, R.-d. Xue, L. Boardman, K. J. Linthicum and D. A. Hahn,  Pest Management Science,  2022-11-24 09:37:31.
Abstract Background The sterile insect technique (SIT) is emerging as a tool to supplement traditional pesticide-based control of Aedes aegypti, a prominent mosquito vector of microbes that have increased the global burden of human morbidity and mortality over the past 50?years. SIT relies on rearing, sterilizing, and releasing large numbers of male mosquitoes that will mate with fertile wild females, thus reducing production of offspring from the target population. In this study, we investigated the effects of ionizing radiation (gamma) on male and female survival, longevity, mating behavior, and sterility of Ae. aegypti in a dose-response design. This work is a first step towards developing an operational SIT field suppression program against Ae. aegypti in St. Augustine, Florida, USA. Results Exposing late-stage pupae to 50 Gy of radiation yielded 99% male sterility while maintaining similar survival of pupae to adult emergence, adult longevity, and male mating competitiveness compared to unirradiated males. Females were completely sterilized at 30 Gy, and when females were dosed with 50 Gy, they had a lower incidence of blood feeding than unirradiated females. Conclusion Our work suggests that an ionizing radiation dose of 50 Gy should be used for future development of operational SIT in our program area because at this dose males are 99% sterile while maintaining mating competitiveness against unirradiated males. Furthermore, females that might be accidentally released with sterile males due to errors in sex sorting are also sterile and less likely to blood feed than unirradiated females at our 50 Gy dose.

Modeling-informed Engineered Genetic Incompatibility strategies to overcome resistance in the invasive Drosophila suzukii

28350
A. Sychla, N. R. Feltman, W. D. Hutchison and M. J. Smanski,  Frontiers in Insect Science,  2. 2022-11-22 10:46:03.
Engineered Genetic Incompatibility (EGI) is an engineered extreme underdominance genetic system wherein hybrid animals are not viable, functioning as a synthetic speciation event. There are several strategies in which EGI could be leveraged for genetic biocontrol of pest populations. We used an agent-based model of Drosophila suzukii (Spotted Wing Drosophila) to determine how EGI would fare with high rates of endemic genetic resistance alleles. We discovered a surprising failure mode wherein field-generated females convert an incompatible male release program into a population replacement gene drive. Local suppression could still be attained in two seasons by tailoring the release strategy to take advantage of this effect, or alternatively in one season by altering the genetic design of release agents. We show in this work that data from modeling can be utilized to recognize unexpected emergent phenomena and a priori inform genetic biocontrol treatment design to increase efficacy.

A CRISPR endonuclease gene drive reveals distinct mechanisms of inheritance bias

24021
S. A. N. Verkuijl, E. Gonzalez, M. Li, J. X. D. Ang, N. P. Kandul, M. A. E. Anderson, O. S. Akbari, M. B. Bonsall and L. Alphey,  Nature Communications,  13:7145. 2022-11-21 09:45:13.
CRISPR/Cas gene drives can bias transgene inheritance through different mechanisms. Homing drives are designed to replace a wild-type allele with a copy of a drive element on the homologous chromosome. In Aedes aegypti, the sex-determining locus is closely linked to the white gene, which was previously used as a target for a homing drive element (wGDe). Here, through an analysis using this linkage we show that in males inheritance bias of wGDe did not occur by homing, rather through increased propagation of the donor drive element. We test the same wGDe drive element with transgenes expressing Cas9 with germline regulatory elements sds3, bgcn, and nup50. We only find inheritance bias through homing, even with the identical nup50-Cas9 transgene. We propose that DNA repair outcomes may be more context dependent than anticipated and that other previously reported homing drives may, in fact, bias their inheritance through other mechanisms.

The effect of mating complexity on gene drive dynamics

23967
P. Verma, R. G. Reeves, S. Simon, M. Otto and C. S. Gokhale,  The American Naturalist,  2022-11-15 12:33:47.
Gene drive technology promises to deliver on some of the global challenges humanity faces today in health care, agriculture, and conservation. However, there is a limited understanding of the consequences of releasing self-perpetuating transgenic organisms into wild populations under complex ecological conditions. In this study, we analyze the impact of three such complexities—mate choice, mating systems, and spatial mating network—on the population dynamics for two distinct classes of modification gene drive systems. All three factors had a high impact on the modeling outcome. First, we demonstrate that distortion-based gene drives appear to be more robust against mate choice than viability-based gene drives. Second, we find that gene drive spread is much faster for higher degrees of polygamy. Including a fitness cost, the drive is fastest for intermediate levels of polygamy. Finally, the spread of a gene drive is faster and more effective when the individuals have fewer connections in a spatial mating network. Our results highlight the need to include mating complexities when modeling the properties of gene drives, such as release thresholds, timescales, and population-level consequences. This inclusion will enable a more confident prediction of the dynamics of engineered gene drives and possibly even inform about the origin and evolution of natural gene drives.

