Genetic Changes in Pink Salmon (Oncorhynchus gorbuscha) Following Their Introduction Into the Great Lakes

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Abstract
Pink salmon (Oncorhynchus gorbuscha) previously thought to require salt water for completion of their life cycle, have been firmly established in the Great Lakes following an accidental introduction into Lake Superior. We compare allelic frequencies at 27 protein-coding loci from collections of pink salmon from the Great Lakes with those from the anadromous population (Lakelse River, British Columbia) from which they were derived. Although the allelic frequencies in the Great Lakes collections are consistent with a single introduction, the frequencies observed in these collections differ substantially from those of the Lakelse River population. Alleles of G3p-1 and Ck-1, rarely observed (frequency < 0.005) in British Columbia population, are present in Great Lakes pink salmon at frequencies between 0.06 and 0.27. Smaller changes were observed at 11 other loci; 14 loci were monomorphic in all collections. Loss of variability in the Great Lakes was reflected by a decrease in average number of alleles per locus. Selection for physiologically tolerant phenotypes may have been necessary to establish this unique, self-perpetuating, freshwater population. The biochemical genetic changes we observed, however, can be adequately explained by genetic drift resulting from bottlenecks occurring at the first and at subsequent generations. Any decreases in survival resulting from freshwater intolerance would have exacerbated the bottlenecks. This adaptively distinct lineage produced by the ecological change coupled with the bottlenecks may be a major step toward speciation.
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