Diversity and evolution of MHC class II DRB gene in the Eurasian badger genus Meles (Mammalia: Mustelidae)

摘要

Major histocompatibility complex (MHC) genes encode proteins that play a key role in the adaptive immune system of vertebrates and are generally highly polymorphic for defence against various pathogens. To understand the diversity and evolution of MHC variation in badgers in the genus Meles (Carnivora, Mustelidae), we analysed sequence variation of the MHC class II DRB gene exon 2 in the Japanese (Meles anakuma), Asian (Meles leucurus), European (Meles meles) and Southwest Asian (Meles canescens) badgers. Variation was higher in the Meles species than in other species in Mustelidae, and altogether 60 alleles were isolated from 28 individuals. The variable number of three to eight putative alleles per individual was observed, indicating the presence of two to four DRB loci per haploid genome. Non-synonymous substitutions exceeded synonymous substitutions at putative antigen-binding sites. Selection analyses of PAML models, fixed-effect likelihood and mixed-effect model evolution, together with the single breakpoint recombination, indicated that recombination and selection could be responsible for driving and maintaining the diversity of Meles DRBs. In a phylogenetic analysis, the DRB sequences from Meles were distributed in several clusters, which were dispersed among sequences of other mustelid family, even five alleles comprised a monophyletic group of Meles DRBs within a canid clade. The data demonstrate trans-species polymorphisms at different taxonomic and temporal scales, transgressing family-, genus- and species-level splits. Some allele sequences were shared by two to four of the Meles species, in line with a close phylogenetic relationship among these species.