Phylogenetic relationships among cetaceans revealed by Y-chromosome sequences

The Y chromosome has recently come into the spotlight as a new and efficient genetic marker for tracing paternal lineages. We reconstructed cetacean phylogeny using a 1.7-kbp fragment of the non-recombining Y chromosome (NRY), including the SRY gene and a flanking non-coding region. The topology of the Y-chromosome tree is robust to various methods of analysis and exhibits high branch-support values, possibly due to the absence of recombination, small effective population size, and low homoplasy. The Y-chromosome tree indicates monophyly of each suborder, Mysticeti and Odontoceti, with high branch support values (BS> or =86%; PP> or =98%). In the Odontoceti clade, three superfamilies, Physeteroidea, Ziphioidea, and Delphinoidea, diverged soon after the split between Mysticeti and Odontoceti. Our analysis allows resolution of this rapid radiation and indicates that Physeteroidea is basal in the Odontoceti clade (BS, 99%; PP, 100%; MBS, 61%). The major split within the superfamily Delphinoidea is between the Delphinidae clade and the Monodontidae+ Phocoenidae clade. The phylogenetic relationships among delphinid species are ambiguous, probably because of the rapid radiation of this family. In the Mysticeti clade, the first major split is between Balaenidae and Balaenopteridae; within Balaenopteridae, a Balaenoptera acutorostrata+B. bonaerensis (minke whales) clade forms a sister clade with the other balaenopterid species. Megaptera novaeangliae is nested within Balaenoptera, making the latter paraphyletic. The low homoplasy exhibited by the Y-chromosome data presented here suggests that an extended data set incorporating longer sequences would provide better resolution of cetacean lower-level pylogeny.