Polymorphism And Evolution Of MHC Class Ⅰ Genes Of Rhscophorus Omeimontis And Rhscophorus Megacephalus

The major histocompatibility complex (MHC) is a kind of cell-surface glycoproteins and play an important role in immune system. MHC molecules encoded by gene of the MHC can bind antigens derived from pathogens or parasite and present them to T-lymphocytes which trigger the appropriate immune response. Based on the key role of MHC in dealing with pathogens, individuals of great polymorphism at MHC genes possibly have high fitness which is expected to confer immunity against a wide range of pathogens. Informed research have proved that MHC gene has high genetic diversity, and balancing selection derived from pathogens probably play an important role in maintaining it.The amphibian adapted to live in water and land are very vulnerable to infected by the pathogen due to their thin and soft skin was exposed to the environment for a long time. Recently, it reported that infection is the main agent lead to sharp decline of number and taxon in amphibian. Given that MHC is extremely important in resistance to the pathogens, there have been certain theoretical and practical significance in studying the evolution mechanism of MHC in amphibians. Therefore, we interpret the evolution mechanism of MHC by analysing MHC polymorphism characteristic in Rhscophorus omeimontis and Rhscophorus megacephalus.This research designed a pair of degenerate primers to investigated an operation Rhscophorus omeimontis by RT-PCR, cloning,sequencing and homology retrieval on BLAST. After determined the amplification sequence is the MHC Ⅰ of Rhscophorus omeimontis, we finished follow-up experiment of26Rhscophorus omeimontis population and11Rhscophorus megacephalus population. According to the sequence alignment, we detected21alleles in27Rhscophorus omeimontis population and7alleles in11Rhscophorus megacephalus population, and the number of alleles per individual can reach to five in both populations, indicating that at least three functional loci exist. The level of nucleotide diversity was0.145and0.127(Xenopus laevis:0.089) respectively, and the variable sites in both populations was46.72%and34.62%(Xenopus laevis:26.76%). Compared with great polymorphism at MHC Ⅰ genes in Xenopus laevis, the alleles of the MHC class Ⅰ have high polymorphism in two fogs.Constructing Phylogenetic Tree with several species’ alleles, it is found that Rhscophorus omeimontis’s and Rhscophorus megacephalus’s did not form monophyly, but mix together, implied the existence of trans-species polymorphism. Furthermore, The greater corrected divergence among amino acid sequences suggests that nonsynonymous substitutions are more frequent than synonymous changes, and we detected the non-synonymous substitution (dN) is significantly higher than the synonymous substitution (ds) in the PBR region. Based on maximum likelihood ratio of random site model in PAML to analysis positive selection, sixteen sites were significantly positively selected under the model M2a and M8in Rhscophorus omeimontis and Rhscophorus megacephalus, as expected for MHC alleles that are currently or have evolved under positive selection. In the same time, recombination events are also detected. Our results indicate that, in the populatin of Rhscophorus omeimontis and Rhscophorus megacephalus, Trans-species, balancing selection and recombination played critical roles in maintaining the diversity of MHC class I gene, which also providing theoretic meaning for explaining the mechanism of maintaining MHC variation in amphibian populations.