Evolution of MHC class I genes in Eurasian badgers, genus Meles (Carnivora, Mustelidae)

摘要

Because of their role in immune defense against pathogens, major histocompatibility complex (MHC) genes are useful in evolutionary studies on how wild vertebrates adapt to their environments. We investigated the molecular evolution of MHC class I (MHCI) genes in four closely related species of Eurasian badgers, genus Meles. All four species of badgers showed similarly high variation in MHCI sequences compared to other Carnivora. We identified 7-21 putatively functional MHCI sequences in each of the badger species, and 2-7 sequences per individual, indicating the existence of 1-4 loci. MHCI exon 2 and 3 sequences encoding domains alpha 1 and alpha 2 exhibited different clade topologies in phylogenetic networks. Non-synonymous nucleotide substitutions at codons for antigen-binding sites exceeded synonymous substitutions for domain alpha 1 but not for domain alpha 2, suggesting that the domains alpha 1 and alpha 2 likely had different evolutionary histories in these species. Positive selection and recombination seem to have shaped the variation in domain alpha 2, whereas positive selection was dominant in shaping the variation in domain alpha 1. In the separate phylogenetic analyses for exon 2, exon 3, and intron 2, each showed three clades of Meles alleles, with rampant trans-species polymorphism, indicative of the long-term maintenance of ancestral MHCI polymorphism by balancing selection.