An MHC class II supertype confers resistance to a sexually transmitted bacterium in females but not in males in a genetically monogamous seabird.

The major histocompatibility complex (MHC) is a gene cluster essential for pathogen recognition in jawed vertebrates. It encompasses the MHC class I genes which primarily recognize intracellular parasites, and the MHC class II genes which primarily recognize extracellular parasites. In wild birds, most studies investigating associations between MHC variants and parasites have been carried out in passerines, and have repeatedly shown that specific MHC class I variants provide resistance to intracellular haemosporidian parasites. In contrast, research on the associations between MHC variants and parasites in non-passerine birds remains limited. In this study, we examined the association between MHC-IIB supertypes and the bacterial load of a sexually transmitted bacterium (named C34) in the black-legged kittiwake (Rissa tridactyla), a genetically monogamous seabird. We focused on MHC class II genes because extracellular parasites are particularly prevalent in non-passerines and may therefore exert strong selection on the studied host. We found that females with lower C34 load had better reproductive performance, and higher probability to carry the MHC-IIB supertype SUP6. In contrast, in males, we observed a positive association between C34 load and reproductive performance, but no association between C34 load and MHC-IIB supertypes. While sexually transmitted diseases are not expected to be a strong selective force in genetically monogamous species, our study suggests that C34 might exert a selective pressure on the evolution of the MHC-IIB. Therefore, further research should explore the influence of sexually transmitted diseases on the reproductive biology of genetically monogamous species.