MOLECULAR SYSTEMATICS AND EVOLUTION OF REPRODUCTIVE TRAITS OF NORTH-AMERICAN FRESH-WATER UNIONACEAN MUSSELS (MOLLUSCA, BIVALVIA) AS INFERRED FROM 16S RIBOSOMAL-RNA GENE-SEQUENCES

North American freshwater unionacean bivalves are a diverse group of n early 300 species. Unionaceans exhibit an array of conchological, anat omical, life history, and reproductive characteristics that have figur ed prominently in proposed classification schemes. Recently, two very different classifications of North American unionaceans have been prop osed. Depending on the classification system utilized, a very differen t evolutionary trajectory of anatomical and reproductive features is o btained. The lack of a robust, well corroborated phylogeny of North Am erican unionacean bivalves hinders the progress of evolutionary and ec ological studies involving these species. Here we present a mitochondr ial DNA (mtDNA) based phylogeny for North American unionacean mussels and compare it to previously proposed classifications. In addition, we present a 'total evidence' phylogeny which incorporates both the mtDN A sequence data and available morphological data. The molecular and to tal evidence phylogenies agree largely with the conclusions of a previ ous study based largely on immunoelectrophoretic data. North American unionaceans can be divided into two families: the Unionidae, which is comprised of most of the species and the Margaritiferidae. Within the Unionidae are two subfamilies, the Anodontinae and Ambleminae. The res ultant phylogeny was used to examine the evolution of several key anat omical features including the number of gills (demibranchs) used by fe males to brood developing embryos, incubation length (bradytictic vs t achytictic), larval (glochidial) tooth structures? and shell texture. Both molecular and total evidence phylogenies indicate several of the aforementioned characters evolved independently or were subsequently l ost or gained in several lineages.