The role of local ecology during hybridisation at the initial stages of ecological speciation in a marine snail

Hybrid zones of ecologically divergent populations are ideal systems to study the interaction between natural selection and gene flow during the initial stages of speciation. Here we perform an AFLP genome scan in parallel hybrid zones between divergent ecotypes of the marine snail Littorina saxatilis, which is considered a model case for the study of ecological speciation. RB (Ridged-Banded) and SU (Smooth-Unbanded) ecotypes are adapted to different shore levels and microhabitats, although they present a sympatric distribution at the mid-shore where they meet and mate (partially assortatively). We used shell morphology, outlier and non-outlier AFLP loci from RB, SU and hybrid specimens captured in sympatry to determine the level of phenotypic and genetic introgression. We found different levels of introgression at parallel hybrid zones and non-outlier loci showed more gene flow with greater phenotypic introgression. These results were independent from the phylogeography of the studied ...