The development, maintenance, and use of immune defences are costly. Therefore, animals face trade-offs in terms of resource allocation within their immune system and between their immune system and other physiological processes. To maximize fitness, evolution may favour investment in one immunological defence or subsystem over another in a way that matches a species broader life history strategy. Here, we used phylogenetically-informed comparative analyses to test for relationships between two immunological components. Natural antibodies and complement were used as proxies for the innate branch; structural complexity of the major histocompatibility complex (MHC) region was used for the acquired branch. We found a negative association between the levels of natural antibodies (i.e., haemagglutination titre) and the total MHC gene copy number across the avian phylogeny, both at the species and family level. The family-level analysis indicated that this association was apparent for both MHC-I and MHC-II, when copy numbers within these two MHC regions were analysed separately. The association remained significant after controlling for basic life history components and for ecological traits commonly linked to pathogen exposure. Our results provide the first phylogenetically robust evidence for an evolutionary trade-off within the avian immune system, with a more developed acquired immune system (i.e., more complex MHC architecture) in more derived bird lineages (e.g., passerines) being accompanied by an apparent downregulation of the innate immune system.

中文翻译：

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鸟类先天免疫防御和后天免疫防御之间的进化权衡

免疫防御的开发、维护和使用成本高昂。因此，动物在免疫系统内以及免疫系统与其他生理过程之间的资源分配方面面临着权衡。为了最大限度地提高适应性，进化可能会倾向于以一种与物种更广泛的生活史策略相匹配的方式对一种免疫防御或子系统进行投资，而不是另一种免疫防御或子系统。在这里，我们使用基于系统发育的比较分析来测试两种免疫学成分之间的关​​系。天然抗体和补体被用作先天分支的代表；主要组织相容性复合体 (MHC) 区域的结构复杂性用于获得的分支。我们发现，在整个禽类系统发育中，天然抗体水平（即血凝滴度）与 MHC 基因总拷贝数之间存在负相关，无论是在物种还是科水平上。家族水平分析表明，当分别分析这两个 MHC 区域内的拷贝数时，这种关联对于 MHC-I 和 MHC-II 都很明显。在控制了基本生活史成分和通常与病原体暴露相关的生态特征后，这种关联仍然显着。我们的结果为禽类免疫系统内的进化权衡提供了第一个系统发育上强有力的证据，在更发达的获得性免疫系统（即更复杂的MHC结构）中，在更衍生的鸟类谱系（例如雀形目）中伴随着明显的先天免疫系统的下调。