Associations between allelic variation at major histocompatibility complex I and inbreeding and fitness components in the house sparrow (Passer domesticus)

Abstract. Major histocompatibility complex (MHC) is a widely studied multigene family that is found in all vertebrates. The main role of MHC genes is in the adaptive immune system, but they are also known to have a role in other processes, for example in mate choice. In this thesis the main goal is to find out how inbreeding affects the number of MHC class I alleles (exon 3 sequences) in house sparrow (Passer domesticus) and whether individual allelic variation at MHC is associated to components of individual fitness. The data used in this thesis was collected between the years 1998 and 2013 in six house sparrow populations in an archipelago of northern Norway. Associations between the nucleotide and amino acid sequence numbers and inbreeding, lifespan and lifetime reproductive success were studied using regression analyses. In total of 127 unique nucleotide and 113 unique amino acid sequences (i.e., alleles) were found at MHC class I exon 3, with on average 12.6 (SD ± 3.5) nucleotide and 11.4 (SD ± 2.9) amino acid alleles per individual. Positive associations were found between the number of nucleotide alleles per individual and lifetime reproductive success, but there was only weak evidence for the association between number of amino acid alleles and reproductive success. There was no evidence for associations between the individual numbers of either type of alleles and inbreeding or lifespan. The positive association found between the number of MHC alleles and reproductive success could be explained by ecological traits that may be positively affected by a higher number of MHC alleles; for example, the ability to defend a wider range of pathogens and thus produce more and healthier offspring. It is also possible that there could be associations between MHC alleles and other fitness-components that are not detected using my analysis methods, for example if specific MHC alleles are advantageous or an intermediate number of alleles is the most beneficial