Potomida littoralis (Bivalvia, Unionidae) evolutionary history. Slow evolution or recent speciation?

Abstract

The distribution of circum-Mediterranean fauna was dramatically affected by palaeogeographical events during the Cenozoic. The biogeographical history of freshwater mussels is related to the development of the continental watershed during the Tertiary and Pleistocene glacial cycles. Our study model is Potomida littoralis (Cuvier, 1798), a Palaearctic freshwater mussel with a patchy circum-Mediterranean distribution encompassing areas of North Africa, south-western Europe, Greece and the Levant. Our phylogenetic reconstructions, network analyses and coalescence estimations indicate that the clade Potomida has a clear geographical structure with four robust lineages from Turkey, Greece, North Africa and Iberian Peninsula plus France. This structure is only disrupted in two cases: specimens from the Ziz River (Morocco) clustered with Tunisian lineages, and samples from the north-eastern Iberian Peninsula were found to have been recently introduced from Morocco. We considered two different calibration strategies of a molecular clock to evaluate divergence times between lineages. The first, based on palaeogeographical events, resulted in substitution rates up to three times lower than the average for molluscs. When such mean values were considered for calibrating the molecular clock, the group's origin and diversification events were incongruent with the fossil record and the likelihood of gene flow between lineages. Rather than a pattern of recent speciation, our results suggest low levels of substitution and a pattern of ‘slow evolution’, probably linked to its long generation time and longevity. From a taxonomic viewpoint, the genus Potomida is currently considered monospecific. Reflecting the data presented herein, we propose the existence of an eastern species, named P. acarnanica (Kobelt, 1879) in Greece and Turkey, and another named P. littoralis (Cuvier, 1798), with a wide distribution ranging from France to Tunisia.