Major histocompatibility genes, polymorphism and balancing selection : the case of parasites and sticklebacks

The extreme polymorphism of genes of the vertebrate major histocompatibility complex is an evolutionary puzzle. Why do high numbers of alleles persist despite selective forces favouring the fittest variants? Overdominant selection can explain the persistance of high numbers of alleles, because MHC heterozygotes can detect more pathogens. Using three-spined sticklebacks and their natural parasite fauna as a model organisms, this thesis addresses some of the key mechanisms that may explain MHC polymorphism. On Population level, diversity of stickleback MHC class IIB genes was correlated with parasite diversity. Within individuals those with an intermediate individual MHC diversity were suffering least from parasite infection in the field and in controlled experiments. Host-parasite co-evolution would further predict that the fitness of genotypes/alleles should fluctuate depending on its frequency. In three populations MHC alleles fluctuate stronger between generations than random. These fluctuations could partly be linked to changes in parasite prevalence and therefore provide support for antagonistic co-evolution between MHC genes and parasites