Molecular phylogenetic analysis of two problematic freshwater mussel genera (Unio and Gonidea) and a re-evaluation of the classification of Nearctic Unionidae (Bivalvia: Palaeoheterodonta: Unionoida)

Abstract

The freshwater mussel genera Unio (Palearctic) and Gonidea (Pacific Nearctic drainages) have been difficult to place in the classification of the Unionidae. This has been principally due to (1) a lack of appreciation for derived vs. ancestral characters and (2) a decoupling of taxonomy from evolutionary theory. To test cladistically the positions of Unio and Gonidea relative to the well-studied Nearctic genera of the Atlantic/Gulf of Mexico drainages, partial nuclear large-ribosomal subunit (28S rDNA) sequences were obtained from representative freshwater mussel lineages, including three genera from Southeast Asia. The phylogenetic reconstruction differs from the traditional placement of Unio among the Ambleminae; instead, Unio falls sister to anodontine mussels. Gonidea is sister to the remaining Nearctic Ambleminae, and these are distinct from the Asian 'amblemine' genera. Based on these results, the classification of the New World Unionidae is updated [i.e., Unioninae (= Unionini + Anodontini) + Ambleminae], and synapomorphies of the family are discussed.