Department of Biology, University of Western Ontario, London, Ontario, Canada.
Heredity (Edinb). 2010 Feb;104(2):155-67. doi: 10.1038/hdy.2009.99. Epub 2009 Jul 29.
The highly diverse genes of the major histocompatibility complex (MHC) are important in the adaptive immune system and are expected to be under selection from pathogens. Thus, the MHC genes provide an exceptional opportunity to investigate patterns of selection within and across populations. In this study, we analyzed genetic variation at the MHC class IIB gene and six microsatellite loci across 10 populations of guppies (Poecilia reticulata) in the northern range of Trinidad. We found a high level of diversity at the MHC, with a total of 43 alleles in 142 individuals. At the population level, we found that neutral evolution could not fully account for the variability found at the MHC. Instead, we found that MHC F(ST) statistics were lower than F(ST) derived from the microsatellite loci; 33 of 45 population pairwise estimates for the MHC were significantly lower than those for the microsatellite loci, and MHC F(ST) estimates were consistently lower than those predicted by a coalescent model of neutral evolution. These results suggest a similar selection acting across populations, and we discuss the potential roles of directional and balancing selection. At the sequence level, we found evidence for both positive and purifying selection. Furthermore, positive selection was detected within and adjacent to the putative peptide-binding region (PBR) of the MHC. Surprisingly, we also found a purifying selection at two sites within the putative PBR. Overall, our data provide evidence for selection for functional diversity at the MHC class IIB gene at both the population and nucleotide levels of guppy populations.
主要组织相容性复合体（MHC）的高度多样化基因在适应性免疫系统中很重要，预计会受到病原体的选择。因此，MHC 基因为研究种群内和种群间的选择模式提供了一个极好的机会。在这项研究中，我们分析了特立尼达北部 10 个孔雀鱼（Poecilia reticulata）种群中 MHC 类 IIB 基因和 6 个微卫星位点的遗传变异。我们在 MHC 上发现了高度的多样性，在 142 个个体中共有 43 个等位基因。在种群水平上，我们发现中性进化不能完全解释 MHC 中发现的可变性。相反，我们发现 MHC F(ST)统计数据低于从微卫星位点得出的 F(ST)；45 对种群间 MHC 的估计中，有 33 对明显低于微卫星位点，MHC F(ST)估计值始终低于中性进化的合并模型预测的值。这些结果表明在种群间存在类似的选择，我们讨论了定向选择和平衡选择的潜在作用。在序列水平上，我们发现了正选择和纯化选择的证据。此外，在 MHC 的假定肽结合区（PBR）内和附近检测到正选择。令人惊讶的是，我们还在假定的 PBR 内的两个位点发现了纯化选择。总的来说，我们的数据为孔雀鱼种群在 MHC 类 IIB 基因的功能多样性选择提供了证据，无论是在种群水平还是核苷酸水平上。
