Rangewide distribution of neutral genetic variation in American shad (Alosa sapidissima)

Species long-term persistence is contingent on the ability of populations to mount variable responses to perturbations; the breadths of which are largely dependent on amounts of heritable variation present at the population level.  However, populations are not necessarily equivalent in their amounts of genetic variation, or in responses to future environmental conditions, and information about the magnitude and spatial distribution of intraspecific genetic variation is integral to conservation planning, and preserving species evolutionary potential.  Using neutral molecular markers, I examined the magnitude and spatial distribution of microsatellite based variation for 33 populations of American shad (Alosa sapidissima) from across the species’ range.  Sequential reductions of genetic variation with latitude were observed among populations from formerly glaciated regions; consistent with stepwise post-glacial range expansion, and successive population founder events.  Different spatial patterns of population structure were detected across the species’ range, and probably reflect different management strategies in Canada (no stocking) and the United States (stocking), alternative glacial histories, or combinations thereof.  Reciprocal patterns of genetic diversity and differentiation across the species’ range suggest that U.S populations contribute more to diversity and less to differentiation than Canadian populations. These results have implications for future shad restoration, and the species’ long-term persistence.