Spatial pattern of intraspecific mitochondrial diversity in the Northern Carpathian endemic spring snail, Bythinella pannonica (Frauenfeld, 1865) (Gastropoda: Hydrobiidae)

Bythinella is a species-rich genus of spring-snails, having a wide range in Europe and Asia Minor. The genus contains several endemic species with narrow ranges, creating interest from a conservation perspective as well as their use as a model for research into biogeographical patterns and evolutionary development. Most of the species of the genus Bythinella are difficult to distinguish by traditional methods due to their similar shell morphology. In previous studies, molecular approaches often came to conclusions that contradicted those from a morphology-based approach, hence the classification of species of Bythinella has been in dispute. Bythinella pannonica has clearly distinct shell morphological features, and consequently is one of the few species of undisputed taxonomic status within this genus. As an important step towards clarifying the systematics of this genus, we have attempted to describe the structure and spatial pattern of diversity of the mitochondrial COI marker within this species, in the hope of applying these findings generally to the whole genus. Molecular results support the monophyly of B. pannonica within the genus. The largest pairwise intraspecific COI sequence difference was almost 5 %, much larger than the value of 1.5 % previously proposed as a barcoding threshold for delimitation of Bythinella species. This finding suggests that incautious application of barcoding might lead to incorrect taxonomic conclusions. Within B. pannonica there are two deeply divergent intraspecific clades, the spatial distribution of which has been shaped by fragmentation, as well as by short and long distance dispersal events. These two clades have not been found syntopically but, as a peculiar feature of this taxon, they are able to persist in nearby habitats. We have demonstrated that the patchy distribution of suitable habitats and the restricted, but non-zero gene flow amongst the populations might play a key role in maintenance of the observed genetic structure of this species.