The enigma of the landlocked Baikal and Caspian seals addressed through phylogeny of phocine mitochondrial sequences

The endemic seals of Lake Baikal (Phoca sibirica) and of the Caspian Sea (Phoca caspica) inhabit ancient continental basins that have remained isolated from primary marine seal habitats for millions of years. The species have been united with the Arctic ringed seal, Phoca hispida, into (sub)genus Pusa, but the age and route of invasions to/from the continental basins remain controversial. A phylogenetic analysis of nine northern phocines based on three mitochondrial genes (Cytb, COI, COII, total 3369 bp) provided no support for the monophyly of the Pusa group. The three species are involved in an apparent polytomy with the boreal harbour seal, Phoca vitulina, and grey seal, Halichoerus grypus. From the average estimated interspecies divergence (4.1%), the radiation of this group plausibly took place in the Late Pliocene 2–3 Mya. This dating does not fit the prevailing hypotheses on the origin of the landlocked taxa in association with Middle Pleistocene glacial events, or of the Caspian seal as a direct descendant of Miocene fossil phocines of the continental Paratethyan basin. The current phocine diversity more likely results from marine radiations, and the continental seals invaded their basins through Plio-Pleistocene (marine) connections from the north. The palaeohydrography that would have enabled the invasions at that time still remains an enigma.