Repeated colonization of the Northern Limestone Alps from the Southern Limestone Alps by the rock-dwelling snail species Cochlostoma henricae

The evolution of biogeographic disjunctions in the Southern and Northern Limestone Alps has been studied only in a few taxa. Cochlostoma henricae (Strobel, 1851) is a land snail species restricted to limestone rocks showing such disjunctions. We studied the phylogeny and population structure of this species using genomic ddRAD data to better understand the differentiation of the species in the Southern Alps and the origin of the isolated occurrences in the Northern Limestone Alps. Phylogenetic and population genetic analyses of the ddRAD data revealed a subdivision of the C. henricae populations into a western cluster and an eastern cluster. These clusters were probably separated by glaciers that extended from the Piave Valley to the margin of the Alps. Such a longitudinal subdivision is a common pattern of several species in the Southern Limestone Alps. However, the boundaries between western and eastern population groups differ between taxa. Cochlostoma henricae colonised the Northern Limestone Alps at least twice. The population from Breitenwang in Tyrol originated from the western group, whereas the occurrences in the Salzkammergut are from the eastern group. These and similar disjunctions of several other land snails show that even species with limited dispersal abilities could cross the Alps repeatedly by passive long-distance dispersal events. A subdivision of C. henricae into three subspecies, proposed based on shell characters, does not correspond to its population genetic structure. Instead, the eastern and western population groups represent distinct evolutionary units that can be classified as subspecies, C. h. henricae and C. henricae insigne.