Populations of the fluted-shell mussel (Lasmigona costata) from the Ohio River and Lake Erie watersheds reveal Pleistocene isolation of two lineages and colonization of current locations from multiple glacial refugia

Freshwater organisms of North America have had their population genetic structure shaped by past events, in particular Pleistocene glaciations.  Life history traits that promote dispersal and gene flow have since continued to shape population genetic structure.  Freshwater mussel larvae are obligate parasites of fish hosts, and thus, fish movement is the major mode of dispersal for mussels.  Little is known about the genetic structure of mussel populations, and more information is needed for conservation.  We used a hierarchical approach to examine the genetic structure of Lasmigona costata by using allozyme electrophoresis and mtDNA sequences.  Using this design, we calculated genetic structure within populations, among populations within rivers, among rivers within watersheds, and among the Lake Erie (Grand, Sandusky, and Sydenham rivers) and Ohio River (Licking, Stillwater, and Walhonding rivers) watersheds.  Both nuclear and mitochondrial data show similar trends in population structure and phylogeography.  We found that the majority of the genetic variation exists among watersheds (F_ST=0.126 from allozymes; 63% among watershed variation from mtDNA).  Significant genetic distances among watersheds and cluster analysis of allozymes indicate historic reproductive isolation between these two regions.  A positive correlation was observed between genetic and geographic distance only when using a historical connection through the Wabash River, indicating geographic structure is due to isolation-by-distance.  Our results are consistent with a hypothesis of recent postglacial dispersal from several refugia.  These results could have important implications for widespread mussel species in the Mississippi River and Great Lakes drainages as significant genetic variation is possible at relatively small geographic scales. Thus, geographic variation must be considered when developing conservation strategies for these organisms.