Co-segregation of recombinant chromatids maintains genome-wide heterozygosity in an asexual nematode

In asexual animals, female meiosis is modified to produce diploid oocytes. Associated with recombination, this is expected to lead to a rapid loss of heterozygosity, with adverse effects on fitness. Many asexuals, however, have a heterozygous genome, the underlying mechanisms being most often unknown. Cytological and population genomic analyses in the nematode Mesorhabditis belari revealed another case of recombining asexual being highly heterozygous genome-wide. We demonstrated that heterozygosity is maintained because the recombinant chromatids of each chromosome pair co-segregate during the unique meiotic division. A theoretical model confirmed that this segregation bias is necessary to account for the observed pattern and likely to evolve under a wide range of conditions. Our study uncovers a new type of cell division involving Directed Chromatid Assortment. One sentence summary Genome wide heterozygosity in the asexual nematode Mesorhabditis belari is achieved by directed assortment of recombinant chromatids during female meiosis ### Competing Interest Statement The authors have declared no competing interest.