Origin of the Lake Ohrid gammarid species flock: ancient local phylogenetic lineage diversification

Aim Lake Ohrid is the oldest existing lake in Europe, dating back to the Tertiary. Given its surface area and the adjusted endemism rate, it seems to hold the greatest biodiversity of any ancient lake. Of all the animal groups endemic to this lake, gammarids form one of the largest species flocks. The goal of our study was (1) to develop the phylogenetic framework for the Ohridian endemic Gammarus species flock and place it within a regional palaeobiogeographical context, and (2) to interpret the data with respect to the putative origin of the Lake Ohrid fauna. Location Lake Ohrid, Balkan Peninsula. Methods Sequences of two mitochondrial genes (cytochrome c oxidase subunit I and 16S ribosomal RNA) and one nuclear gene (28S ribosomal RNA) of the Ohridian endemic Gammarus species and of other Balkan gammarids from 69 localities were analysed. The phylogenetic relationships of the group were studied using Bayesian methods. The biogeographical history and the chronology of diversification events were investigated using a relaxed molecular clock with two calibration points. Results It appears that the Lake Ohrid gammarids derive from an old local lineage of the Gammarus balcanicus complex. This lineage arrived from the northern part of the Proto-Balkans, after its emergence during the Tethys/Paratethys regression, and differentiated during the last 18 Myr of the Dinaric uplift (Alpine orogeny). It is also closely affiliated with the biota endemic to the neighbouring area of the former Miocene lake system and to lineages from the upper Vardar and Crni Drim river systems. Main conclusions The Lake Ohrid endemic fauna is closely affiliated to the local biota and its roots are more ancient than the lake itself.