Abstract
Freshwater mollusk shell morphology exhibits clinal variation along a stream continuum that has been termed the Law of Stream Distribution. We analyzed phylogenetic relationships and morphological similarity of two freshwater mussels (Bivalvia: Unionidae), Obovaria jacksoniana and Villosa arkansasensis, throughout their ranges. The objectives were to investigate phylogenetic structure and evolutionary divergence of O. jacksoniana and V. arkansasensis and morphological similarity between the two species. Our analyses were the first explicit tests of phenotypic plasticity in shell morphologies using a combination of genetics and morphometrics. We conducted phylogenetic analyses of mitochondrial DNA (1416 bp; two genes) and morphometric analyses for 135 individuals of O. jacksoniana and V. arkansasensis from 12 streams. We examined correlations among genetic, morphological, and spatial distances using Mantel tests. Molecular phylogenetic analyses revealed a monophyletic relationship between O. jacksoniana and V. arkansasensis. Within this O. jacksoniana/V. arkansasensis complex, five distinct clades corresponding to drainage patterns showed high genetic divergence. Morphometric analysis revealed relative differences in shell morphologies between the two currently recognized species. We conclude that morphological differences between the two species are caused by ecophenotypic plasticity. A series of Mantel tests showed regional and local genetic isolation by distance. We observed clear positive correlations between morphological and geographic distances within a single drainage. We did not observe correlations between genetic and morphological distances. Phylogenetic analyses suggest O. jacksoniana and V. arkansasensis are synonomous and most closely related to a clade composed of O. retusa, O. subrotunda, and O. unicolor. Therefore, the synonomous O. jacksoniana and V. arkansasensis should be recognized as Obovaria arkansasensis (Lea 1862) n. comb. Phylogenetic analyses also showed relative genetic isolation among drainages, suggesting no current gene flow. Further investigation of in-progress speciation and/or cryptic species within O. arkansasensis is warranted followed by appropriate revision of conservation management designations.In this study, we found Obovaria jacksoniana and Villosa arkansasensis are synonomous. We suggest that morphological differences between the two species are caused by ecophenotypic plasticity, where V. arkansasensis is the upstream morphotype and O. jacksoniana is the downstream morphotype of a single species.
Highlights
Preservation of biological diversity is the fundamental goal of conservation biology; the total biodiversity on Earth is still undetermined
Among Obovaria, O. olivaria comprised the basal clade in the maximum likelihood analysis (ML) tree followed by a clade comprised of O. retusa, O. unicolor, and O. subrotunda, and a clade of O. jacksoniana and V. arkansasensis
Shell morphology is often used as the key characteristic for identification and taxonomy of freshwater mussel species, morphologies can be extremely variable in different habitats
Summary
Preservation of biological diversity is the fundamental goal of conservation biology; the total biodiversity on Earth is still undetermined. Freshwater ecosystems represent a small portion of the global area, they harbor a disproportionally high level of diversity (Dudgeon et al 2006; Strayer and Dudgeon 2010). Freshwater ecosystems may be the most threatened due to human use and associated activities causing worldwide habitat degradation. Examples of habitat degradation include a decline in water quality by pollution,. Ecology and Evolution published by John Wiley & Sons Ltd. Ecology and Evolution published by John Wiley & Sons Ltd
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