Data from: The potential for adaptive maintenance of diversity in antimicrobial peptides

Genes involved in immune defence are among the fastest evolving in the genomes of many species. Interestingly, however, genes encoding antimicrobial peptides (AMPs) have shown little evidence for adaptive divergence in arthropods, despite the centrality of these peptides in direct killing of microbial pathogens. This observation, coupled with a failure to detect phenotypic consequence of genetic variation in AMPs, has led to the hypothesis that individual AMPs make minor contributions to overall immune defence and that AMPs instead act as a collective cocktail. Recent data, however, have suggested an alternative explanation for the apparent lack of adaptive divergence in AMP genes. Molecular evolutionary and phenotypic data have begun to suggest that variant AMP alleles may be maintained through balancing selection in invertebrates, a pattern similar to that observed in several vertebrate AMPs. Signatures of balancing selection include high rates of non-synonymous polymorphism, trans-species amino acid polymorphisms, and convergence of amino acid states across the phylogeny. In this review, we revisit published literature on insect AMP genes and analyse newly available population genomic datasets in Drosophila, finding enrichment for patterns consistent with adaptive maintenance of polymorphism.

参与免疫防御的基因，在诸多物种的基因组中属于进化速率最快的基因之列。然而值得注意的是，尽管抗菌肽（antimicrobial peptides, AMPs）在直接杀灭微生物病原体的过程中发挥核心作用，但在节肢动物中，编码这类肽的基因几乎未表现出适应性分化的相关证据。这一观察结果，加之未能在AMPs中检测到遗传变异带来的表型效应，催生了这样一种假说：单个AMPs对整体免疫防御的贡献微不足道，而这类肽类实则以集体鸡尾酒的形式发挥功能。但近期的研究数据为AMPs基因适应性分化的明显缺失提供了另一种解释。分子进化与表型相关数据已逐渐表明，无脊椎动物体内的变异AMPs等位基因或许可通过平衡选择得以保留，这一模式与在部分脊椎动物AMPs中观察到的情况相似。平衡选择的特征包括高频率的非同义多态性、跨物种氨基酸多态性，以及系统发育中氨基酸状态的趋同演化。在本综述中，我们重新梳理了已发表的昆虫AMPs基因相关研究，并分析了果蝇属（Drosophila）中新公开的群体基因组数据集，结果发现了与多态性适应性维持相符的模式富集现象。