Genetic differentiation in walleye pollock (Theragra chalcogramma) in response to selection at the pantophysin (PanI) locus

Samples of walleye pollock (Theragra chalcogramma) from the North Pacific Ocean and Bering Sea were screened for variation at the pantophysin (PanI) locus. Global genetic differentiation across samples (F-ST = 0.038) was considerably greater than reported in previous population studies using allozymes, mtDNA, or microsatellite loci and significantly greater than F-ST distributions of neutral loci simulated over a large range of locus heterozygosity. PanI allele frequencies varied over a broad latitudinal gradient and were correlated with estimated mean surface temperatures, resulting in the greatest levels of genetic divergence between the northern Bering Sea and the southernmost locations in the temperate Pacific Ocean (Puget Sound, Japan). The discordance between estimates of population differentiation estimated from PanI and other neutral marker classes, both in magnitude and in geographic patterns, could arise from temperature-mediated effects of natural selection over broad geographic scales. Our empirical results suggest that loci subject to directional selection may prove to be useful markers for stock identification in weakly structured marine fishes.