Natural selection and the genetic differentiation of coastal and Arctic populations of the Atlantic cod in northern Norway: a test involving nucleotide sequence variation at the pantophysin (PanI) locus

To examine the role of contemporary selection in maintaining significant allele frequency differences at the pantophysin (PanI) locus among populations of the Atlantic cod, Gadus morhua, in northern Norway, we sequenced 127 PanIA alleles sampled from six coastal and two Barents Sea populations. The distributions of variable sites segregating within the PanIA allelic class were then compared among the populations. Significant differences were detected in the overall frequencies of PanIA alleles among populations within coastal and Arctic regions that was similar in magnitude to heterogeneity in the distributions of polymorphic sites segregating within the PanIA allelic class. The differentiation observed at silent sites in the PanIA allelic class contradicts the predicted effects of widescale gene flow and suggests that postsettlement selection acting on cohorts cannot be responsible for the genetic differences described between coastal and Arctic populations. Our results suggest that the marked differences observed between coastal and Arctic populations of G. morhua in northern Norway at the PanI locus reflect the action of recent diversifying selection and that populations throughout the region may be more independent than suggested by previous studies.