Contrasting patterns of genetic population structure in walleye pollock (Theragra chalcogramma)

Two classes of nuclear DNA markers yielded contrasting patterns of genetic differentiation among populations of walleye pollock, Theragra chalcogramma . Samples across the natural geographic range were screened for variation at seven microsatellite loci and the pantophysin (Pan I) locus. Results from microsatellites revealed weak neutral genetic structure (global F_ST = 0.002) among samples over large geographic scales. Indirect measures of gene flow were correlated with geographic distances, exhibiting an isolation-by-distance pattern consistent with a stepping-stone model of gene flow. Evidence for the effects of selection at the pantophysin (Pan I) locus in walleye pollock was examined using DNA sequence data. Positive selection at the Pan I locus was strongly inferred by high ratios of nonsynonymous to synonymous substitutions per site (d_N/d_S) in two intravesicular domains of the protein and from results of maximum likelihood analyses. Replacement mutations segregating in the second intravesicular domain were screened in the same samples examined using microsatellites. Genetic differentiation among populations at the Pan I locus was considerably greater than for microsatellites (global F_ST = 0.036) and did not conform to an isolation-by-distance model. Geographic distributions of Pan I alleles appeared to follow a latitudinal gradient, resulting in the largest genetic divergence between northern Bering Sea and temperate Pacific Ocean populations. The discordant estimates of genetic structure from the two marker classes most likely results from effects of natural selection at the Pan I locus in causing higher levels of spatial heterogeneity.; An additional study examined the evolution of a complex microsatellite locus consisting examined in six gadid fish species. Locus size distributions generally conformed to expectations for a stepwise mutation model (SMM) of evolution and there was an inverse relationship between array interruptions and allelic polymorphism. A process of slipped-strand mispairing between the microsatellite repeat array and flanking sequence may be responsible for restoring lost polymorphism to decaying microsatellites in several genera. Phylogenetically informative mutational features were found within the microsatellite arrays and in non-repetitive flanking sequence. Phylogenies constructed from these characters revealed allelic paraphyly in one species, suggesting that reciprocal monophyly has not been achieved in all lineages of Pacific gadids.