Natural selection on immune defense: a field experiment

Predicting the evolution of phenotypic traits requires an understanding of natural selection on them. Despite its indispensability in the fight against parasites, selection on host immune defense has remained understudied. Theory predicts immune traits to be under stabilizing selection due to associated trade-offs with other fitness-related traits. Empirical studies, however, report mainly positive directional selection. This discrepancy could be caused by low phenotypic variation in the examined individuals and/or variation in host resource level that confounds trade-offs in empirical studies. In a field experiment where we maintained Lymnaea stagnalis snails individually in cages in a lake, we investigated phenotypic selection on two immune defense traits, phenoloxidase (PO)-like activity and antibacterial activity, in hemolymph. We used a diverse laboratory population and manipulated snail resource level by limiting their food supply. For six weeks, we followed immune activity, growth, and two fitness components, survival and fecundity of snails. We found that PO-like activity and growth were under stabilizing selection, while antibacterial activity was under positive directional selection. Selection on immune traits was mainly driven by variation in survival. The form of selection on immune defense apparently depends on the particular trait, possibly due to its importance for countering the present parasite community.

预测表型性状的演化，需明确其经受的自然选择作用。尽管宿主免疫防御在对抗寄生虫的过程中不可或缺，但针对宿主免疫防御的选择作用迄今仍未得到充分研究。理论层面，由于免疫性状与其他适合度相关性状存在适应性权衡，免疫性状应受到稳定选择（stabilizing selection）。然而，现有实证研究报告的大多为正向定向选择（positive directional selection）。这种理论与实证的差异，可能源于受试个体的表型变异度偏低，或是宿主资源水平的变异混淆了实证研究中的适应性权衡效应。在一项野外实验中，我们将静水椎实螺（Lymnaea stagnalis）单独饲养于湖中的笼内，以此探究其血淋巴（hemolymph）中两种免疫防御性状——类酚氧化酶（phenoloxidase, PO）活性与抗菌活性——所受的表型选择作用。本研究使用遗传多样性丰富的实验室种群，并通过限制食物供应调控螺类的资源供给水平。实验周期为六周，期间我们追踪了免疫活性、生长状况以及两项适合度组分：螺类的存活率与繁殖力（fecundity）。研究结果表明，类酚氧化酶活性与生长受到稳定选择，而抗菌活性则受到正向定向选择。免疫性状所受的选择作用主要由存活率的变异驱动。宿主免疫防御所受选择的形式显然取决于具体的免疫性状，这可能与其在对抗当前栖息环境中的寄生虫群落时的重要性密切相关。