Optimizing MHC diversity in mate choice of ring-necked pheasants

Females of different vertebrates selectively choose males on the basis of the Major Histocompatibility Complex (MHC) genes, although the patterns of assortative mating according to MHC genes vary widely among species. The expression of male epigamic traits in vertebrates largely depends on androgens, which have immunosuppressive side-effects. Because exposure to androgens during early ontogeny may have long-lasting consequences for adult phenotype, the expression of male epigamic traits and reproductive success may be expected to depend on the combined (interaction) effects between MHC individual profile and variation in exposure to androgens during embryonic life. Here, we studied MHC-based mate preferences in the pheasant (Phasianus colchicus), and how these were influenced by in ovo testosterone (T) treatment. Females did not choose males on the basis of male secondary sexual traits or social rank. However, copulations occurred more frequently than expected by chance between individuals sharing an intermediate number of MHC alleles. This result supports the „Optimal Dissimilarity Model" of mate choice. Conversely, there was no conclusive evidence of in ovo T-treatment differentially affecting mate choice in relation to partners" MHC profile. Thus, mating preference for males with an intermediate level of MHC diversity may have evolved as the results of a trade-off between the benefits of the maximization of a broad immune response and the risk of autoimmune defences.