Adapative Evolution Of TLRs In Cetaceans

Cetaceans including dolphins, porpoises and whales are a kind of fully adapted to aquatic life of marine mammals, with special evolution history. About 50 million years ago, Cetaceans, evolved from terrestrial mammals, have evolved a series of adaptations back to aquatic environment. Cetaceans evolved rapidly to occupy nearly all oceans of the world, some even to the freshwater environment. During the habitat conservation, cetaceans must have been faced to unprecedented challenge from ever-changing environment pathogens. Innate immune widely exsit in invertebrates and vertebrates, lie on the body’s first line of defense. Identification of pathogenic microorganisms’ specific conservative structure of pathogen associated molecular patterns (PAMPs) by pattern recognition receptors (PRRs). However, innate immune genetic mechanism of cetaceans is not clear in the course of the secondary adaption to aquatic environment. TLRs is an important component of the innate immune, essential for activating the adaptive immune system, is the bridge of innate immunity and acquired immunity. This research based on TLRs gene to discuss:1) whether cetaceans TLRs have been undergone adaptive change during the process of secondary back to aquatic environment? 2) If different types of TLRs such as viral TLRs and Non-viral TLRs show different evolutionary patterns? 3) If different selections acting on various branches and what formed this pattern?This study measured 10 TLRs gene family members (TLR1-10) of the 16 cetaceans represent species, compared with other terrestrial mammal homologous sequence, bioinformatics analysis showed that generally positive selection acted on cetaceans TLRs in the whole evolution course, particularly in three stages:ancestors cetaceans shifted from land to marine environment and dolphin family fast differentiation radiation adaptation. Results suggested that adaptation of innate immune system happened when the habitat of cetaceans shifted from land to the oceans. In addition, our research suggested that different types of TLRs such as viral TLRs and Non-viral TLRs under different intensity of selection pressure. It imply that specific amino acid sites change of each TLRs gene locus generate some new function to recognize and resist pathogenic microorganisms in different environments. This provide the possible of cetaceans survived in the new environment.It’s interesting to note that TLR5 is pseudogene in four cetaceans species, we speculated that selection pressure relaxed in cetaceans may due to functional redundancy of TLR5.