Adaptive Evolution Of Corms MCPH1, ASPM, CENPJ And STIL And Their Relationship With Increased Brain Volume

The unusually enlarged brain mass is one of the remarkable feature of modern cetaceans. Odontocete, especially dolphin, have the EQ higher than any of the nonhuman anthropoid primates and are second only to those of modern humans. Fossil evidence shows that brain size increased significantly in two critical phases in the evolution of cetaceans:the first increase occurred with the origin of odontocetes and the second in the origin of Delphinoidea. Although it allows cetaceans are convergence with primates in neuroanatomy, cognitive competence, social behaviour and self-awareness, the possible evolutionary mechanisms responsible for cetacean brain enlargement still need more efforts. To explore the possible genetic mechanism of cetacean brain development, microcephaly (MCPH) genes were selected as candidate genes for evolutionary analysis.In this study, we comprehensive investigated four microcephaly loci of 14 representative cetaceans along with other mammals. First, we detect whether these genes are under positive selection among cetacean lineage and whether the selections are associated with twice enlargement events. Second, as the primates also evolved large brain size, we test whether cetacean and primate have suffered the similar selection pressure and involved convergent evolution. Finally, we explore whether or not the molecular evolution rate of microcephaly genes are association with the brain size or EQ of cetacean.The results show that four microcephaly genes were positive selected, the selection of ASPM and STIL were consistent with twice steep extremely enlargement events, and the selection of CENPJ was associated with the second enlargement events in cetacean. However, the selection of MCPH 1 was also detected in mysticeti, whose brain didn’t increase. It is of particular interest to find that cetaceans and primates subjected similar selective pressure and positive selection of these genes was restricted to cetaceans and primates, who are distant lineages but characterized by the expansion of brain size. The linear-regression analysis shown that the evolution of ASPM and STIL showed a significant positive correlation with EQ which indicated the microcephaly genes play an important function in brain development. Our result of this study strongly implicates the important role of ASPM, CENPJ and STIL genes in cetacean brain development.