Research On The Adaptive Evolution Of Milk Fat And Milk Protein-related Genes In Marine Mammals

Marine mammals is a kind of mammals that live in marine environment with specific evolution characteristics,such as viviparity,lactation,constant temperature,lung breath,streamlined body shape and flippers.The study of physiological and ecology of marine mammals showed its lactation patterns,milk composition and energy output is significantly different from terrestrial mammals.The milk fat and milk protein content of marine mammals is significantly higher than that of the terrestrial relatives.Its physiological significance mainly contains two aspects:1)the higher milk fat content is useful for the pups to form body fat rapidly,which can keep warm,insulate heat and improve survival rate,so as to adapt to the low temperature underwater;2)thick and rich nutrition of the milk is the result of adaption to the breastfeeding difficulty in water to improve the survival rate.However,its adaptive molecular mechanism remains unclear.Here,we comprehensively investigated six mlik fat genes(ACACA,FASN,SCD,DGAT1,LPL and ATGL)and two milk protein genes(CSN2 and CSN3)of 11 representative species of cetaceans and 3 species of pinnipeds along with other terrestrial mammals to discuss the following two scientific questions:1)whether adaptive evolution occurs on the milk fat and protein genes across marine mammals of cetaceans and pinnipeds phylogeny?2)whether the milk fat and milk protein genes of cetaceans and pinnipeds have convergent(or parallel)evolution at the molecular level?The selective test implemented in PAML showed significant evidence of positive selection acting on four milk fat genes(ACACA,ATGL,DGAT1 and LPL)across cetaceans lineage and two milk protein genes(CSN2 and CSN3)across both cetaceans and pinnipeds.Among them,gene ACACA and DGAT1 are separately the speed limit enzymes of fatty acid(FA)and triglyceride(TAG)in the biosynthesis pathway.The evidence for positive selection of ACACA and DGAT1 suggested that the adaptive evolution of both genes has helped to strengthen the synthesis ability of FA and TAG to improve cetacean milk fat and milk protein content,thus,which can meet the needs of energy for cetaceans pups.Milk protein genes CSN2 and CSN3 are the key enzyme for the synthesis of casein which contains all the essential amino acids that is the most nutritional protein for newborn pups.Thus,the two milk genes identified to be under positive selection across both cetaceans and pinnipeds suggested marine mammals enhance the synthesis ability of milk protein in order to promote the fast growth of infants and increase the survival rate in the marine environment.In addition,we detected 58 positive selected sites in six milk genes using four kinds of Maximum Likelihood(ML)methods,of which 24 sites were picked out by two ML methods.Furthermore,TreeSAAP analysis showed the overwhelming majority of these positive sites were identified as having undergone radical changes.More importantly,radical amino acid changes in positively selected sites were localized in,or close to,the functional region in the predicted 3D structures.This further indicated the milk fat and milk protein genes of marine mammals had evolved adaptive function change.Interestingly,we identified 4 parallel and 2 convergent amino acids sites in milk fat genes(ATGL and FASN)and milk protein gene CSN2 in both cetaceans and pinnipeds,two mammalian groups highly diverged or distantly related with each other.However,the both groups showed similar lactation patterns and lactation strategies.Hence,convergent sites identified in both groups suggested that convergent evolution might have occurred in cetaceans and pinnipeds in order to adapt to the marine environment.This is the first comprehensive analysis of the molecular evolution of milk fat and milk protein genes in marine mammals.Our analysis revealed that marine mammals may improve its milk fat and milk protein content by strengthening the synthesis of milk fat and milk protein,which could meet the needs of energy for marine mammalian pups.In addition,cetaceans and pinnipeds evolved convergent molecular evolution of milk fat and milk protein genes because of the adaption to the similar lactation patterns and lactation strategies.