Analysis of mitochondrial DNA (mtDNA) sequence variation among 244 American shad (Alosa sapidissima) from 14 rivers spanning the (Florida-Quebec) range of the species revealed several unusual features of shad mtDNA polymorphism. Two types of heteroplasmy, one involving a length polymorphism and the other a restriction site are common in shad. The length polymorphism involves a novel tandem triplication of a 1.5-kb sequence in the D-loop region. Both forms of heteroplasmy stem from multiple mutational events. The mtDNA data indicate that shad populations are reproductively discrete, and suggest that differences in the reproductive traits of northern and southern shad populations have evolved since the Pleistocene. Low mtDNA sequence variation in shad may stem at least in part from Pleistocene population reductions. A fossil calibration supports an mtDNA divergence rate in shad at least one order of magnitude slower than the prevailing estimate for vertebrates.

No description available.