Abstracts (first author)
A young neo-sex chromosome is the site of divergence between incipient stickleback species
Sex chromosomes turn over rapidly in some taxonomic groups, where closely related species have different sex chromosomes. However, little is known about the functional roles of sex chromosome turnover in phenotypic diversification and speciation. We use a sympatric pair of threespine sticklebacks to understand the roles of sex chromosome turnover in phenotypic divergence and speciation. Previously, we found that the Japan Sea sticklebacks have a neo-sex chromosome system resulting from a fusion between an ancestral Y chromosome and an autosome, while sympatric Pacific Ocean sticklebacks have a XY sex chromosome system. Furthermore, we demonstrated that the Japan Sea neo-X chromosome plays an important role in phenotypic divergence and reproductive isolation between these sympatric species. Here, we conducted additional QTL mapping of other morphological and behavioral traits to confirm that both old-X and neo-X chromosomes have significant QTL important for morphological divergence, while the old-X, but not the neo-X chromosome, contributed to hybrid courtship abnormality. Next, we conducted whole genome sequencing of these two sticklebacks to find that the Japan Sea neo-Y chromosome has no large-scale deletion, but some regions on the neo-Y have started to accumulate deleterious mutations. Importantly, significant QTL on the neo-X chromosome were located at regions even outside the regions where deleterious mutations accumulate. Furthermore, comparison of genome sequences between these two species revealed that the neo-X chromosome showed faster protein evolution than autosomal genes. Thus, a young neo-sex chromosome is the site of divergence between incipient species.