Pathogens are one of the main forces driving the evolution and maintenance of the highly polymorphic genes of the vertebrate major histocompatibility complex (MHC). Although MHC proteins are crucial in pathogen recognition, it is still poorly understood how pathogen-mediated selection promotes and maintains MHC diversity, and especially so in host species with highly duplicated MHC genes. Sedge warblers (Acrocephalus schoenobaenus) have highly duplicated MHC genes and using data from high-throughput MHC genotyping we were able to investigate to what extent avian malaria parasites explain temporal MHC class I supertype fluctuations in a long-term study population. We investigated infection status and infection intensities of two different strains of Haemoproteus, i.e. avian malaria parasites that are known to have significant fitness consequences in sedge warblers. We found that prevalence of avian malaria in carriers of specific MHC class I supertypes was a significant predictor of their frequency changes between years. This finding suggests that avian malaria infections partly drive the temporal fluctuations of the MHC class I supertypes. Furthermore, we found that individuals with a large number of different supertypes had higher resistance to avian malaria, but there was no evidence for an optimal MHC class I diversity. Thus, the two studied malaria parasite strains appear to select for a high MHC class I supertype diversity. Such selection may explain the maintenance of the extremely high number of MHC class I gene copies in sedge warblers, and possibly also in other passerines where avian malaria is a common disease.

Biedrzycka_etal_MHC_parasitesThis file contains data on prevalence and infection intensity of Haemoproteus strains detected in sedge warbler. For every individual the number of alleles in each supertype is given. The age and sex for every individual is also given.Sedge_warbler_MHCI_genotypesThe file contains the results of genotyping of analysed individuals with use of AmpliSAS pipeline. The supertypes assigned for each allele are also given.Biedrzycka_etal_R_sciptsThe file contains R scripts used to analyse effects of quantitative and qualitative disease resistance to Haemoproteus.