Multiple fossil calibrations, nuclear loci and mitochondrial genomes provide new insight into biogeography and divergence timing for true seals (Phocidae, Pinnipedia)

Aim To better understand the historical biogeography of the true seals, Phocidae, by combining nuclear DNA (nDNA) and mitochondrial DNA (mtDNA) in a divergence time analysis using multiple fossil calibrations.

Location Arctic, Antarctic, Pacific and Atlantic Oceans, Lake Baikal, Caspian Sea.

Methods Fifteen nuclear genes totalling 8935 bp plus near-complete mitochondrial genome sequences were used in a Bayesian divergence time analysis, incorporating eight soft-bound fossil calibrations across the phylogeny. All species of true seals were included, plus the walrus, three otariids and seven carnivore outgroups. The majority of the nuclear sequences and four phocid mitochondrial genomes (plus three non-phocid mitochondrial genomes) were newly generated for this study using DNA extracted from tissue samples; other sequences were obtained from GenBank.

Results Using multiple nuclear genes and multiple fossil calibrations resulted in most divergence time estimations within Phocidae being much more recent than predicted by other molecular studies incorporating only mtDNA and using a single calibration point. A new phylogenetic hypothesis was recovered for the Antarctic seals.

Main conclusions Incorporating multiple nuclear genes and fossil calibrations had a profound effect on the estimated divergence times. Most estimated divergences within Phocinae (Arctic seals) correspond to Arctic oceanic events and all occur within the last 12 Myr, a time when the Arctic and Atlantic oceans were freely exchanging and perennial Arctic sea ice existed, indicating that the Arctic seals may have had a longer association with ice than previously thought. The Monachinae (‘southern’ seals) split from the Phocinae c. 15 Ma on the eastern US coast. Several early trans-Atlantic dispersals possibly occurred, leaving no living descendants, as divergence estimates suggest that the Monachus (monk seal) species divergences occurred in the western Atlantic c. 6 Ma, with the Mediterranean monk seal ancestor dispersing afterwards. The tribes Lobodontini (Antarctic seals) and Miroungini (elephant seals) are also estimated to have diverged in the eastern Atlantic c. 7 Ma and a single Lobodontini dispersal to Antarctica occurred shortly afterwards. Many of the newly estimated dates are used to infer how extinct lineages/taxa are allied with their living relatives.