Genetic drift shaped MHC IIB diversity of an endangered anuran species within the Italian glacial refugium

Highly polymorphic genes of the major histocompatibility complex (MHC) encode proteins involved in the immune response that protect vertebrates from parasites and pathogen infections. MHC variation is shaped by a complex and poorly understood interplay of selective and demographic forces. Studies of MHC variation provide biologically meaningful insights useful for conservation of threatened taxa such as amphibians, which are globally declining partly due to the impact of emerging infectious diseases (e.g. chytridiomycosis). Here, we characterized MHC class IIB variation in the Apennine yellow-bellied toad (Bombina pachypus), an endangered anuran, across its whole distribution range within the Italian peninsula. We compared MHC IIB diversity between refugial (REF) and post-glacial expansion (PGE) areas and tested for the correlation between MHC and neutral variation to quantify the role of drift in shaping MHC diversity. Overall MHC polymorphism was limited compared to other amphibian species. Despite the clear evidence of historical positive selection acting on antigen-binding sites, we found a significant correlation between the diversity of MHC and that of putatively neutral microsatellite markers, which suggests that genetic drift has contributed extensively to shaping MHC variation. MHC diversity was higher in the REF populations and decreased northwards into the PGE area, in accordance with the ‘southern richness and northern purity’ pattern. Past demographic events likely determined the reduction of MHC variation in the PGE area; in particular, the northern populations appeared to survive for a long time with a depleted MHC variation. We discussed recent population declines of B. pachypus in light of our findings.