MHC polymorphism in the three-spined stickleback and its role in host-parasite co-evolution

Natural genetic variation is the raw material for evolution and in vertebrates the most polymorphic part of the genome is represented by the major histocompatibility complex. This complex is part of the adaptive immune system and plays a key role in the recognition of invading pathogens. The exceptional polymorphism of these genes is potentially the result of an ongoing arms race between vertebrate hosts and their parasites, also called the Red Queen Hypothesis, which is facilitated by genetic recombination and therefore maintains sexual reproduction. Genes in this region are known to influence parasite resistance and mating decisions in several species, including the three-spined stickleback Gasterosteus aculeatus. This fish has long been a model species for behavioural studies and is currently also becoming a model system in evolutionary ecology and genetics. With the available knowledge about its behavioural ecology, parasite community and first insights into the genetics of the stickleback MHC, it provides a perfect tool to investigate and disentangle the mechanisms that drive host-parasite co-evolution and potentially the maintenance of sexual reproduction. Previous work on the MHC in the three-spined stickleback has mainly focussed on overall genetic diversity, but already found first hints for allele-specific host-pathogen interactions. To follow up on these results and further unravel the mechanisms of host-parasite co-evolution, I started my PhD with the development of a new MHC typing protocol, which is optimised for highly polymorphic loci and based on reference strand-mediated conformation analysis. The new protocol includes new primers that reliably amplify all known MHC class IIB alleles and was thoroughly verified by cloning and sequencing. It enables reliable detection and recognition of individual MHC alleles to the sequence level and therefore provides an important new tool for evolutionary studies in the stickleback. The first pilot screens gave already interesting results in terms of chromosomal organisation of the MHC with tight linkage between amplified MHC class IIB loci and potential polymorphism in the number of loci between haplotypes. The allele specificity also allowed a new experimental level, which was applied in an experiment with semi-natural enclosures. We selected wild-caught individual sticklebacks with specific MHC IIB genotypes and let them reproduce under semi-natural conditions where they encountered their natural parasites, but were protected from predators. During the full reproductive period, we collected eggs and determined parenthood from microsatellite typing. Parental assignment in combination with MHC genotyping revealed mate choice for intermediate genetic distance between mates, but not for specific haplotypes. Further analysis also showed effects of MHC IIB diversity and a haplotype on individual conditional and immunological parameters as well as parasite resistance. Sticklebacks are known for their potential to adapt to new environments and show high diversification in many different habitats across their distributional range. This diversification potentially also includes adaptation to new parasite communities. In another experiment, we therefore tested two local stickleback populations for levels of local immunogenetic adaptation. These populations had been shown in previous studies to be differentiated and reproductively isolated and it has been suggested that this is based on locally specialised cycles of host-parasite co-evolution. We reciprocally exposed lab-bred hybrid and pure line F2 fish to their native and to the foreign habitat. We found MHC-linked as well as genome-wide local adaptation. However the interactions turned out to be more complex than expected and were dependent on the life-history strategy of the parasites. These results show that more focussed experiments are needed to understand the interaction of host and parasite genotypes. In the last and still ongoing project, we are increasing the focus and test experimentally, whether frequency-dependent selection between a host and a single parasite can occur. Although this is a widely hypothesised mechanism for the maintenance of MHC polymorphism, it could not be verified in vertebrates yet. Here we infected two groups of lab bred fish each with one of two sympatric nematode parasites, let them reproduce in a common garden set up and re-infected their offspring. We hypothesize that under natural selection and amplified by sexual selection, the more resistant genotypes will show increased reproductive success and that the resistance is transmitted to the offspring. This will lead to an increase of the resistant host genotype and a decrease of the parasite in the offspring generation and by that fulfil the definition of negative frequency-dependent selection. The data collection has been finished, but the analysis is still ongoing. We will also include host MHC genotypes in the analysis to understand the extent of host allele-parasite associations, which could ultimately explain the exceptional polymorphism in the MHC. The here presented studies provide a more detailed view on the extensive polymorphism of the MHC in the three-spined stickleback and give further insights into the key role that this gene complex has in the arms race between hosts and parasites, mediated through its pleiotropic role in mate choice and parasite resistance.