Climate factors and selection shape MHC class II diversity in a montane wild sheep (Ovis ammon)

Understanding how environmental heterogeneity drives spatially varying selection is key to deciphering adaptive evolution. The Major Histocompatibility Complex (MHC) provides an ideal model to study this process. We investigated the MHC class II DQA gene across seven subspecies of wild argali sheep (Ovis ammon) in China. High-throughput sequencing of 98 individuals revealed high diversity (25 alleles), with balancing selection maintaining allelic richness and trans-species polymorphism indicating long-term balancing selection. Molecular evolutionary analyses uncovered heterogeneous positive selection on antigen-binding sites among subspecies (ω = 1.181-3.000), mirroring variable pathogen-driven pressures. Mantel tests detected no significant isolation-by-distance or isolation-by-environment. However, Spearman correlations identified significant associations between specific alleles (e.g., Ovam-DQA*08) and climatic variables, as well as between allele frequencies and MaxEnt-predicted suitability. Redundancy analysis showed that temperature and precipitation explained 54.7% of allele frequency variance. Ecological niche modeling suggested that annual mean temperature was associated with allele distribution, indicating that abiotic factors may shape immunogenetic variation, likely by structuring pathogen communities. Our study provides the first MHC characterization in argali and elucidates how balancing and positive selection interact to maintain functional diversity, offering a foundation for predicting evolutionary responses to environmental change.