MOLECULAR SYSTEMATICS OF THE FRESH-WATER MUSSEL GENUS POTAMILUS (BIVALVIA, UNIONIDAE)

Few explicit hypotheses for the relationships of unionid mussels exist . The absence of explicit phylogenetic hypotheses is problematic and i s in part responsible for the lack of taxonomic stability seen in this group. In this paper we examine the relationships of mussels in the g enus Potamilus, based upon the DNA sequences of a 600 base pair portio n of the first subunit of the mitochondrial cytochrome c oxidase (COI) gene. We also examine the genetic distinctiveness of populations of t he inflated heelsplitter P. inflatus. The molecular phylogeny indicate s that Potamilus is paraphyletic with Leptodea fragilis and Lampsilis ornata nested between P. capax and the remaining Potamilus species. Wi th the exception of Fl capax, the remaining Potamilus species are depi cted as monophyletic and form three distinct clades: (1) a reciprocall y monophyletic P. inflatus clade; (2) a Fl ohiensis/P. amphichaenus cl ade; and (3) a P. purpuratus/P. p. coloradoensis/P. alatus clade. Whil e bootstrap values indicate a high degree of support for these three c lades, relationships among these three clades are not as strongly supp orted. The genetic distinctiveness of two populations of the inflated heelsplitter exceeds that seen between some other species in the genus . These populations represent geographically isolated, genetically dis tinct entities, and we therefore recommend the recognition of both the Amite and the Black Warrior populations of P. inflatus as separate sp ecies.