Genetic Variation at the Malic Enzyme-2 Locus and Age at Maturity in Sea-run Atlantic Salmon (Salmo salar)

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Abstract
Variation in the diallelic polymorphism at the malic enzyme-2 locus (Me-2) in Atlantic salmon is used extensively in descriptions of the population structure of this species. Genotypes at the Me-2 locus were examined in adult Atlantic salmon captured at the Girnock Burn, a tributary of the Aberdeenshire Dee, over 3 yr (1986–88), and in salmon caught in the commercial fishery in the estuary of the River Tay in 2 yr (1987–88). Initial observations on the Girnock Burn data suggested that Me-2 genotype frequency distributions differed between one-sea-winter fish (grilse) and multi-sea-winter fish (salmon) groups from the same hatch-year class. Data from the River Tay supported hypotheses constructed on the basis of the Girnock Burn data. Genotype frequencies in some grilse groups at both sites differed significantly from those expected under Hardy–Weinberg equilibrium, while those in salmon groups did not. The frequency of heterozygotes was generally significantly higher in grilse than in salmon from the same hatch-year class. The data suggest a variable association between Me-2 genotype and age at maturity, and a selective influence on the Me-2 locus. The implications of selective influences on this locus for estimates of gene flow among Atlantic salmon populations are discussed.