RESEARCH ARTICLE Genetic Variation of the Major Histocompatibility Complex (MHC Class II B Gene) in the Threatened Hume’s Pheasant,

Major histocompatibility complex (MHC) genes are the most polymorphic genes in verte-brates and encode molecules that play a crucial role in pathogen resistance. As a result of their diversity, they have received much attention in the fields of evolutionary and conserva-tion biology. Here, we described the genetic variation of MHC class II B (MHCIIB) exon 2 in a wild population of Hume’s pheasant (Syrmaticus humiae), which has suffered a dramatic decline in population over the last three decades across its ranges in the face of heavy ex-ploitation and habitat loss. Twenty-four distinct alleles were found in 73 S. humiae speci-mens. We found seven shared alleles among four geographical groups as well as six rare MHCIIB alleles. Most individuals displayed between one to five alleles, suggesting that there are at least three MHCIIB loci of the Hume’s pheasant. The dN ⁄ dS ratio at putative an-tigen-binding sites (ABS) was significantly greater than one, indicating balancing selection is acting on MHCIIB exon 2. Additionally, recombination and gene conversion contributed to generating MHCIIB diversity in the Hume’s pheasant. One to three recombination events and seventy-five significant gene conversion events were observed within the Hume’s pheasant MHCIIB loci. The phylogenetic tree and network analysis revealed that the Hume’s pheasant alleles do not cluster together, but are scattered through the tree or net-work indicating a trans-species evolutionary mode. These findings revealed the evolution of the Hume’s pheasant MHC after suffering extreme habitat fragmentation