Sequence Variation of Mitochondrial 12S rRNA Gene and Inferred Molecular Systematics of River Dolphins

Partial DNA sequences of mitochondrial 12S rRNA gene from Lipotes vexillifer,Platanista gangetica,and Pontoporia blainvillei,were amplified and determined to address the systematics of river dolphins.The sequence divergence among different genera of river dolphins,estimated either by transition+transversion or transversion only,is greater than those among other odontocete families,far greater than those among families Delphinidae,Monodontidae,and Phocoenidae within superfamily Delphinoidea.Thus,the river dolphin genera should be referred into different families,I.e.Lipotidae,Platanistidae,Iniidae,and Pontoporiidae.The phylogenetic analysis by using neighbor-joining method (NJ) and maximum parsimony method (MP) suggested that the Platanistidae was the earliest divergent clade in the cetaceans,whereas the other three river dolphin families constituted a monophyletic group having sister relationship with Delphinoidea.The phylogeny of river dolphins should be polyphyletic,rather than monophyletic or paraphyletic.The Platanistidae should be referred into a separated superfamily Platanistoidea,whereas the superfamily classification of other three river dolphin families should be further studied.