| The quantity and kinds of the amphibians were in sharp decrease with the deterioration of ecological environment, especially for the reason that the pathogenic microorganisms existed widely and expand in a wide region around the world.The degree of sensitivity to microorganisms depended on immune system itself. Studies on vertebrates had revealed that MHC molecules in process of immune defense played an important role that they could presenting pathogen, that had laid the foundation of further exploration of MHC molecules polymorphism and genetic diversity of species. In addition, we can provide reliable basis of molecular biology for the research of disease resistance if MHC gene polymorphisms and it’s maintaining mechanism are all clear.Vibrissaphora boringii and Vibrissaphora leishanensis are the two main research objects in this paper. The individual number of the two species in this research is28. Molecular evolution analysis for sequences of the MHC I gene exon3found that nucleotide and amino acid polymorphism levels both are very high in two kinds of mustache toad. Among individuals the most alleles is6, that is to say, each contains at least three function sites. In the antigen-binding areas, the non-synonymous replacement frequency of nucleotides in two species are higher than the synonymy replacement frequency, indicating that balancing selection plays an important role in MHC evolution process. Also recombination events are not detected respectively in these two species, and the evolutionary tree can intuitively show there exists trans-species polymorphisms. Studies have shown that MHC gene polymorphism are relatively high in many animals,such like mammals-, birds and amphibians.The number of alleles in some fish may be up to10.The function sites of MHC gene in Xenopus laevis and Bombina orientalis are only1or2.In addition to selection and trans-species polymorphisms, recombination also is very critical in MHC I gene polymorphism maintenance of the salmonidae fish. |
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