Species boundaries, geographic distribution and evolutionary history of the Western Palaearctic freshwater mussels Unio (Bivalvia: Unionidae)

Delimiting species boundaries is a fundamental yet challenging task that has been especially problematic in morphologically conserved lineages. The use of genetic markers and phylogenetic approaches has proven of paramount importance to resolve these cases. Using the genus Unio as a case study, we have analysed two mitochondrial markers, cytochrome c oxidase subunit I (COI) and 16S rRNA (16S), phylogenetically and phylogeographically to explore species limits and evolutionary lineages within the group. We followed two different approaches to define species boundaries: the generalized mixed Yule coalescence model in a Bayesian framework and the Poisson tree process model. Phylogenetic relationships among species and divergence times were also assessed using relaxed molecular clock analyses. Taken together, we provide a sound systematic framework for Western Palaearctic Unio species, addressing phylogenetic relationships, and the taxonomy, biogeographic patterns and evolutionary history of this group in this region. The Unio clade from the studied area had a clear phylogenetic structure with four robust lineages that include the following species: (1) Unio durieui + Unio gibbus; (2) Unio tumidus; (3) Unio pictorum, Unio delphinus + Unio foucauldianus, Unio elongatulus, Unio mancus, Unio ravoisieri, Unio tigridis; and (4) Unio tumidiformis + (Unio bruguierianus + (Unio crassus + Unio ionicus)). Deeper analyses, including filling gaps in distributions, should be conducted to disentangle the U. crassus, U. pictorum and U. tigridis species complexes.