Micro- and macrogeographic allozyme variation in Littorina fabalis; do sheltered and exposed forms hybridize?

Earlier studies of Swedish populations of the marine snail Littorina fabalis show that snails from different microhabitats (with a greater and lesser exposure to wave action) have almost diagnostic differences in one allozyme locus (arginine kinase, Ark), and differ in adult size. Snails with ‘sheltered’ and ‘exposed’ Ark genotypes occur in sympatry in intermediary exposed sites and here adult sizes remain distinct. Approaching the microgeographic differentiation we studied the parts of two populations where the frequency of Ark changes dramatically over zones 50–120 m wide. The aim was to test if the transitional zones are best described as areas of mixing of two genetically separate populations, or if hybridization between the exposed and sheltered groups occurs. Heterozygotes were in deficiency along both clines but were still roughly twice as common as expected from a pure mixing of ‘sheltered’ and ‘exposed’ groups suggesting hybridization. Hybridization was also supported by the observation that snails homozygous for sheltered and exposed alleles mated at random with each other in both populations. On the macrogeographic scale, we found populations from exposed and sheltered sites in France and Wales being fixed for the same exposed and sheltered Ark alleles as found in Sweden. However, variation in three other highly polymorphic loci indicated geographic affinity rather than habitat similarity being the main factor of genetic coherence. These observations support a hypothesis of gene flow between exposed and sheltered populations of L. fabalis. Two Spanish populations were remarkably different with unique alleles at high frequencies in three of four strongly polymorphic loci.