The mitochondrial genome of the sperm whale and a new molecular reference for estimating eutherian divergence dates

Extant cetaceans are systematically divided into two suborders: Mysticeti ( baleen whales) and Odontoceti (toothed whales). Tn this study, we have sequ enced the complete mitochondrial (mt) genome of an odontocete, the sperm wh ale (Physeter macrocephalus), and included it in phylogenetic analyses toge ther with the previously sequenced complete mtDNAs of two mysticetes (the f in and blue whales) and a number of other mammals, including five artiodact yls (the hippopotamus, cow. sheep, alpaca, and pig). The most strongly supp orted cetartiodactyl relationship was: outgroup,((pig, alpaca),((cow, sheep ),(hippopotamus,(sperm whale,(baleen whales))))). As in previous analyses o f complete mtDNAs, the sister-group relationship between the hippopotamus a nd the whales received strong support, making both Artiodactyla and Suiform es (pigs, peccaries, and hippopotamuses) paraphyletic. In addition, the ana lyses identified a sister-group relationship between Suina (the pig) and Ty lopoda (the alpaca), although this relationship was not strongly supported. The paleontological records of both mysticetes and odontocetes extend into the Oligocene, suggesting that the mysticete and odontocete lineages diver ged 32-34 million years before present (MYBP). Use of this divergence date and the complete mtDNAs of the sperm whale and the two baleen whales allowe d the establishment of a new molecular reference, O/M-33, for dating other eutherian divergences. There was a general consistency between O/M-33 and t he two previously established eutherian references, A/C-60 and E/R-50. Ceta cean (whale) origin, i.e., the divergence between the hippopotamus and the cetaceans, was dated to approximate to 55 MYBP, while basal artiodactyl div ergences were dated to greater than or equal to 65 MYBP. Molecular estimate s of Tertiary eutherian divergences were consistent with the fossil record.