Zoological Studies

Phylogenetic Analyses of MHC Class II Genes in Bottlenose Dolphins and Their Terrestrial Relatives Reveal Pathogen-Driven Directional Selection

Wei-Cheng Yang, Jer-Ming Hu, and Lien-Siang Chou (2010) The mammalian order Cetacea is believed to have made the challenging evolutionary transition from land to sea early in the Eocene, > 50 million years ago (Mya). With the shift in habitat, cetaceans had to evolve a range of specializations, including immune response. The major histocompatibility complex (MHC) multigene families have proven to be ideal candidates for research on the timing of selection and the spatial heterogeneity of selection pressures experienced by the ancestors of cetaceans because MHC multigene families consist of a mixture of divergent genes and play key roles in immune responses. Herein, we report the evolutionary relationship and estimates of divergence times of cetartiodactyls using MHC class II β-chain genes. In both the DQB and DRB phylogenies, cetaceans (Tursiops truncatus and T. aduncus) and artiodactyls (pig, hippo, and ruminants) formed 2 distinct clades, and the estimated divergence time was about 60 Mya or slightly earlier. Furthermore, our results showed that T. truncatus and T. aduncus diverged about 24 Mya, which greatly predates the emergence of the oldest dolphin (approximately 11 Mya). These findings are explained by postulating gene duplications in MHC genes and pathogen-driven directional selection during cetacean evolution. This research provides new information for studying the dynamics of MHC-pathogen co-evolution.