Evolutionary dynamics of major histocompatibility genes in Arctic charr

Lead Research Organisation: University of Aberdeen
Department Name: Inst of Biological and Environmental Sci

Abstract

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.

Publications

10 25 50
 
Description Arctic charr exhibit phenotypic variation with isolated lochs with two morphs being present, namely pelagic and benthic. We showed using neutral DNA markers that such morphs show almost complete reproductive isolation. Our finding suggest that the splitting between morphs has occurred independantly in each loch. We also examined MHC class I gene sequence variation which revealled this gene to be highly polymorphic, with ~190 alleles shared between 290 fish. With this very high variation it has not yet been possible to obtain any signatures of selection between the two morphs.
Exploitation Route The main users of the data will be evolutionary biologists who will be able to use the MHC class I sequences as a base line for future studies. The population structuring of the Scottish arctic charr populations will be of great use to Scottish Natural Heritage and other conservation bodies including cairngorm national parks in which as number f the samples were collected. Other users could include Marine Science Scotland that is responsible for Scotlands freshwater fish populations.
No direct exploitation
Sectors Environment

 
Description Data generated during this project is now being shared with University of waterloo, Canada for data analysis 
Organisation University of Waterloo
Country Canada 
Sector Academic/University 
PI Contribution Data analysis of arctic charr MHC class I genes
Collaborator Contribution New way to analyses the MHC class I genes
Impact Non as yet still carrying out data analysis
Start Year 2013