Search

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. [ABSTRACT FROM AUTHOR]

Mustela nivalis, the smallest member of the family Mustelidae, is widely distributed in the Holarctic region and shows high geographical variation. To further understand the molecular evolution of major histocompatibility complex (MHC), we sequenced part of MHC class II DRB gene exon 2, including codons encoding the antigen binding site (ABS), from 35 individuals from Finland, Bulgaria, Russia, and Japan. We detected 27 species-specific alleles (Muni- DRBs), some of which were broadly distributed and others geographically restricted. The ratio of nonsynonymous to synonymous substitution rates for predicted ABS codons exceeded 1, though not statistically significant. In addition, the single break point recombination (SBP) and mixed-effects model evolution (MEME) analyses demonstrated a recombination site and positive selection sites that could be committed to maintain the diversity of Muni-DRBs. In a Bayesian phylogenetic tree, all Muni-DRBs grouped within a Mustelidae clade. The Muni-DRBs showed transspecies polymorphism in related Mustela species that could have appeared to be evolved under long-lasting balancing selection. However, the sister-group patterns suggested that Mustela itatsi and Mustela sibirica alleles are much more closely related to one another than either are to M. nivalis alleles. This result correlates with the large genetic distance separating M. nivalis from M. itatsi or M. sibirica, data for which we have previously reported. [ABSTRACT FROM AUTHOR]