Adaptive Evolution Of Mammalian Viral TLRs Gene And Functional Verification Of Cetacean TLR8

Mammals are the highest form of vertebrates and hold a dominant position on the earth. Most of them are with hair, constant temperature and a variety of feeding habits. The living environments of mammals are various including terrene, ocean, sky and cave. Due to the vast and diversity of mammals, some special groups of them are bound to happen significantly different adaptive change. Innate immunity, as a system that most animals use against pathogenic microorganism invasion, is relatively old and the body’s first line of defense against pathogenic microorganisms. Toll-like receptors (TLRs) belong to an old family which mediates innate immunity, divided into viral TLRs and non-viral TLRs. Among them, TLR3, TLR7, TLR8 and TLR9 belong to viral TLRs. They are located on the membranes of endosomes, and identify the viral nucleic acids. Because of the great differences in living environments and feeding habits of mammals, the viruses’types are also different. However, the research on the evolution mechanism of viral TLRs is generally considered to be conservative. To explore the possible molecular mechanism of the evolution of mammals’viral TLRs, TLR3, TLR7, TLR8 and TLR9 were chosen for evolutionary analysis in 27 representative mammals.The selective test implemented with Site Model, Branchsite Model in PAML and SLAC, FEL, REL in Datamonkey discovers a great number of positive selected sites in the four viral TLRs of mammals and the most in TLR9. Most of these positive selected sites are located in the LRR domain in the extracellular domain. LRR domain is the region which direct combine with PAMPs, and has been under strong positive selection. Through the analysis of the Branchsite Model and free-ratio model, it is showed that viral TLRs has been under strong positive selected pressures in cetaceans and Chiroptera. This indicates that the types and number of viruses of cetaceans and Chiroptera have changed due to the marine environment and the sky, caves environment in the long process of evolution, and the selected pressure of their viral TLRs has changed, too. We also find positive selected signals in the other branches such as Cetartiodactyla and Primates. It is showed the mammalian viral TLRs have been under universal positive selection. We detected the most common positive selected signals in TLR9, suggesting that it may be the only TLR to identify the viral DNA. This may be result of long-term co-evolution with viruses.In addition, functional verification has been carried on bottlenose dolphin TLR8 gene on the cell level in this study. The results showed that bottlenose dolphin TLR8 is a functional gene which can be activated by R848 but is not as sensitive as bovine TLR8. This may be due to cetaceans in order to adapt to the marine environment, the innate immunity gene TLR8 has occurred adaptive evolution.