Genetic variation at the Me-2 locus in the Atlantic salmon within and between rivers: evidence for its selective maintenance

Spatial variation at the diallelic Me-2 locus in the Atlantic salmon, Salmo salar L., was analysed using data from 95 river basins. Gene diversity was apportioned as follows: 63% within samples, 20% between North America and Europe, 14% between regions within continents and 3% within and among rivers within regions. On both continents the variation between rivers was clinal with latitude and highly correlated with summer temperatures. The correlation was detectable within and between rivers. These correlations strongly suggest that variation at the locus is subject to the direct or indirect effects of natural selection, and that caution is required when interpreting between-location differentiation at the locus as evidence for distinct stocks.