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Diversification and hybrid incompatibility in auto-pseudogamous species of Mesorhabditis nematodes.

Caroline Launay, Marie-Anne Félix, Joris Dieng, and Marie Delattre (2020)

BMC Evol Biol, 20(1):105.

BACKGROUND: Pseudogamy is a reproductive system in which females rely on the spermof males to activate their oocytes, generally parasitizing males of other species,but do not use the sperm DNA. The nematode Mesorhabditis belari uses a specific formof pseudogamy, where females produce their own males as a source of sperm. Malesdevelop from rare eggs with true fertilization, while females arise by gynogenesis.Males thus do not contribute their genome to the female offspring. Here, we exploredthe diversity of reproductive mode within the Mesorhabditis genus and addressedspecies barriers in pseudogamous species. RESULTS: To this end, we established acollection of over 60 Mesorhabditis strains from soil and rotting vegetal matter. Wefound that males from pseudogamous species displayed a reduced size of their body,male tail and sperm cells compared to males of sexual Mesorhabditis species, asexpected for males that face little competition. Using rDNA sequences and crosses,we could define 11 auto-pseudogamous biological species, with closely relatedspecies pairs and a possible single origin of pseudogamy in the Mesorhabditis genus.Most crosses between males and females of different species did not even producefemale progeny. This surprising species barrier in pseudogamous egg activation waspre or postcopulatory depending on the species pair. In the latter case, when hybridembryos were produced, most arrested before the first embryonic cell division.Hybrid incompatibility between auto-pseudogamous species was due to defectiveinteraction between sperm and oocyte as well as defective reconstitution of zygoticcentrosomes. CONCLUSIONS: We established a collection of sexual and pseudo-sexualspecies which offer an ideal framework to explore the origin and consequences oftransition to asexuality. Our results demonstrate that speciation occurs in thepseudogamous state. Whereas genomic conflicts are responsible for hybridincompatibility in sexual species, we here reveal that centrosomes constitute keyorganelles in the establishment of species barrier.

 
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