Driving lessons: a brief (personal) history of centromere drive

24030
H. S. Malik,  Genetics,  2022-11-15 09:41:41.
Meiosis is an important specialized cell division in many eukaryotic species, including fungi, plants, and animals. Meiosis results in the production of haploid gametes starting from a diploid cell via 1 round of replication and 2 rounds of cell division. In an influential article published in 1957, Sandler and Novitski first pointed out that meiosis is also an intense battleground, in which gametes vie for evolutionary supremacy with each other, often poisoning their competition to gain a fratricidal advantage (Sandler and Novitski 1957). This competition, which they termed “meiotic drive,” operates as an evolutionary force that can cause an increase in frequency of the allele that is favored during meiotic transmission. Unlike alleles that rise in frequency because they confer a fitness advantage to their carriers, meiotic drivers can rise in frequency even while conferring significant fitness disadvantages on their carriers. Thus, meiotic drivers can be viewed as the quintessential selfish genes; it is the best interest of the rest of the genome to counteract their action to restore organismal fitness.

CRISPR-Mediated Cassette Exchange (CriMCE): A Method to Introduce and Isolate Precise Marker-Less Edits

24026
I. Morianou, A. Crisanti, T. Nolan and A. M. Hammond,  The CRISPR Journal,  2022-11-15 09:33:31.
The introduction of small unmarked edits to the genome of insects is essential to study the molecular underpinnings of important biological traits, such as resistance to insecticides and genetic control strategies. Advances in CRISPR genome engineering have made this possible, but prohibitively laborious for most laboratories due to low rates of editing and the lack of a selectable marker. To facilitate the generation and isolation of precise marker-less edits we have developed a two-step method based on CRISPR-mediated cassette exchange (CriMCE) of a marked placeholder for a variant of interest. This strategy can be used to introduce a wider range of potential edits compared with previous approaches while consolidating the workflow. We present proof-of-principle that CriMCE is a powerful tool by engineering three single nucleotide polymorphism variants into the genome of Anopheles gambiae, with 5?41???higher rates of editing than homology-directed repair or prime editing.

A Wolbachia factor for male killing in lepidopteran insects

24016
S. Katsuma, K. Hirota, N. Matsuda-Imai, T. Fukui, T. Muro, K. Nishino, H. Kosako, K. Shoji, H. Takanashi, T. Fujii, S.-i. Arimura and T. Kiuchi,  Nature Communications,  13:6764. 2022-11-14 12:59:35.
Bacterial symbionts, such as Wolbachia species, can manipulate the sexual development and reproduction of their insect hosts. For example, Wolbachia infection induces male-specific death in the Asian corn borer Ostrinia furnacalis by targeting the host factor Masculinizer (Masc), an essential protein for masculinization and dosage compensation in lepidopteran insects. Here we identify a Wolbachia protein, designated Oscar, which interacts with Masc via its ankyrin repeats. Embryonic expression of Oscar inhibits Masc-induced masculinization and leads to male killing in two lepidopteran insects, O. furnacalis and the silkworm Bombyx mori. Our study identifies a mechanism by which Wolbachia induce male killing of host progeny.

