Genetic Differentiation in the Face of Gene Flow: A Study of Mussel Populations from a Single Nova Scotian Embayment

Abstract

There are significant genetic differences among populations of the blue mussel, Mytilus edulis, from six localities in St. Margaret's Bay, N.S., Canada, despite the presence of gene flow. The populations are differentiated into two groups, those at the head of the bay where ambient conditions fluctuate widely during the year, and those at the mouth of the bay where conditions (particularly salinity) fluctuate to a lesser degree. Three isoenzyme loci, i.e. leucine aminopeptidase 1, peptidase 2, and phosphoglucose mutase, show a clear shift in the predominant frequencies, from slow migrating alleles at the mouth of the bay to faster migrating alleles at the head of the bay. This shift is a microgeographic parallel of the pattern we observed in 1980 on a macrogeographic scale. A genetic comparison of these populations with those studied previously shows that allele "populations" cluster according to their environment and not according to geographical proximity. We conclude that differences in allele frequencies among localities index heterogeneity among environmental conditions, and that the sensitivity to environmental selective forces varies from locus to locus.