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

Sophie Tandonnet, Georgios D Koutsovoulos, Sally Adams, Delphine Cloarec, Manish Parihar, Mark L Blaxter, Andre Pires-daSilva

Research output: Contribution to journalArticlepeer-review


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 In the trioecious nematode Auanema rhodensis, males have one X chromosome (XO), whereas females and hermaphrodites have two (XX). The female versus 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 (doublesex/mab-3) domain transcription factor dmd-10/11 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 A. rhodensis, instead of the typical six found in other rhabditine species, derives from fusion/rearrangements events involving three Nigon elements. The A. rhodensis 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.

Original languageEnglish
Pages (from-to)1211-1230
Number of pages20
JournalG3: Genes | Genomes | Genetics
Issue number4
Early online date15 Feb 2019
Publication statusPublished - 9 Apr 2019


  • Auanema
  • Genome
  • Genetic map
  • Nigon elements
  • sex determination

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