[
International Worm Meeting,
2021]
Sexuality is widespread and asexuality is a derived character. Despite the recognized costs associated with sex, asexuals remain rare, which constitutes one of the most intriguing paradox in evolutionary biology. To understand the adaptative value of sex, there is clearly a need for further exploration of the asexual world, and in particular of the transition to asexuality. We recently characterized the nematode genus Mesorhabditis, in which regular sexual species are found, as well as species featuring progressive loss of males and sexuality. These pseudo-sexual species are composed of 90% females and only 10% males. Males and sperm are needed for most eggs to develop by gynogenesis (the sperm is needed to activate the oocytes but its DNA is not used, i.e pseudogamy). In the few cases where the sperm DNA is incorporated, the eggs develop as males, because only the Y-bearing sperm of males are competent to fertilize (Grosmaire & al. Science 2019; Launay & al. BMC Evol Bio 2020). In this intriguing system, females are thus produced asexually whereas males are produced sexually. Mesorhabditis nematodes represent an ideal system to study the evolutionary consequences of transition to asexuality in closely related species and to explore the molecular origins of asexuality. There are many ways to lose sex. Here we asked which modifications to meiosis led to the production of diploid oocytes in asexuals. Using cytological descriptions, we found that meiosis of asexual females is similar to sexual species up to anaphase I. Chromosome pairing and crossing-over occur and bivalent forming chiasmata are present in diakinesis. Next, the first meiotic spindle forms and bivalent chromosomes initiate their segregation. However, chromosome segregation eventually stops and all univalents realign at metaphase of meiosis II. Meiosis II then proceeds normally with the segregation of sister chromatids and formation of the single polar body. Hence, meiosis in these asexuals is characterized by the maintenance of recombination and the assortment of non-sister chromatids in the diploid oocyte. In parallel, we are comparing the level of recombination in the sexual and asexual species of this genus. For that, we have sequenced the genome of a dozen strain for one sexual and one asexual species, to measure linkage desequilibrium. The theoretical consequence of such modified meiosis is genome-wide homozygosity, except maintenance of heterozygosity on chromosome centers. This will be contrasted with the actual measure of heterozygosity.