Patterns of genetic diversity and structure in Pressodonta viridis, Slippershell Mussel, in streams of the U.S. Midwest.

Pressodonta viridis (Bivalvia: Unionida) is a species of freshwater mussel considered rare and imperiled in 14 U.S. states and Ontario, Canada. Limited population-level genetic information is available for P. viridis despite the species' imperilment status. We used 13,661 single nucleotide polymorphisms (SNPs) derived from double digest restriction site-associated sequencing (ddRAD-seq) to examine patterns in population genetic diversity, structure, and differentiation among P. viridis populations in the Upper Mississippi River, Ohio and Wabash rivers, and Great Lakes watersheds. Structure analyses revealed significant differentiation related to major drainage among groups of P. viridis populations. Evidence of founder effects in Great Lakes populations by colonization routes from Upper Mississippi River and Ohio/Wabash River systems was found in admixture analyses. Limited evidence of isolation-by-distance effects was found, suggesting that multiple colonization routes and glacial refugia are responsible for current P. viridis genetic structure. A combination of ancient river hydrological patterns and more recent headwater stream capture events following the last glacial retreat likely correspond to considerable admixture found among populations of P. viridis in the Upper Mississippi, Wabash, and Ohio Rivers. Population genetic variation, structure, and differentiation described in this study should be used to inform conservation efforts of P. viridis in the future, particularly if recovery actions include population augmentation from translocations or hatchery propagation.