Mitochondrial DNA reveals historical maternal lineages and a postglacial expansion of the grey seal in European waters

Three different evolutionarily significant units (ESU) can be discerned in the grey seal Halichoerus grypus: in the northwest Atlantic, the northeast Atlantic and the Baltic Sea. The northeast Atlantic population has been strongly affected by hunting around the European mainland, where it is only in the last decades that breeding colonies have reappeared. The southern-most settlement of the northeast Atlantic ESU is found in the Molene archipelago, northwest France. We studied polymorphisms of the mitochondrial control region (MCR) in grey seals sampled along the Atlantic coast of France for over a decade. MCR polymorphisms highlighted the existence of 4 major MCR haplogroups. Comparison of MCR sequences with those in the GenBank database yielded highly significant differentiation between grey seals sampled in France and grey seals originating from the North Sea and the north and east of the British Isles, as well as from the Baltic Sea. MCR haplogroups were also identified in Baltic Sea grey seals, with some being in common with our samples, while others were not. The times of separation between the MCR haplogroups were estimated to be between 84 000 and 20 500 years ago, and differentiation between Baltic Sea and French coast haplotypes was estimated to have occurred approximately 8000 years ago. These data, added to positive expansion signals, suggest that MCR haplogroups correspond to ancestral maternal lineages, isolated in different grey seal refugia during the last ice age and prior to the expansion of the species in the North Atlantic and the separation between the East Atlantic and the Baltic Sea ESUs.