Résumé
Closely related species are key models to investigate mechanisms leading
to reproductive isolation and early stages of diversification, also at the
genomic level. The brittle star cryptic species complex Ophioderma
longicauda encompasses the sympatric broadcast spawning species C3 and the
internal brooding species C5. Here we used de novo transcriptome
sequencing and assembly in two closely related species displaying
contrasting reproductive modes to compare their genetic diversity and to
investigate the role of natural selection in reproductive isolation. We
reconstructed 20,146 and 22,123 genes for C3 and C5, respectively, and
characterized a set of 12,229 orthologs. Genetic diversity was 1.5-2 times
higher in C3 compared to C5, confirming that species with low parental
investment display higher levels of genetic diversity. Forty-eight genes
were the targets of positive diversifying selection during the evolution
of the two species. Notably, two genes (NHE and TetraKCNG) are
sperm-specific ion channels involved in sperm motility. Ancestral sequence
reconstructions show that natural selection targeted the two genes in the
brooding species. This may result from an adaptation to the novel
environmental conditions surrounding sperm in the brooding species, either
directly affecting sperm, or via an increase in male/female conflict. This
phenomenon could have promoted prezygotic reproductive isolation between
C3 and C5. Finally, the sperm receptors to egg chemoattractants differed
between C3 and C5 in the ligand-binding region. We propose that mechanisms
of species-specific gamete recognition in brittle stars occur during sperm
chemotaxis (sperm attraction towards the eggs), contrary to other marine
invertebrates where prezygotic barriers to interspecific hybridization
typically occur before sperm-egg fusion.