Balkan refugium;Cyprinidae;Mediterranean endemism;Ponto-Caspian region;Gene flow;Genetic drift

South-east Europe, along with the adjacent region of south-west Asia, is an important biodiversity hotspot with high local endemism largely contributed by contemporary continental lineages that retreated to southern refugia during colder Quaternary periods. We investigated the genetic diversity of the European bitterling fish (Rhodeus amarus) species complex (Cyprinidae) across its range in the western Palearctic, but with a particular emphasis in the region of Balkan, Pontic and Caspian refugia. We genotyped 12 polymorphic microsatellite loci and a partial sequence of mitochondrial gene cytochrome b (CYTB) for a set of 1,038 individuals from 60 populations. We used mtDNA sequences to infer phylogenetic relationships and historical demography, and microsatellite markers to describe fine-scale genetic variability and structure. Our mtDNA analysis revealed six well-supported lineages, with limited local co-occurrence. Two lineages are distributed throughout central and western Europe (lineages “A” and “B”), with two zones of secondary contact. Another two lineages were restricted to the Ponto-Aegean region of Greece (lineages “C” and “D”) and the final two lineages were restricted south of the Caucasus mountains (lineage “E” from the Black Sea watershed and lineage “F” from the Caspian watershed). A signal of recent expansion was revealed in the two widespread lineages and the Ponto-Aegean lineage “C”. The geographic distribution of clusters detected by nuclear microsatellites corresponded well with mitochondrial lineages and demonstrated finely sub-structured populations. A profound population structure suggested a significant role of genetic drift in differentiation among lineages. Lineage divergence in the Ponto-Aegean and Caspian regions are substantial, supporting the validity of two described endemic species (Rhodeus meridionalis as lineage “D” and Rhodeus colchicus as lineage “E”) and invite taxonomic evaluation of the other two southern lineages ...