Independent evaluation of Wolbachia infected male mosquito releases for control of Aedes aegypti in Harris County, Texas, using a Bayesian abundance estimator

23969
S. Lozano, K. Pritts, D. Duguma, C. Fredregill and R. Connelly,  PLOS Neglected Tropical Diseases,  16:e0010907. 2022-11-14 12:37:36.
Among disease vectors, Aedes aegypti (L.) (Diptera: Culicidae) is one of the most insidious species in the world. The disease burden created by this species has dramatically increased in the past 50 years, and during this time countries have relied on pesticides for control and prevention of viruses borne by Ae. aegypti. The small number of available insecticides with different modes of action had led to increases in insecticide resistance, thus, strategies, like the “Incompatible Insect Technique” using Wolbachia’s cytoplasmic incompatibility are desirable. We evaluated the effect of releases of Wolbachia infected Ae. aegypti males on populations of wild Ae. aegypti in the metropolitan area of Houston, TX. Releases were conducted by the company MosquitoMate, Inc. To estimate mosquito population reduction, we used a mosquito abundance Bayesian hierarchical estimator that accounted for inefficient trapping. MosquitoMate previously reported a reduction of 78% for an intervention conducted in Miami, FL. In this experiment we found a reduction of 93% with 95% credibility intervals of 86% and 96% after six weeks of continual releases. A similar result was reported by Verily Life Sciences, 96% [94%, 97%], in releases made in Fresno, CA.

Centromere drive: chromatin conflict in meiosis

23856
P. Talbert and S. Henikoff,  Current Opinion in Genetics and Development,  77:102005. 2022-11-11 10:41:54.
Centromeres are essential loci in eukaryotes that are necessary for the faithful segregation of chromosomes in mitosis and meiosis. Centromeres organize the kinetochore, the protein machine that attaches sister chromatids or homologous chromosomes to spindle microtubules and regulates their disjunction. Centromeres have both genetic and epigenetic determinants, which can come into conflict in asymmetric female meiosis in seed plants and animals. The centromere drive model was proposed to describe this conflict and explain how it leads to the rapid evolution of both centromeres and kinetochores. Recent studies confirm key aspects of the centromere drive model, clarify its mechanisms, and implicate rapid centromere/kinetochore evolution in hybrid inviability between species.

Influence of public hesitancy and receptivity on reactive behaviours towards releases of male Wolbachia-Aedes mosquitoes for dengue control

23860
M. O. Lwin, Z. Ong, C. Panchapakesan, A. Sheldenkar, L. T. Soh, I. Chen, X. Li, W. Niah, K. Vasquez, S. Sim and L.-C. Ng,  PLOS Neglected Tropical Diseases,  16:e0010910. 2022-11-11 10:06:20.
Singapore, a highly urbanized Asian tropical country that experiences periodic dengue outbreaks, is piloting field releases of male Wolbachia-carrying Aedes aegyptimosquitoes with the aim of suppressing urban populations of the primary dengue vector Aedes aegypti. This study proposes and assesses a model to explain the roles of hesitancy and receptivity towards Project Wolbachia–Singapore in influencing reactive mosquito prevention behaviors (reactive behaviors) towards the release of Wolbachia-Aedes mosquitoes for residents living in the release sites. Interestingly, both hesitancy and receptivity predicted greater instances of reactive behaviors. The model also examines the roles of general knowledge about Wolbachia technology, perceived severity of mosquito bites, perceived density of mosquitoes, and social responsibility as predictors of hesitancy, receptivity, and reactive behaviors towards the release of Wolbachia-Aedes mosquitoes. Hesitancy towards the project mediated the effects of general knowledge, perceived severity of mosquito bites, and perceived density of mosquitoes on reactive behaviors towards the releases, although receptivity towards the project did not. Having less knowledge about Project Wolbachia–Singapore was associated with higher hesitancy towards the project and higher likelihood of performing reactive behaviors towards the releases. Individuals who perceive mosquito bites to be more severe and think that there are more mosquitoes in their living environments were also more likely to be hesitant about the project and practice reactive behaviors. However, both hesitancy and receptivity towards the project mediated the effect of social responsibility on reactive behaviors. Receptivity towards the project was driven by social responsibility, which was also associated with reduced hesitancy towards the project. Our findings suggest that, to address the hesitancy reported by a minority of participants, future outreach efforts should focus on strengthening the public’s sense of social responsibility

Wolbachia inhibits ovarian formation and increases blood feeding rate in female Aedes aegypti

23858
M.-J. Lau, P. A. Ross, N. M. Endersby-Harshman, Q. Yang and A. A. Hoffmann,  PLOS Neglected Tropical Diseases,  16:e0010913. 2022-11-11 09:59:48.
Author summary Wolbachia bacteria reduce the transmission of dengue and other arboviruses transmitted by their mosquito hosts once they have invaded host populations. This invasion process is being undertaken in Aedes aegypti mosquito populations throughout the world but can be slowed by deleterious fitness costs that the Wolbachia bacteria have on their mosquito hosts. Here we investigate in detail a recently identified cost where one of the Wolbachia strains being used in invasions causes infertility in females when the females develop from eggs that have been stored for a period. We show that slowed larval development (through food reduction) can cause the same phenomenon and that the emerged females lack functional ovaries. These infertile females also take blood meals more often. The results have implications for the successful invasion of Wolbachia under ecological circumstances where the mosquito eggs have to persist in the absence of water for a period and where larval food is limiting.

