Am Nat. 2018 Dec;192(6):745-760. doi: 10.1086/700115. Epub 2018 Oct 10.
An effective immune response is expected to confer fitness benefits through improved resistance to parasites but also incur energetic costs that negatively impact fitness-related traits, such as reproduction. The fitness costs and benefits of an immune response are likely to depend on host age, sex, and levels of parasite exposure. Few studies have examined the full extent to which patterns of natural selection on immune phenotypes vary across demographic groups and environments in the wild. Here, we assessed natural selection on plasma levels of three functionally distinct isotypes (IgA, IgE, and IgG) of antibodies against a prevalent nematode parasite measured in a wild Soay sheep population over 26 years. We found little support for environment-dependent selection or reproductive costs. However, antibody levels were negatively associated with parasite egg counts and positively associated with subsequent survival, albeit in a highly age- and isotype-dependent manner. Raised levels of antiparasite IgA best predicted reduced egg counts, but this did not predict survival in lambs. In adults increased antiparasite IgG predicted reduced egg counts, and in adult females IgG levels also positively predicted overwinter survival. Our results highlight the potential importance of age- and sex-dependent selection on immune phenotypes in nature and show that patterns of selection can vary even among functionally related immune markers.
预计有效的免疫反应将通过提高对寄生虫的抵抗力来带来适应度的好处，但也会产生负面影响适应度相关特征的能量成本，例如繁殖。免疫反应的适应度成本和收益可能取决于宿主的年龄、性别和寄生虫暴露水平。很少有研究调查在野外的不同人口群体和环境中，免疫表型的自然选择模式在多大程度上发生变化。在这里，我们评估了在 26 年的时间里，针对一种普遍的线虫寄生虫，在野生的设得兰绵羊群体中测量的三种功能不同的同种型（IgA、IgE 和 IgG）的血浆抗体水平的自然选择。我们几乎没有发现与环境相关的选择或生殖成本的证据。然而，抗体水平与寄生虫卵数呈负相关，与随后的生存呈正相关，但这种相关性高度依赖于年龄和同种型。升高的抗寄生虫 IgA 水平最能预测减少卵数，但这并不能预测羔羊的生存。在成年羊中，增加的抗寄生虫 IgG 水平预测卵数减少，而在成年雌性羊中，IgG 水平也与越冬生存呈正相关。我们的研究结果强调了年龄和性别依赖的选择对自然中免疫表型的潜在重要性，并表明选择模式即使在功能相关的免疫标志物之间也可能有所不同。
