Expressed sequence tag-linked microsatellites as a source of gene-associated polymorphisms for detecting signatures of divergent selection in atlantic salmon (Salmo salar L.).

The prediction that selection affects the genome in a locus-specific way also affecting flanking neutral variation, known as genetic hitchhiking, enables the use of polymorphic markers in noncoding regions to detect the footprints of selection. However, as the strength of the selective footprint on a locus depends on the distance from the selected site and will decay with time due to recombination, the utilization of polymorphic markers closely linked to coding regions of the genome should increase the probability of detecting the footprints of selection as more gene-containing regions are covered. The occurrence of highly polymorphic microsatellites in the untranslated regions of expressed sequence tags (ESTs) is a potentially useful source of gene-associated polymorphisms which has thus far not been utilized for genome screens in natural populations. In this study, we searched for the genetic signatures of divergent selection by screening 95 genomic and EST-derived mini- and microsatellites in eight natural Atlantic salmon, Salmo salar L., populations from different spatial scales inhabiting contrasting natural environments (salt-, brackish, and freshwater habitat). Altogether, we identified nine EST-associated microsatellites, which exhibited highly significant deviations from the neutral expectations using different statistical methods at various spatial scales and showed similar trends in separate population samples from different environments (salt-, brackish, and freshwater habitats) and sea areas (Barents vs. White Sea). We consider these ESTs as the best candidate loci affected by divergent selection, and hence, they serve as promising genes associated with adaptive divergence in Atlantic salmon. Our results demonstrate that EST-linked microsatellite genome scans provide an efficient strategy for discovering functional polymorphisms, especially in nonmodel organisms.