Sex chromosomes and chromosomal rearrangements contribute to behavioral sexual isolation in Jaera albifrons marine isopods.

We focus on the evolution of behavioral sexual isolation through the empirical study of two marine isopod species. The males of Jaera albifrons and Jaera praehirsuta engage females in tactile courtship by brushing the female's back, but they do so with divergent sets of specialized setae, and female choice results in strong reproductive isolation. Using RADseq-derived genotypes of individuals from natural populations and controlled crosses, we found that secondary contacts between J. albifrons and J. praehirsuta resulted in different levels of heterospecific gene flow. Comparison of the genomic landscapes of differentiation in the two most contrasted situations (extremely low heterospecific gene flow in one geographic region, but strong introgressive hybridization in another) allowed us to conclude that genomic regions resistant to interspecies gene flow are primarily located on the sex chromosomes or on rearranged chromosomes (several fusion-scissions and one reciprocal translocation). These genomic regions show low recombination, and in two cases quantitative trait locus analyses found genetic variation associated with male courtship traits. These results suggest that a period of allopatry may have allowed the divergent coevolution of male traits and female preferences, with genetic bases located at least in part in nonrecombining regions on sex chromosomes and rearranged chromosomes.