Genetic discontinuity associated with an environmentally induced barrier to gene exchange in the marine snail Littorina saxatilis

The geographical genetic structure of 7 populations of the intertidal marine snail Littorina saxatilis with limited dispersal was studied in NW Spain using 3 classes of genetic markers: 840 amplified fragment length polymorphism (AFLP) loci, 3 microsatellites, and 1 morphological trait (shell size). A sharp genetic division at Cape Finisterre, as evidenced by all 3 markers, delimits 2 regions, This genetic discontinuity coincides with an abrupt change in plankton composition, climate and oceanographic characteristics, suggesting that contemporary influences on gene flow associated with the presence of an ecological barrier maintain the genetic subdivision in NW Spain. However, the genetic structure also accords to some extent with an isolation-by-distance model despite the heterogeneous gene flow due to this ecological barrier. Microsatellites and shell size showed a strong and significant covariation in pairwise differences (F-ST and Q(ST)) among populations, but no correlation was found between these markers and AFLPs. Similarly, both microsatellites and shell size showed a higher differentiation between the 2 regions than within each region, but this pattern was reversed for AFLPs. Differences in genomic sampling error and homoplasy could account for the discrepancy between different kinds of markers. These results point to the possibility that other marine species may show a similar pattern of intra-specific genetic divergence in NW Spain.