Female house sparrows "count on" male genes: experimental evidence for MHC-dependent mate preference in birds

Overall, we found no evidence that females preferred males with high individual MHC diversity. Yet, when we considered individual MHC allelic diversity of the females, we found that females with a low number of alleles were most attracted to males carrying a high number of MHC alleles, which might reflect a mating-up preference by allele counting.This is the first experimental evidence for MHC-dependent mating preferences in an avian species to our knowledge. Our findings raise questions about the underlying mechanisms through which birds discriminate individual MHC diversity among conspecifics, and they suggest a novel mechanism through which mating preferences might promote the evolution of MHC polymorphisms and generate positive selection for duplicated MHC loci.Darwin suggested that female choice can help explain the evolution of extravagant secondary sexual characters in males, but he struggled over how to understand why females evolve mating preferences for such males [1]. Jerram Brown decided to "put aside the idea that there is a best male and that he is best for every female," and instead, he argued that females should prefer genetically compatible or heterozygous males to increase offspring heterozygosity or genetic diversity [2] (also see [3-5]). He was inspired by studies on house mice (Mus musculus) that found disassortative mating preferences for genes of the major histocompatibility complex (MHC) [6,7]. MHC genes are a multigene family in vertebrates that encode cell-surface glycoproteins (class I and II molecules) that control antigen presentation to T-lymphocytes, and through this mechanism MHC genes play a pivotal role in immune recognition of pathogens and parasites. MHC-disassortative mating preferences may function to increase offspring heterozygosity - MHC or genome-wide - as both can enhance resistance to infectious diseases [8-12]. Furthermore, MHC-disassortative mating preferences can also help to explain the extraordinary polymorphism of MH