Leveraging a natural murine meiotic drive to suppress invasive populations

23835
L. Gierus, A. Birand, M. D. Bunting, G. I. Godahewa, S. G. Piltz, K. P. Oh, A. J. Piaggio, D. W. Threadgill, J. Godwin, O. Edwards, P. Cassey, J. V. Ross, T. A. A. Prowse and P. Q. Thomas,  Proceedings of the National Academy of Sciences,  119:e2213308119. 2022-11-08 09:36:13.
Invasive rodents are a major cause of environmental damage and biodiversity loss, particularly on islands. Unlike insects, genetic biocontrol strategies including population-suppressing gene drives with biased inheritance have not been developed in mice. Here, we demonstrate a gene drive strategy (t(CRISPR)) that leverages super-Mendelian transmission of the t haplotype to spread inactivating mutations in a haplosufficient female fertility gene (Prl). Using spatially explicit individual-based in silico modeling, we show that t(CRISPR) can eradicate island populations under a range of realistic field-based parameter values. We also engineer transgenic t(CRISPR) mice that, crucially, exhibit biased transmission of the modified t haplotype and Prl mutations at levels our modeling predicts would be sufficient for eradication. This is an example of a feasible gene drive system for invasive alien rodent population control.

Gene drive technologies: navigating the ethical landscape

24074
N. d. Graeff,  Utrecht University,  2022-11-04 10:41:56.
Gene drives are technologies that modify a particular genetic element in animals or insects so that this genetic element does not follow the typical rules of heredity, and is passed onto future generations with an increased likelihood. Gene drive technologies could be used to tackle intractable problems such as vector-borne diseases like malaria or the biodiversity impact of invasive species. At the same time, the development and governance of gene drives raise a range of ethical questions and concerns that warrant proactive ethical evaluation. In the PhD thesis ?Gene drive technologies: navigating the ethical landscape?, Nienke de Graeff analyzes these questions and concerns. In Part I, she outlines the ?ethical landscape? of gene drive technologies by identifying the associated ethical challenges through literature review and empirical ethical research. Important challenges concern how the uncertainty and risks of these technologies should be navigated, whether it is morally permissible to intervene in nature in this way, and how the development, governance, and potential deployment of gene drive technologies should be guided. In Part II, De Graeff normatively analyzes various of these challenges and provides guidance to navigate them. In Part III, she stipulates recommendations for researchers and policymakers in the gene drive field as well as lessons learned for ethics parallel research as an approach for early ethical guidance of new and emerging technologies more generally.

Making waves: Comparative analysis of gene drive spread characteristics in a continuous space model

23816
M. Pan and J. Champer,  bioRxiv,  2022.11.01.514650. 2022-11-02 10:23:47.
With their ability to rapidly increase in frequency, gene drives can be used to modify or suppress target populations after an initial release of drive-containing individuals. Recent advances in this field have revealed many possibilities for different types of drives, and several of these have been realized in experimental demonstrations. These drives all have unique advantages and disadvantages related to their ease of construction, confinement, and capacity to act as a modification or suppression system. While many properties of these drives have been explored in modelling studies, assessment of these drives in continuous space environments has been limited, often focusing on outcomes rather than fundamental properties. Here, we conduct a comparative analysis of many different gene drive types that have the capacity to form a wave of advance against wild-type alleles in one-dimensional continuous space. We evaluate the drive wave speed as a function of drive performance and ecological parameters, which reveals substantial differences between drive performance in panmictic versus spatial environments. In particular, we find that suppression drive waves are uniquely vulnerable to fitness costs and undesired CRISPR cleavage activity that can form resistance alleles in embryos by maternal deposition. Some drives, though, retain robust characteristics even with widely varying performance characteristics. To gain a better understanding of drive waves, we compare panmictic performance of drives across the full range of drive frequencies. We find that rates of wild-type allele removal in panmictic setting is correlated with drive wave speed, though this is also affected by a range of other factors. Overall, our results provide a useful resource for understanding the performance of drives in continuous spatial environments, which may be most representative of potential drive deployment in many relevant scenarios.Competing Interest StatementThe authors have declared no competing interest.

