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 balanoid es (L.) could be due to selection by ice scour acting through variatio n 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 Islan d, the Gulf of St. Lawrence, and on the Atlantic coasts of Nova Scotia and New Brunswick, Canada, in order to test these hypotheses. Presump tive genetic variation at 2 enzyme loci, Mpi and glucose-6-phosphate i somerase (Gpi), was quantified by isoelectric focusing. All population s were in Hardy-Weinberg equilibrium, and no linkage disequilibrium be tween loci was observed. We found significant spatial variation in all ele frequencies at both loci. In particular, S. balanoides populations within the Gulf of St. Lawrence were not genetically homogeneous at e ither locus. A discontinuity in allele frequencies at both the Gpi and Mpi loci occurred in the vicinity of the Miramichi estuary, in the so uthern Gulf. Comparisons of allele frequencies between barnacle recrui ts and adults suggested this population structure may have resulted fr om selection, in the case of Mpi, possibly associated with environment al temperature. Selection by ice scour does not appear to affect spati al variation in allele frequencies in the Gulf of St. Lawrence.