Geographically distinct patterns of reproductive isolation and hybridisation in two sympatric species of the Jaera albifrons complex (marine isopods)

Sympatric species that in some populations hybridize and in other populations remain reproductively isolated open interesting research possibilities for the study of hybridization and speciation. Here we test for such a situation in two littoral isopods (Jaera albifrons and J. praehirsuta) that occur in mixed populations and where past morphological descriptions suggested that the two species are generally reproductively isolated except in rare populations where hybridization may be happening. Using field surveys and microsatellite genetic structure analyses in two regions from France (Normandy and Brittany), we confirmed that introgressive hybridization occurs in a subset of mixed J. albifrons / J. praehirsuta populations (region Normandy) where the two species are found in the same habitat (pebbles on the shore). Moreover, we found that introgression in these populations is differential, 21 out of 23 microsatellite markers showing little genetic divergence between species (hierarchica...