Data from: 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
(hierarchical analysis of molecular variance FCT=0.017) while the
remaining two loci were strongly differentiated (FCT=0.428). By contrast,
J. albifrons and J. praehirsuta in mixed populations from region Brittany
occupied distinct habitats (pebbles and seaweeds, respectively) with
little overlap and showed stronger genetic divergence (FCT=0.132). In
hybridizing populations, the majority of individuals show morphological
traits that are characteristic of one or the other species. This raises
the question of the forces that act to maintain this polymorphism, noting
that hybridizing populations seem to be geographically isolated from
potential source parental populations and show no detectable habitat
divergence between species.