Use of multiple markers demonstrates a cryptic western refugium and postglacial colonisation routes of Atlantic salmon (Salmo salar L.) in northwest Europe

Glacial and post-glacial processes are known to be important determinants of contemporary population structuring for many species. In Europe, refugia in the Italian, Balkan and Iberian peninsulas are believed to be the main sources of species colonising northern Europe after the glacial retreat, however, there is increasing evidence of small, cryptic refugia existing north of these for many cold-tolerant species. This study examined the glacial history of Atlantic salmon in Western Europe using two independent classes of molecular markers, nuclear microsatellites and mitochondrial DNA variation. Alongside the well-documented refuge in the Iberian Peninsula, evidence for a cryptic refuge in northwest France is also presented. Critically, methods utilised to estimate divergence times between the refugia indicated that salmon in these two regions had diverged a long time before the last glacial maximum; coalescence analysis (as implemented in the program IMa2) estimated divergence times at...