Molecular Mechanisms Of The Adaptive Evolution Of Flippers In Marine Mammals And Polyphalanges In Cetaceans

The emergence of five-toe-appendage was an important innovation in vertebrate evolution,which solved the difficulty about supporting body and movement on land,and provided conditions for vertebrates to adapt to land environment.Cetaceans and manatee degenerated hind limbs and specialized forelimbs as soft-tissue flippers that five fingers are completely connected forming webbing digit with complete aquatic life.In contrast,pinnipeds possessed both fore and hind flipper in order to locomote both on land and in water for amphibious living habits.It was noted that cetaceans were unique to evolve hyperphalangy with increased numbers of phalanges,which was not found in other mammals.The hyperphalangy are conducive to form different shape of flipper,and may smooth leading edge flipper contour to distribute force in the flipper.However,the genetic base of flipper in marine mammals and hyperphalangy in cetaceans remain poorly explored.In the present study,the evolution of the 11 limb development related genes were investigated in marine mammals using bioinformatics and comparative genomics.First,we tested whether different evolution pattern acted on these appendage development related genes in different marine mammal groups,and whether unique evolution pattern occurred in cetaceans.Second,we examined whether convergent molecular evolution for the flipper in marine mammals.A total of 10 representative species of marine mammals and 18 terrestrial mammals were analyzed used in our study.The coding sequences were obtained by download from database and performed BLAST program.Five maximum likelihood methods(ML),including branchsite model,site model,FEL(fixed effects likelihood),REL(random effects likelihood),FUBAR(fast unconstrained bayesian approximation),were used in the selective pressure analysis.Our result showed that positively selected genes are limited to cetaceans lineages based on all mammal dataset using branchsite model.A total of 13 condons from 5 genes(Shh,Ptch1,Smo,Gli3 and Lmbr1)were detected to be under positive selection using 5 ML methods.Further,10 positive sites were detected by 2 ML methods at least and nine sites(90%)were changed with radical amino acid properties.Previous studies of developmental biology have confirmed that Shh protein can induce the formation of extra phalange in the bud of chicken embryo,and the mutations of Ptch1,Smo,Gli3 and Lmbr1 genes are related to the phenotype of polydactyly.Therefore,the positively selected genes and mutations in specific amino acid sites of cetaceans might be associated with the formation hyperphalangy.The evolution rate of Lmbr1 and Ptch1 in marine mammals was about two times of other mammals with accelerated evolution.Studies have confirmed that both Ptch1 and Lmbr1 mutations cause a webbed digit in the toes of mice.Therefore,the accelerated evolution of these two genes in marine mammals may be related to the formation of flippers.And a total of fifteen parallel non-synonymous amino acid substitutions were detected in Fgf10,Gli3,Ptch1 genes among marine mammals.The mutation of these genes were related to the disease of polydactyly or syndactyly(webbing digit)in human or mice,which was similar to flipper of marine mammals.These parallel/convergent sites still provided molecular evidence for the phenotypes of marine mammal flippers.Our result revealed the molecular convergent mechanism of marine mammal flipper and cetaceans hyperphalangy based on the analysis of SHH signal pathway and limb development related genes,and provided new insights into secondary adaptation to aquatic environment of cetaceans and even marine mammals.