Mitochondrial DNA Polymorphism, Population Structure, and Life History Variation in American Shad (Alosa sapidissima)

Abstract

Restriction endonuclease analysis of mitochondrial DNA (mtDNA) variation was used to assess genetic differentiation and population structure in American shad (Alosa sapidissima) sampled from 14 rivers spanning the native range of the species (Florida to Quebec). Estimated mtDNA sequence divergences among 52 shad surveyed with 16 endonucleases were relatively low (mean 0.2%). The low level of mtDNA variation in shad may be a consequence of population bottlenecks that occurred during Pleistocene glacial maxima. A survey of 243 shad with four enzymes revealed several genotypes that were distributed across the range of the species. Three genotypes, a length variant and two single-enzyme genotypes, exhibited nonrandom, geographically clumped distributions. The distributions of shad mtDNA genotypes may have been influenced primarily by founder effects in the northern (glaciated) part of the range, and gene flow in the southern part of the range. The mtDNA data suggest that differences in the reproductive traits of northern and southern populations of shad, if genetically mediated, are likely to have evolved since the Pleistocene. The results of this study support theoretical predictions that mtDNA analysis is a highly sensitive means of examining population structures.