Selection and population genetic structure of the barnacle Semibalanus balanoides in the northwest Atlantic and Gulf of St. Lawrence*

It has been proposed that population genetic structure at the mannose-6-phosphate isomerase (Mpi) locus in the barnacle Semibalanus balanoides (L.) could be due to selection by ice scour acting through variation in larval settlement behavior. Homogeneity of Mpi allele frequencies for barnacles within the Gulf of St. Lawrence could be maintained by a combination of selection and limited gene flow from populations of S. balanoides on the Atlantic coast of Canada south of the Cabot Strait. We sampled S. balanoides from 19 sites in Greenland and Baffin Island, the Gulf of St. Lawrence, and on the Atlantic coasts of Nova Scotia and New Brunswick, Canada, in order to test these hypotheses. Presumptive genetic variation at 2 enzyme loci, Mpi and glucose-6-phosphate isomerase (Gpi ) , was quantified by isoelectric focusing. A11 populations were in Hardy-Weinberg equilibrium, and no Linkage disequilibrium between loci was observed. We found significant spatial variation in allele frequencies at both loci. In particular, S. balanoides populations within the Gulf of St. Lawrence were not genetically homogeneous at either locus. A discontinuity in allele frequencies at both the Gpj and Mpi loci occurred in the vicinity of the Mirarmchi estuary, in the southern Gulf. Comparisons of allele frequencies between barnacle recruits and adults suggested this population structure may have resulted from selection, in the case of Mpi, possibly associated with environmental temperature. Selection by ice scour does not appear to affect spatial variation in allele frequencies in the Gulf of St. Lawrence.