Polymorphism And Evolutionary Mechanisms Of MHC Class Ⅰ Genes In The Caudata Amphibians

Major histocompatibility complex (MHC) genes are a multigene family widespread in vertebrate, and they are central to immune system, the products are cell surface glycoproteins that can recognize and present peptides to T lymphocytes, and then induce immune response. The high level of MHC gene polymorphism is associated with its pathogens-defense ability. Therefore, studies on the polymorphism and evolutionary mechanisms of MHC genes can reveal the evolution of adaptive immunity. Current studies on MHC evolution are mainly conducted in mammals, birds, and fish. As the first terrestrial vertebrates, amphibians live in amphibious environment, which make them contact diverse microorganisms; thus, it is very important to study the evolution of MHC genes in amphibians. However, related studies on MHC class I genes have been restricted to model species (Xenopus lavies) and a few other ampibians. A systematic knowledge of MHC evolution in amphibians, especially in caudate is imperative. Here, we selected five caudate species (family Salamandridae:Tylototriton verrucoosus, Pachytriton labiatus; family Hynobiidae:Pachyhynobius shangchengensis, Ranodon shihi; family Cryptobrachidae:Andrias davidianus) to explore the polymorphisms and ecolutionary mechanisms of caudate MHC class I genes. Main results are as below:(1) We isolated 1,100 sequences (about 730 bp) from the five species and identified 26 MHC class I alleles, which contained partial exon 2 and exon 4 and the entire region of exon 3. The nucleotide diverstity was lower than Anuran amphibians. Considering that different species living in diverse habitats, we deduced that the adaptability of amphibians to environment was relevant with the MHC gene diversity.(2) The number of MHC class I alleles varied in five species, these five species possessed at least 1-2 MHC class la loci. The number variation of loci across caudate numbers across caudate species may result from the occurrence of gene duplication or loss in the process of MHC evolution. The birth and death model could be used to explain this pattern of evolution.(3) MHC class I genes in the five species displayed higher amino-acid divergence compared to nucleotide divergence, implying the occurrence of positive selection. The ratio of non-synonymous substitutions in putative antigen-binding sites (ABS) was greater than 1. Moreover, four codon-based methods have detected some amino acids under positive selection, and the deduced sites mostly located in the ABS and the vicinity. All the results mentioned above indicated that positive selection occurred in the ABS regions. Considering the function of ABS, we deduced that the MHC class I genes were driven by pathogen-mediated balancing selection.(4) We identified several breakpoints locating near the boundary of exon 2 and exon 3, implying the exchange of entire exons during gene recombination. This recombination pattern, exon shuffling, was similar with that occurred in bony fishes and anurans. We concluded that recombination acted an important role during the evolution of MHC class I genes in caudata.(5) The phylogenetic relationships of MHC class I alleles varied in different regions across five species, and it may be relevant with the gene recombination occurring at the boundary of exons. The trans-species polymorphisms in MHC class I alleles of Pachytriton labiatus and Andrias davidianus implied that MHC class I genes as old genes existed before the two families differentiated.In conclusion, we found that MHC class I genes of five caudata species had high level of polymorphisms. Gene duplication, gene recombination, and balancing selection were all important mechanisms to maintain the high polymorphisms.