Analysis of the Genetic Structure of Northwestern Bering Sea Walleye Pollock, Theragra chalcogramma

The intraspecific structure of major populations of the walleye pollock Theragra chalcogramma from the northwestern part of the Bering Sea was studied. Specimens from Navarin, Olyutor and Shirshov shoals were sampled in the 2001 spawning season. Preliminary cluster analysis (TREECON) of PCR-RAPD data revealed the existence of cluster with low level of bootstrap support, which generally corresponds to geographic localization of the shoals. Processing of microsatellite sequence data (loci Tch12, Tch14, Tch15 and Tch18) with GENEPOP demonstrated the deviation from HWE and differentiation between the samples. The value of the inter-population variance (F_st = 0.02) corresponded to published data on marine stocks, which were subject to high levels of gene flow. The Shirshov group was found to be equidistant from the Navarin and Olyutor groups, with the genetic distance between the latter two being significantly less. Analysis of the microsatellite loci inferred disparities in their selective capacities. Loci Tch12 and Tch15 are found to be highly homozygous with low levels of polymorphism. Tch12 clearly follows the ‘3 band’ pattern. Tch15 shows sign of linkage disequilibrium. Comparison of F_st values for the loci with the mean standardized variance f₀ suggests that Tch12 is under pressure of disruptive selection, while Tch15 is rather neutral and Tch14 and Tch18 are under the influence of balance selection. Also a discussion is conducted in the context of undeterminate group migration.