Evolutionary dynamics of major histocompatibility genes in Arctic charr

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Arctic charr (Savelinus alpinus), in common with many salmonid species, show great variability within and across localities. A number of morphotypes of charr coexist in various freshwater systems and differ in characteristics such as body size and spawning coloration, external and internal morphology and parasite fauna. Differences in parasite faunas are likely to create differences in the types of natural selection experienced by different morphs and subsequent differentiation at loci involved in the immune response. The highly polymorphic major histocompatibility (MH) genes are likely targets for such selection. MH class I and class II molecules are instrumental in eliciting an immune response, which results in elimination or control of the pathogen. We will isolate and characterize the MH class I and class II genes from Arctic charr and use the DNA sequence information to genotype charr populations from different localities. Samples will be collected from Scottish lochs holding polymorphic and monomorphic charr populations, with lochs chosen to represent different sizes of charr populations and temperature regimes. We will use analysis of morphology to assign individual charr to different morphotypes and to identify possible hybrids. In addition, we will score parasite loads and calculate a parasite index, which takes into account the type of intermediate host of the parasite as this is important in relation to differences in feeding habits of the different morphotypes. The individual charr will be genotyped for the MH class I and class II genes and a panel of neutral microsatellite markers. In addition, we will assess mitochondrial DNA variation to test the assumption that the different morphs within a locality have arisen after a single colonisation event. The MH and microsatellite genotypes will be used to test for associations between differences in parasite faunas and the presence of MH class I and class II alleles between morphs within and across localities. The MH analyses will be contrasted with analyses of neutral microsatellite markers. Our hypothesis is that differentiation at immune response loci is higher than at neutral loci and that this divergence is driven by differences in parasite faunas between morph within localities and among morphs across localities. MH class I and class II genes have different immunological functions and are associated with either cellular (class I) or antibody (class II) responses. We hypothesize, based on the importance of the antibody response in controlling parasite infections, that class II alleles are partitioned among morphotypes within and between localities, whereas class I alleles are shared. To test this we will determine and compare the MH allelic content of the different polymorphic populations. MH genes are highly polymorphic and new alleles arise from point mutations and recombination events. The latter mechanism is particularly relevant for class I genes where exon shuffling leads to highly divergent alleles. We will test for associations between the frequency of these recombinant alleles and population size to determine whether these are favoured in small populations. We will repeat all our analyses in two years to check for temporal stability of the patterns we observe and will use the temporal variation in microsatellite allele frequencies to estimate the effective size of each of the charr populations.