Post-glacial dispersal patterns of Pyganodon grandis (Bivalvia: Unionidae) into the lower Great Lakes watershed

Pyganodon grandis (Say, 1829) and other unionid mussels arrived in the Laurentian Great Lakes only after the last glacial maximum in North America. The mussel assemblage is thought to have entered Lake Erie as a wave of expansion from west to east, and moved upstream within tributary rivers. A similar process for most mussels occurred in Lake Ontario, but by eastern species, as the two lake systems are separated by Niagara Falls. Only P. grandis is abundant in both lakes. We applied variation in a fragment of the mtDNA Carboxylase I (COI) gene to identify potential historical paths for P. grandis to enter these watersheds. Nearly complete monomorphy characterized the Lake Ontario population for the most common Lake Erie allele, which is concordant with sequential founder effects across the Great Lakes. A OST including populations from both Lakes, the intervening Niagara River, tributaries of Lake Erie, and additional samples from below the Laurentian Divide, was 0.10. The southern tributaries of Lake Erie had greater genetic diversity, although few haplotypes were shared among regions or even neighboring streams. Tributary and lake populations differed significantly, yet variation across this divide was not significant. This pattern likely arose from stream capture of upper Ohio River tributaries that once flowed south, and then population isolation, with upstream dispersal from Lake Erie limited to the lower reaches.