Adaptive Evolution Of Genes Related To Cetacean Body Size

Cetaceans,a group of secondarily adapted marine mammals that have evolved a fully aquatic lifestyle,approximately 53Mya,cetacean ancestors transformed from a fully terrestrial quadruped to an obligate aquatic habitat,which represents one of the most amazing events in mammalian evolutionary history.Extant cetaceans can be subdivided into two suborders(Odontoceti and Mysticeti),and they eahibited a particularly wide range of body size from under less than 2 meters in porpoises to over 30 meters in large whales.The largest living animal bule whale(Balaenoptera musculus)has approximately 30 meters in body length and 150 tons in body mass.By contrast,dolphins and porpoises are generally small,such as the vaquita(Phocoena sinus),the smallest cetaceans,with 1.4 meters in body length and 42 kilograms in body mass..However,the genetic bases of dramatic body size variation in cetaceans remain poorly explored.The present study examined the coding sequences of 20 candidate body size related genes and compared the ortholigous genes among different cetacean species with contrastedbody size to explore:1)whether adaptive evolution of these genes occurred during cetacean evolution;2)whether different selective pressures acting on small cetaceans or large whales;3)whether there is an association between evolutionary rate and morphological variables of cetacean body size.A total of 7 representative cetacean species and 6 related terrestrial mammals were used in our study.All the exons of 20 body size related genes were obtained by download and BLASTN algorithm.A toatal of 67 codons from 12 genes were detected to be under positive selection when we used five ML methods(PAML:site model,branch site model;Datamonkey:FEL,REL,FUBAR).Twenty-two sites(32.84%)was further found to be under radical changes when we employed a complementary protein-level approach implemented in TreeSAAP,which provides additional evidence for the positive selection on cetaceans.In addition,67 positively sites scattered throughout most of the cetacean phylogeny,suggesting that body size related genes appears to have undergone adaptive evolution across cetacean phylogeny.ACAN gene,encodes aggrecan,is essential for cartilage structure,mutation of this gene leading to dwarfism phenotype occurred in different species.The analysis of regression showed that a significant negative association between ACAN evolution and body length/mass.Furthermore,most of positively selected sites of this gene(88.89%)were focused on species of Odontocete,special for the lineage of the common ancestor of Odontoceti which are characterized by decreases in body mass from the ancestral crown cetacean.Thus,positive selection at ACAN was suggested to play a key role in restricting body size increase.Extensively positive selection was found in FBN1 gene across different cetacean species,suggesting that FBN1 plays a crucial role in driving cetaceans to evolve proper measures to fit the change of environment.It was noted that twenty-nine sites under positive selection were detected at FBN1 in three large whales.More than 1800 mutations have been identified in FBN1 that accounted for disease of Marfan syndrome(MFS),and two of them 1136 and 2262 were also identified that subjected to positive selection in sperm whale,suggesting that the two sites may exert function in increasing the body length of sperm whale.When we divided cetaceans into large whales and small cetaceans,we obsvered that co value estimated for small cetacean was nearly three times than that for large whale at growth suppressor(GRB10).Of course,we should further confirm the function of these positively selected sites or genes in cetaceans in future.In a word,the evolutionary changes in body size related genes provide new insights into the molecular basis of significant body size change in different cetacean species during its transition from land to water.