Evolutionary dynamics of major histocompatibility genes in Arctic charr

与许多鲑鱼物种一样，北极沙鼠在地区内和地区之间表现出很大的变异性。在不同的淡水系统中共存着多种形态的炭，它们的特征不同，如身体大小和产卵颜色、外部和内部形态以及寄生虫区系。寄生虫区系的差异可能会造成不同形态的自然选择类型的不同，以及随后在涉及免疫反应的基因座上的分化。高度多态的主要组织相容性(MH)基因可能是这种选择的目标。MH I类和II类分子有助于引发免疫反应，从而消除或控制病原体。我们将从北极Charr中分离和鉴定MH I类和II类基因，并利用DNA序列信息对来自不同地点的Charr群体进行基因分型。样本将从拥有多态和单态碳种群的苏格兰湖中收集，选择代表不同碳种群大小和温度制度的湖。我们将使用形态分析将不同形态类型的个体炭化，并鉴定可能的杂交种。此外，我们将对寄生虫负载进行评分，并计算寄生虫指数，该指数考虑到寄生虫的中间宿主类型，因为这与不同形态类型的摄食习性的差异有关。将对单个字符进行MH I类和II类基因以及一组中性微卫星标记的基因分型。此外，我们将评估线粒体DNA变异，以测试假设在一个单一的殖民事件后，一个地方出现了不同的形态。MH和微卫星基因型将被用来测试寄生虫区系的差异与MH I类和II类等位基因在地区内和地区之间的存在之间的关联。MH分析将与中性微卫星标记的分析进行对比。我们的假设是，免疫反应位点的分化高于中性位点，这种差异是由地区内不同变种之间和不同地区之间的寄生虫区系差异驱动的。MH第I类和第II类基因具有不同的免疫功能，与细胞(第I类)或抗体(第II类)反应有关。基于抗体反应在控制寄生虫感染中的重要性，我们假设II类等位基因在地区内和地区之间的不同形态之间划分，而I类等位基因是共享的。为了检验这一点，我们将测定和比较不同多态群体的MH等位基因含量。MH基因是高度多态的，新的等位基因来自点突变和重组事件。后一种机制与I类基因特别相关，在I类基因中，外显子改组导致高度差异的等位基因。我们将测试这些重组等位基因的频率与种群大小之间的关联，以确定它们是否在小种群中受到青睐。我们将在两年内重复我们的所有分析，以检查我们观察到的模式的时间稳定性，并将使用微卫星等位基因频率的时间变化来估计每个CharR种群的有效大小。

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.