Molecular Evolution Of Amphibian TLR Gene Family

The immune system is the latest physiological system in the history of biological evolution.It is a set of defense systems that can be identified and resisted by foreign pathogens during the long-term evolution of the organism.It is important for the species to cope with environmental external sources and to broden the niche as well as to ensure the daptive evolution on top.The innate immune system is a universal,and ancient defense mechanism during evoluiton.It is the first line of defense against external pathogenic microorganisms.In particular,toll-like receptors play an important role in innate immune responses.It also plays an important part in identifying the invariant and unique molecular characteristics of invasive microorganisms and activating the adaptive immune system.Amphibians are the "bridge" group from aquatic to terrestrial vertebrate evolution.They need to undergo the metamorphosis developmental process of larval aquatic to adult landscaping.At the same time,they face a more complicated dual pathogen environment both aquatic and terrestrial,and their pathogen identification.The pathogen identification defense and immune response systems will inevitable to face great challenges.Based on genomic and transcriptome data this study takes amphibian species as research objects,combining with molecular ecology and bioinformatics,to discuss the molecular evolution mechanism of amphibian TLR innate immune receptor gene family.Through the NCBI public database,401 TLR gene sequences were collected from amphibians genome and transcriptome data.Analyzing all TLR gene sequences identified,mimicking protein hydrophobicity,predicting protein three-dimensional structure,phylogeny and the selection of stress.This study first predicted the three-dimensional structure of TLR protein of tropical xenopus laevis and found that all three-dimensional structures of TLR protein contain a three-dimensional horseshoe-shaped structure,while TLR5 L protein lacks an intracellular domain that mediates downstream signal transduction.Through the prediction of the three-dimensional structure of the TLR5 S protein of bony fish,it is found that TLR5 S also lacks the intracellular domain,suggesting that the origin of the amphibian TLR5 L gene may be directly related to the fish's TLR5 S gene.The vertebrate Toll-like receptor gene family is reclassified into eight subfamilies by phylogenetic analysis.The amphibian Toll-like receptor gene family is divided into seven subfamilies.It also proves that the composition of the Toll-like receptor gene family of amphibians is between fish and amniotic animals,indicating that the composition of the TLR gene family of anura and urodela is roughly similar,while the TLR4 gene is only identified in the anura.Nothingis found in the urodela,suggesting that the TLR4 gene was lost in the common anceser of the urodela.Through the point model in PAML and the FEL and REL models in Datamonkey,it is found that all TLR genes are detected positive selection sites at both the klasse level and the order level.Through the study of viral TLRs and non-viral TLRs,the ? values of both are significantly different,indicating that the viral TLRs suffers more strongly selection pressure than the non-viral TLRs,which is correlated to functional redundancy of non-viral TLRs.At the level of the class or the level of the order,the viral TLR7 gene has the smallest ? value and fewer positive selection sites,while the TLR8 gene,which is also a viral TLRs,is detected the most positive selection sites.It is related to the difference in the structure and function of the ligands recognized by the two.The number of positive selection sites detected in the anurais much more than that in the urodela,indicating that the TLR gene of the anura is affected more strongly by positive selection than that of the urodela,and the positive selection sites detected is significantly located extra cellular LRR region related to pathogen identification,indicating that the region in which the TLR gene affected by positive selection is mainly the antigen-binding region of the TLR gene protein.Through the analysis of the branch model at the order level,it is found that more species of the anurans are positively selected and select stress relaxation.Through the analysis of the branch point model,it is found that the positive selection effect is detected to different extents in anurans and urodelas.Further analysis reveals that the numbers of the urodelas TLR genes with positive selection sites are more than those of the anurans.The TLR19 gene has been strongly positively selected in amphibians of both anurans and urodelas,and the TLR19 gene may have an important role in the immune function of amphibians.