Institute of Evolutionary Biology, University of Edinburgh, Edinburgh, UK.
PLoS Genet. 2009 Oct;5(10):e1000698. doi: 10.1371/journal.pgen.1000698. Epub 2009 Oct 23.
It is estimated that a large proportion of amino acid substitutions in Drosophila have been fixed by natural selection, and as organisms are faced with an ever-changing array of pathogens and parasites to which they must adapt, we have investigated the role of parasite-mediated selection as a likely cause. To quantify the effect, and to identify which genes and pathways are most likely to be involved in the host-parasite arms race, we have re-sequenced population samples of 136 immunity and 287 position-matched non-immunity genes in two species of Drosophila. Using these data, and a new extension of the McDonald-Kreitman approach, we estimate that natural selection fixes advantageous amino acid changes in immunity genes at nearly double the rate of other genes. We find the rate of adaptive evolution in immunity genes is also more variable than other genes, with a small subset of immune genes evolving under intense selection. These genes, which are likely to represent hotspots of host-parasite coevolution, tend to share similar functions or belong to the same pathways, such as the antiviral RNAi pathway and the IMD signalling pathway. These patterns appear to be general features of immune system evolution in both species, as rates of adaptive evolution are correlated between the D. melanogaster and D. simulans lineages. In summary, our data provide quantitative estimates of the elevated rate of adaptive evolution in immune system genes relative to the rest of the genome, and they suggest that adaptation to parasites is an important force driving molecular evolution.
据估计，在果蝇中，很大一部分氨基酸替换已经被自然选择所固定，而且由于生物体面临着不断变化的病原体和寄生虫，它们必须适应，因此我们研究了寄生虫介导的选择作为可能的原因。为了量化这种影响，并确定哪些基因和途径最有可能参与宿主-寄生虫的军备竞赛，我们对两种果蝇的 136 个免疫基因和 287 个位置匹配的非免疫基因的种群样本进行了重测序。利用这些数据，以及 McDonald-Kreitman 方法的一个新扩展，我们估计自然选择在免疫基因中固定有利的氨基酸变化的速度几乎是其他基因的两倍。我们发现免疫基因的适应进化速度也比其他基因更具可变性，一小部分免疫基因受到强烈选择。这些基因可能代表宿主-寄生虫协同进化的热点，它们往往具有相似的功能或属于相同的途径，如抗病毒 RNAi 途径和 IMD 信号途径。这些模式似乎是两种物种免疫系统进化的一般特征，因为在黑腹果蝇和拟暗果蝇谱系中，适应进化的速度是相关的。总之，我们的数据提供了相对于基因组其他部分，免疫系统基因的适应性进化率升高的定量估计，并且表明对寄生虫的适应是驱动分子进化的重要力量。
