Stronger population differentiation at infection-sensing than infection-clearing innate immune loci in songbirds: different selective regimes for different defenses

Parasite-mediated selection is widespread at loci involved in immune defence, but different defences may experience different selective regimes. For defences involved in clearing infections, purifying selection favouring a single most efficacious allele likely predominates. However, for defences involved in sensing and recognizing infections, evolutionary arms races may make positive selection particularly important. This could manifest primarily within populations (e.g., balancing selection maintaining variation) or among them (e.g., spatially varying selection enhancing population differences in allele frequencies). We genotyped three toll-like receptors (TLR; involved in sensing infections) and three avian beta-defensins (involved in clearing infections) in 96 song sparrows (Melospiza melodia) from three breeding populations that differ in disease resistance. Variation-based indicators of selection (proportion of variable sites, proportion of nonsynonymous SNPs, proportion of sites b...

寄生虫介导选择（Parasite-mediated selection）在免疫防御相关基因座中普遍存在，但不同免疫防御机制所承受的选择压力模式或有所不同。对于参与清除感染的防御机制而言，倾向于保留单一最优功效等位基因的纯化选择（purifying selection）大概率占据主导地位。然而，对于参与感知与识别感染的防御机制而言，进化军备竞赛（evolutionary arms races）可能使得正向选择（positive selection）的作用尤为关键。这种选择效应主要可分为两类：一类发生在种群内部，例如维持遗传变异的平衡选择（balancing selection）；另一类则发生在种群之间，例如增强等位基因频率种群差异的空间异质性选择（spatially varying selection）。我们以来自3个抗病性存在显著差异的繁殖种群的96只歌带鹀（Melospiza melodia）为研究对象，对3个Toll样受体（TLR，参与感染感知）与3个禽β-防御素（参与感染清除）进行了基因分型。基于遗传变异的选择指示指标包括可变位点占比、非同义单核苷酸多态性占比、位点b...