Clinal and Size-Dependent Variation at the Lap Locus in Mytilus Edulis

Samples of the mussel, Mytilus edulis, were taken from 16 sites on the northern shore of Cape Cod. The mussels were measured and their genotypes at the leucine-amino-peptidase (LAP) locus determined by electrophoresis.2. At two separate estuarine localities, Sandwich Harbor and Scorton Creek, a pronounced cline in slow allele frequencies in the larger mussels was found, with upstream sites showing characteristically low frequencies (about 15%) and downstream (entrance) sites having high frequencies (45% to 55%). Mussels smaller than 26 mm had intermediate (22% to 35%) slow frequencies.3. This clinal divergence with increasing size strongly argues for an adaptive polymorphism. Mechanisms for the observed excess of homozygote genotypes in almost all samples are also discussed.