Data from: Experimental evolution reveals differences between phenotypic and evolutionary responses to population density

Group living can select for increased immunity, given the heightened risk of parasite transmission. Yet, it also may select for increased male reproductive investment, given the elevated risk of female multiple mating. Trade-offs between immunity and reproduction are well documented. Phenotypically, population density mediates both reproductive investment and immune function in the Indian meal moth, Plodia interpunctella. However, the evolutionary response of populations to these traits is unknown. We created two replicated populations of P. interpunctella, reared and mated for 14 generations under high or low population densities. These population densities cause plastic responses in immunity and reproduction: at higher numbers, both sexes invest more in one index of immunity (phenoloxidase (PO) activity) and males invest more in sperm. Interestingly, our data revealed divergence in PO and reproduction in a different direction to previously reported phenotypic responses. Males evolving at low population densities transferred more sperm and both males and females displayed higher PO than individuals at high population densities. These positively correlated responses to selection suggest no apparent evolutionary trade-off between immunity and reproduction. We speculate that the reduced PO activity and sperm investment when evolving under high population density may be due to the reduced population fitness predicted under increased sexual conflict and/or to trade-offs between pre- and post-copulatory traits.鉴于寄生虫传播风险升高，群居生活可对宿主的免疫增强产生选择压力；而鉴于雌性多次交配的风险上升，群居生活也可能对雄性的生殖投入增加产生选择压力。免疫与繁殖之间的权衡关系已有大量文献记载。从表型层面来看，种群密度可调控印度谷螟（Plodia interpunctella）的生殖投入与免疫功能，但目前学界对该物种种群针对这些性状的进化响应仍不清楚。本研究构建了两个重复的印度谷螟（P. interpunctella）种群，分别在高、低种群密度条件下饲养并交配至第14代。此前研究表明，这两种密度会引发免疫与繁殖的可塑性响应：种群密度更高时，雌雄两性的免疫指标之一——酚氧化酶（phenoloxidase, PO）活性——表达量更高，且雄性的精子投入也更多。有趣的是，本研究的数据显示，印度谷螟在PO活性与繁殖性状上的进化分歧方向，与此前报道的表型响应恰好相反。在低种群密度条件下进化的雄性，其精子传递量更高；且相较于高种群密度下的个体，无论雌雄，其PO活性均显著更高。这种经选择产生的正向相关响应，表明免疫与繁殖之间并不存在明显的进化权衡。我们推测，在高种群密度条件下进化的种群，其PO活性与精子投入降低的现象，可能源于性冲突加剧时种群适合度下降，亦或是交配前与交配后性状间存在权衡关系。