Department of Biology, Florida State University, Tallahassee, Florida 32306, USA.
Evolution. 2010 Jul;64(7):2086-96. doi: 10.1111/j.1558-5646.2010.00965.x. Epub 2010 Feb 9.
Understanding genetic diversity in natural populations is a fundamental objective of evolutionary biology. The immune genes of the major histocompatibility complex (MHC) are excellent candidates to study such diversity because they are highly polymorphic in populations. Although balancing selection may be responsible for maintaining diversity at these functionally important loci, temporal variation in selection pressure has rarely been examined. We examine temporal variation in MHC class IIB diversity in nine guppy (Poecilia reticulata) populations over two years. We found that five of the populations changed significantly more at the MHC than at neutral (microsatellite) loci as measured by F(ST), which suggests that the change at the MHC was due to selection and not neutral processes. Additionally, pairwise population differentiation measures at the MHC were higher in 2007 than in 2006, with the signature of selection changing from homogenizing to diversifying selection or neutral evolution. Interestingly, within the populations the magnitude of the change at the MHC between years was related to the change in the proportion of individuals infected by a common parasite, indicating a link between genetic structure and the parasite. Our data thereby implicate temporal variation in selective pressure as an important mechanism maintaining diversity at the MHC in wild populations.
了解自然种群中的遗传多样性是进化生物学的一个基本目标。主要组织相容性复合体 (MHC) 的免疫基因是研究这种多样性的极好候选者，因为它们在种群中高度多态。尽管平衡选择可能负责维持这些功能重要基因座的多样性，但选择压力的时间变化很少被研究。我们在两年内检查了 9 个孔雀鱼（Poecilia reticulata）种群中 MHC 类 IIB 多样性的时间变化。我们发现，与中性（微卫星）基因座相比，有 5 个种群在 MHC 上的变化显著更大，这表明 MHC 的变化是由于选择而不是中性过程。此外，2007 年 MHC 上的种群间分化测量值高于 2006 年，选择的特征从均质化变为多样化选择或中性进化。有趣的是，在种群内，MHC 上两年间的变化幅度与受一种常见寄生虫感染的个体比例的变化有关，这表明遗传结构与寄生虫之间存在联系。因此，我们的数据表明，选择压力的时间变化是维持野生种群 MHC 多样性的重要机制。
