Data from: Life-history strategy determines constraints on immune function

1) Determining the factors governing investment in immunity is critical for understanding host-pathogen ecological and evolutionary dynamics. Studies often consider disease resistance in the context of life-history theory, with the expectation that investment in immunity will be optimized in anticipation of disease risk. Immunity, however, is constrained by context-dependent fitness costs. How the costs of immunity vary across life-history strategies has yet to be considered.
2) Pea aphids are typically unwinged but produce winged offspring in response to high population densities and deteriorating conditions. This is an example of polyphenism, a strategy used by many organisms to adjust to environmental cues. The goal of this study was to examine the relationship between the fitness costs of immunity, pathogen resistance, and the strength of an immune response across aphid morphs that differ in life-history strategy but are genetically identical.
3) We measured fecundity of winged and unwinged aphids challenged with a heat-inactivated fungal pathogen, and found that immune costs are limited to winged aphids. We hypothesized that these costs reflect stronger investment in immunity in anticipation of higher disease risk, and that winged aphids would be more resistant due to a stronger immune response. However, producing wings is energetically expensive. This guided an alternative hypothesis—that investing resources into wings could lead to a reduced capacity to resist infection.
4) We measured survival and pathogen load after live fungal infection, and we characterized the aphid immune response to fungi by measuring immune cell concentration and gene expression. We found that winged aphids are less resistant and mount a weaker immune response than unwinged aphids, demonstrating that winged aphids pay higher costs for a less effective immune response.
5) Our results show that polyphenism is an understudied factor influencing the expression of immune costs. More generally, our work shows that in addition to disease resistance, the costs of immunity vary between individuals with different life-history strategies. We discuss the implications of these findings for understanding how organisms invest optimally in immunity in light of context-dependent constraints.

1) 明确调控免疫投入的关键因素，对于解析宿主-病原体的生态与进化动态至关重要。现有研究多基于生活史理论探讨疾病抗性，默认免疫投入会针对预期的疾病风险实现优化配置。然而，免疫功能受限于情境依赖的适合度成本，而不同生活史策略下免疫成本的变化规律尚未得到关注。
2) 豌豆蚜（pea aphid）通常为无翅型，但在种群密度过高、环境恶化时会产生有翅后代，这属于多型现象（polyphenism）——这是众多生物用以响应环境信号的适应性策略。本研究旨在探究：在生活史策略存在差异但遗传背景一致的蚜虫型之间，免疫适合度成本、病原体抗性与免疫应答强度三者之间的关联。
3) 我们对经热灭活真菌病原体（heat-inactivated fungal pathogen）刺激的有翅与无翅豌豆蚜进行了繁殖力（fecundity）测定，结果显示免疫成本仅存在于有翅蚜虫中。我们曾提出两个假说：其一，免疫成本反映了个体针对更高疾病风险所投入的更强免疫资源；其二，有翅蚜虫因免疫应答更强而具备更高的病原体抗性。但翅的发育本身具有较高的能量成本，这引导我们提出另一假说：将资源投入翅发育，会削弱个体的感染抗性能力。
4) 我们测定了活真菌感染后蚜虫的存活率与病原体负荷（pathogen load），并通过检测免疫细胞浓度与基因表达（gene expression），表征了蚜虫对真菌的免疫应答特征。结果发现，相较于无翅蚜虫，有翅蚜虫的抗性更弱、免疫应答更弱，这表明有翅蚜虫虽启动了低效的免疫应答，却为此付出了更高的适合度成本。
5) 本研究结果表明，多型现象（polyphenism）是影响免疫成本表达的一个尚未得到充分关注的因素。更广泛而言，本研究证实：除疾病抗性外，免疫成本在不同生活史策略的个体间存在显著差异。我们还讨论了上述发现对于解析生物如何在情境依赖的约束下优化免疫投入的启示意义。