Asymmetric Inheritance: The Diversity and Evolution of Non-Mendelian Reproductive Strategies

L. Ross, A. J. Mongue, C. N. Hodson and T. Schwander,  Annual Review of Ecology, Evolution, and Systematics,  53:1-23. 2022-11-02 08:36:15.
The ability to reproduce is the key trait that distinguishes living organisms from inorganic matter, and the strategies used to achieve successful reproduction are almost as diverse as the organisms themselves. In animals, the most widespread form of reproduction involves separate male and female sexes: Each sex produces haploid gametes via meiosis, and two gametes fuse to form a new diploid organism. In some cases, both parents contribute equally to the nuclear and cytoplasmic genomes of their offspring. However, such fully symmetric reproduction of both parents represents the extreme end of a continuum toward complete asymmetry, where offspring inherit their nuclear and cytoplasmic genomes from only one of the two parents. Asymmetries also occur with respect to the fate of maternally and paternally inherited genomes and which sex is affected by non-Mendelian inheritance. In this review, we describe the diversity of animal reproductive systems along different axes with a symmetry?asymmetry continuum and suggest evolutionary routes that may have led to increased levels of asymmetry.

GMOs in Africa: Status, adoption and public acceptance

27490
L. A. Gbadegesin, E. A. Ayeni, C. K. Tettey, V. A. Uyanga, O. O. Aluko, J. K. Ahiakpa, C. O. Okoye, J. I. Mbadianya, M. A. Adekoya, R. O. Aminu, F. P. Oyawole and P. Odufuwa,  Food Control,  141:109193. 2022-11-01 13:46:04.
Globally, genetically modified (GM) crops contribute to food security by increasing crop yield, quality and shelf-life. The commercialisation and adoption of GM crops in many developed countries raised hope of improving food security and livelihood. Africa, a developing continent facing malnutrition, food crises and inadequate food production technologies has been slow to accept GM crops. The hesitancy to accept GM crops emanates from unfavourable policies shaped by public opinion, despite its potential for achieving the zero-hunger agenda. Impeding factors hampering the adoption of GM technology necessitate biosecurity regulations on GM crops to monitor the crop biosafety, environmental and health concerns. Herein, we reviewed GM crops status and adoption in Africa and possible constraints to their acceptance amidst some commercialised GM crops in African countries. Efforts aimed at improving GM adoption in Africa should include the provision of adequate monitoring and surveillance system, science-based policies, political will and a robust public education on GM technology.

A sterile insect technique pilot trial on Captiva Island: defining mosquito population parameters for sterile male releases using mark–release–recapture

23818
D. O. Carvalho, R. Morreale, S. Stenhouse, D. A. Hahn, M. Gomez, A. Lloyd and D. Hoel,  Parasites and Vectors,  15:405. 2022-11-01 10:41:32.
The sterile insect technique (SIT), which involves area-wide inundative releases of sterile insects to suppress the reproduction of a target species, has proven to be an effective pest control method. The technique demands the continuous release of sterilized insects in quantities thatThe sterile insect technique (SIT), which involves area-wide inundative releases of sterile insects to suppress the reproduction of a target species, has proven to be an effective pest control method. The technique demands the continuous release of sterilized insects in quantities that ensure a high sterile male:wild male ratio for the suppression of the wild population over succeeding generations. Over the course of seven mark-release-recapture studies using single- and multiple-point releases, 190,504 sterile marked males were released, for which the recapture rate was 1.5% over a mean period of 12 days. The mean distance traveled by sterile males of the local strain of Ae. aegypti that has colonized Captiva Island was 201.7 m from the release point, with an observed maximum traveled distance of 404.5 m. The released sterile mosquitoes had a probability of daily survival of 0.67 and an average life expectancy of ~ 2.46 days.s These data together with the population size estimate and sterile:wild ratio provide a solid basis for planning the SIT operational phase which is aimed at mosquito population suppression. ensure a high sterile male:wild male ratio for the suppression of the wild population over succeeding generations.

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