Humoral and cellular immune responses provide animals with major defences against harmful pathogens. While it is often assumed that immune genes undergo rapid diversifying selection, this assumption has not been tested in many species. Moreover, it is likely that different classes of immune genes experience different levels of evolutionary constraint, resulting in varying selection patterns. We examined the evolutionary patterns for a set of 91 canonical immune genes of North American monarch butterflies (Danaus plexippus), using as an outgroup the closely related soldier butterfly (Danaus eresimus). As a comparison to these immune genes, we selected a set of control genes that were paired with each immune for approximate size and genomic location. As a whole, these immune genes had a significant but modest reduction in Tajima's D relative to paired-control genes, but otherwise did not show distinct patterns of population genetic variation or evolutionary rates. When further partitioning these immune genes into four functional classes (recognition, signalling, modulation, and effector), we found distinct differences among these groups. Relative to control genes, recognition genes exhibit increased nonsynonymous diversity and divergence, suggesting reduced constraints on evolution, and supporting the notion that coevolution with pathogens results in diversifying selection. In contrast, signalling genes showed an opposite pattern of reduced diversity and divergence, suggesting evolutionary constraints and conservation. Modulator and effector genes showed no statistical differences from controls. These results are consistent with patterns found in immune genes in fruit flies and Pieris butterflies, suggesting that consistent selective pressures on different classes of immune genes broadly govern the evolution of innate immunity among insects.

中文翻译：

---

种群基因组学揭示了帝王蝶（Danaus plexippus）免疫基因进化的可变模式

体液和细胞免疫反应为动物提供了抵御有害病原体的主要防御能力。虽然通常假设免疫基因会经历快速多样化的选择，但这种假设尚未在许多物种中得到验证。此外，不同类别的免疫基因很可能会经历不同程度的进化约束，从而导致不同的选择模式。我们研究了北美帝王蝶 ( Danaus plexippus ) 的一组 91 个典型免疫基因的进化模式，使用密切相关的士兵蝴蝶 ( Danaus eresimus ) 作为外群）。作为与这些免疫基因的比较，我们选择了一组对照基因，这些基因与每个免疫基因配对，以获得大致的大小和基因组位置。总的来说，这些免疫基因显着但适度地减少了田岛的D相对于配对控制基因，但在其他方面没有显示出群体遗传变异或进化率的不同模式。当进一步将这些免疫基因分为四个功能类别（识别、信号传导、调节和效应子）时，我们发现这些组之间存在明显差异。相对于对照基因，识别基因表现出增加的非同义多样性和分歧，表明对进化的限制减少，并支持与病原体共同进化导致选择多样化的观点。相比之下，信号基因显示出相反的多样性和差异减少模式，表明进化限制和保护。调节基因和效应基因与对照无统计学差异。Pieris蝴蝶，表明对不同类别免疫基因的一致选择压力广泛地控制着昆虫先天免疫的进化。