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Mysterious males: Asexual female nematodes produce males for sperm, not genes

American Association for the Advancement of Science

Getting at why nematodes engaged in a unique female-favoring reproduction strategy produce males at all, researchers report that the asexual females produce limited numbers of male offspring to exploit them for their sperm in order to make more males, and in a ratio meaning the resultant sons are more likely to mate with their sisters. The careful process ensures that male genes never re-enter the female gene pool and it keeps the overall population stable. Pseudogamy, also called sperm-dependent parthenogenesis, is an asexual reproductive strategy in which females use the sperm of males - usually from other species - to activate their eggs. However, the sperm DNA is not used. Instead, embryos develop only from maternal DNA and become females themselves. As such, females are the only reproductively successful sex. While it would seem unnecessary and perhaps even wasteful for these species to produce male offspring, the pseudogamous nematode Mesorhabditis belari is known to produce males strictly for reproductive purposes. Manon Grosmaire and colleagues isolated M. belari from soil and rotting plant matter to study their unique reproductive strategy. While most embryos develop without using sperm DNA to produce only daughter nematodes, Grosmaire et al. discovered that in roughly 9% of fertilizations, male DNA is used. When parental DNA is mixed in such cases, only males are produced, likely because the Y-chromosome-bearing sperm is more competent and much more likely to penetrate an egg than X-chromosome-bearing sperm, according to the authors. The study's results imply that M. belari females produce males whose genetic material is not transmitted to asexual females but exploit their sperm to activate their eggs. What's more, the authors show that the resultant males are more likely to mate with their siblings, which according to a game theory model, allows the unique reproductive arrangement to be an evolutionarily stable strategy.

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