Chromosome-Wide Evolution and Sex Determination in the Three-Sexed Nematode .

Trioecy, a mating system in which males, females and hermaphrodites co-exist, is a useful system to investigate the origin and maintenance of alternative mating strategies. In the trioecious nematode , males have one X chromosome (XO), whereas females and hermaphrodites have two (XX). The female hermaphrodite sex determination mechanisms have remained elusive. In this study, RNA-seq analyses show a 20% difference between the L2 hermaphrodite and female gene expression profiles. RNAi experiments targeting the DM (/) domain transcription factor suggest that the hermaphrodite sexual fate requires the upregulation of this gene. The genetic linkage map (GLM) shows that there is chromosome-wide heterozygosity for the X chromosome in F2 hermaphrodite-derived lines originated from crosses between two parental inbred strains. These results confirm the lack of recombination of the X chromosome in hermaphrodites, as previously reported. We also describe conserved chromosome elements (Nigon elements), which have been mostly maintained throughout the evolution of Rhabditina nematodes. The seven-chromosome karyotype of , instead of the typical six found in other rhabditine species, derives from fusion/rearrangements events involving three Nigon elements. The X chromosome is the smallest and most polymorphic with the least proportion of conserved genes. This may reflect its atypical mode of father-to-son transmission and its lack of recombination in hermaphrodites and males. In conclusion, this study provides a framework for studying the evolution of chromosomes in rhabditine nematodes, as well as possible mechanisms for the sex determination in a three-sexed species.