Sexuality is the most ancestral and common reproductive system but several forms of reproduction without sex exist. This requires modifications of female meiosis, for the production of unreduced gametes. The genomic consequences of asexuality have been well studied but much less is known about the cellular and molecular changes at the origin of new types of meiosis in asexuals.
In my thesis, I first explored how asexual females are produced in the nematode species Mesorhabditis belari. I discovered that during meiosis, homologous chromosomes undergo crossing-overs. However, meiosis I is abortive due to a default in anaphase B. Meiosis II next proceeds normally giving rise to diploid oocytes with an assortment of non-sister chromatids. The theory predicts that species undergoing such modified meiosis with recombination should have a widely homozygous genome. However, our collaborators demonstrated that the genome of M. belari is widely heterozygous, which raised a paradox. Using a combination of cytology and genomics we uncovered this species undergoes a new type of meiosis, which we named Directed Chromatid Assortment (DCA), whereby the two recombinant chromatids of a given pair of chromosomes co-segregate during the meiotic division. We demonstrated that DCA allows the maintenance of genome-wide heterozygosity in an asexual, despite recombination.
In parallel, I searched for genes involved in oocyte determination, using molecular and cytological tools. In doing so, I developed the CRISPR Cas9 technique in M. belari. Finally, I wrote a review which summarizes my readings and thoughts on the importance of cytological approaches for the study of asexuality.