MHC and genomic diversity in Atlantic salmon (Salmo salar L.)

Sammanfattning: In this thesis I present studies concerning genomic diversity and genetic heterogeneity as well as the evolution and maintenance of genetic variation at a specific immune defence gene in Atlantic salmon (Salmo salar, L.). I here present data on nucleotide diversity showing that human activities as well as different post-glacial colonisation events have had considerable effects on the genomic diversity in different salmonid populations. Also, significant within-river heterogeneity in an unmanaged naturally reproducing population of Atlantic salmon is described. In this natural river, the subpopulation genetic structure was significantly correlated to the geographical distance between subpopulations suggesting that gene flow between subpopulations within this natural Atlantic salmon population corresponds to an isolation-by-distance process. However, major histocompatibility complex (MHC) alleles, encoding molecules crucial for the presentation of foreign antigens to T-cells in the adaptive immune system, are homogeneously distributed along the same river. This indicates that selection maintains genetic homogeneity at this specific MHC locus over large areas, probably due to a uniformly distributed pathogen fauna within the river, which is not the situation at neutral loci where genetic drift is counterbalanced by migration. Moreover, the within-river spatial genetic heterogeneity was almost as large as between-river differentiation showing that significant local adaptation probably exists even between regions within rivers. In this thesis I also show that genetic variation at specific genes may have pronounced fitness effects. Different alleles within the MHC rendered significant differences in disease resistance during controlled experimental infections of Atlantic salmon. Nucleotide differences as small as one single base substitution between different MHC alleles had considerable effects on susceptibility. Since MHC heterozygosity did not affect disease resistance these findings suggest that frequency-dependent selection, rather than overdominant selection, is acting at on the MHC. These results are in opposition to the widely assumed hypothesis that overdominant selection is the main contributor to maintenance of MHC diversity.