Genetic characterization of flat periwinkles (Littorinidae) from the Iberian Peninsula reveals interspecific hybridization and different degrees of differentiation

Abstract

The flat periwinkles, Littorina fabalis and L. obtusata, are two intertidal gastropods that offer a promising system for local adaptation and ecological speciation studies. Taking advantage of 17 recently developed specific microsatellite loci, we performed the first comprehensive genetic characterization of these sister species in the Iberian Peninsula. Special attention was given to L. fabalis by sampling populations representing the three morphotypes/ecotypes described from the region, which mainly differ in the habitat they are confined to (i.e. algae/seagrass ‘host’ species and level of wave exposure). Our analysis revealed, for the first time, the existence of hybrids between L. fabalis and L. obtusata in a natural population, which suggests that despite their strong genetic differentiation, reproductive isolation is not complete. Furthermore, the differentiation between L. fabalis ecotypes is lower than between these and L. obtusata; and their population genetic structure appears to be mainly governed by geography. However, ecological and demographic factors seem to have also played an important role, as suggested by the higher population differentiation in one of the ecotypes. Altogether, this work provides an important baseline for further studies on flat periwinkles as a model to explore how reproductive barriers evolve and interact with each other across a speciation continuum.