Phylogeography and cryptic diversity of Gammarus balcanicus Schäferna, 1922 in Europe
According to the classical point of view, the genus Gammarus had originated in inland waters and subsequently some of its representatives colonized marine habitats. Very recent studies point out reverse scenario with the Tethys Ocean and Paleocene as a spatiotemporal frame for the origin of the genus, followed by divergence of main Gammarus species groups later in Tertiary. Diversity, and phylogeny, within these lineages has been scarcely studied so far. Yet, already it is known that that the conventionally recognized morphospecies hide substantial cryptic diversity. That is especially true in Southern European glacial refugia were high genetic diversity of many animal species in these areas is commonly attributed to effect of Pleistocene glaciations. Our study aimed to reveal cryptic diversity and phylogeography of G. balcanicus, species widely distributed in Europe. We used one nuclear (28S rRNA) and two mitochondrial (COI, 16S rRNA) markers sequenced for ca. 1000 individuals from 112 sampling sites in South-Eastern Europe, ranging from Alps to Crimean Peninsula and from northern Carpathians to southern Balkan Peninsula. In result, the morphospecies appeared to be complexes of dozens of highly divergent lineages, some dating back to the lower Neogene. Taking into account spatial distribution of these lineages and paleogeography of the area, the view that main Gammarus lineages diverged in freshwaters seems much simplified. Within G. balcanicus, several independent colonizations of inland waters in various areas of its present range can be assumed and related to emergence of once isolated land masses from the regressing Paratethys. Shallower phylogenetic divisions can be attributed to the Alpide orogeny, repetitively reshaping hydrography of the area.