Genetic Basis Of Cetacean Dietary Shifts

Fossil evidence suggests that cetaceans evolve from artiodactyla which feed on vegetarian,thus,there is a major dietary change from herbivorous to carnivorous accompanied by cetaceans from terrestrial into aquatic environment.However,the molecular mechanism of dietary switch is not fully understood.Thus,16 digestion-related genes were selected,including five protease family genes(Pepsin,Carboxypeptidase,Trypsin,Chymotrypsin,Elastase),two H+,K+-ATPase genes(ATP4A,ATP4B),and four lipase genes(PLRP2,CEL,CLPS,PLA2G1B)and 1 gastrointestinal hormone Gene(GAST)to compare with other mammalian homologous sequences,respectively.First of all,by reconstructing the phylogenetic tree of the above genes,we will explore the evolutionary history of these genes.Then,we will explore whether these digestion-related genes were suffered from selected pressures(positively,negatively or neutrally)in the process of cetacean adaptation to aquatic life.In addition,the analysis of cetacean gene family's evolutionary patterns during the adaption of dietary switch was performed.Finally,by reconstructing ancestral sequences,it was determined whether cetaceans had convergent amino acid replacement sites compared with carnivores.We found that for the four pancreatic proteinase families(including carboxypeptidase,trypsin,chymotrypsin and elastase),each family included a functional gene and further selective pressure analysis showed that PRSS1 of trypsin,CTRC of chymotrypsin and CELA3B of elastase were subject to an intensely positive selection in cetaceans,other genes in the family had the presence of pseudogenes in cetaceans.Thus,it suggested that for the four pancreatic proteinase families,in order to ensure effective hydrolysis of dietary protein accompanied by ingesting more protein in cetaceans,family genes retain one key gene which has evolved a stronger ability to digest dietary protein.To the contrary,other genes in the family have become pseudogenes to save energy and resources.Then,among the genes related to digestive dietary protein in the stomach.PGA was under positive selection.It indicated that although cetaceans still retain a multiple chambered stomach which is common in artiodactyl,the gastric PGA gene appears to produce adaptive evolution in response to the dietary switch.In addition,lipase genes of PLRP2,LIPF,PNLIP and CYP7A1 also detected intensely positive selection in cetaceans according to previous and this study.We speculated that cetaceans might evolve a complex and effective mechanism to respond to high-fat food.Finally,10 of 11 functional genes found specific parallel/convergent amino acid substitution between the cetaceans and carnivores.It suggested that the cetaceans might have produced similarly evolutionary mechanism with carnivores at the molecular level in response to animal foods.In summary,this study reveals the molecular mechanism of dietary changes of cetaceans in their origin and differentiation and provides important informattion for revealing the evolutionary rules of digestive enzymes in mammals.