Data from: Breakdown of a defensive symbiosis, but not endogenous defenses, at elevated temperatures

Environmental factors, including temperature, can have large effects on species interactions, including mutualisms and antagonisms. Most insect species are infected with heritable bacterial symbionts with many protecting their hosts from natural enemies. However, many symbionts or their products are thermally sensitive hence their effectiveness may vary across a range of temperatures. In the pea aphid, Acyrthosiphon pisum, the bacterial symbiont Hamiltonella defensa, and its associated APSE bacteriophages confer resistance to this aphid's dominant parasitoid, Aphidius ervi. Here we investigate the effects of temperature on both endogenous and symbiont-based protection against this parasitoid. We also explored the defensive properties of the X-type symbiont, a bacterium hypothesized to shape aphid defense when co-occurring with H. defensa. We show that H. defensa protection fails at higher temperatures, although some aphid genotype and H. defensa strain combinations are more robust than others at moderately warmer temperatures. We also found that a single X-type strain neither defended against parasitism by A. ervi nor rescued lost H. defensa protection at higher temperatures. In contrast, endogenous aphid resistance was effective across temperatures, revealing that these distinct defensive modes are not equally robust to changing environments. Through a survey of field-collected pea aphids we found a negative correlation between H. defensa frequencies and average daily temperatures across North American locales, fitting expectations for reduced symbiont benefits under warm climates. Based on these findings, we propose that rising global temperatures could promote the widespread breakdown of defensive mutualisms, a prospect with implications for both human and ecosystem health.

环境因子（包括温度）会对物种互作产生显著影响，涵盖互利共生与拮抗互作两类。多数昆虫类群均携带有可遗传的细菌共生体，其中不少共生体可帮助宿主抵御天敌。然而诸多共生体及其代谢产物具有热敏感性，因此其防御效能会随温度变化而产生波动。本研究以豌豆蚜（Acyrthosiphon pisum）为研究对象，其体内的汉密尔顿菌（Hamiltonella defensa）及其伴随的APSE噬菌体（APSE bacteriophages）可赋予该蚜虫对抗其主要寄生蜂——埃氏蚜茧蜂（Aphidius ervi）的抗性。我们在此探究了温度对内源防御以及基于共生体的防御在对抗该寄生蜂时的影响。此外，我们还分析了X型共生菌的防御特性——该类细菌被推测可在与汉密尔顿菌共存时调控蚜虫的防御能力。研究结果显示，汉密尔顿菌介导的防御在高温环境下会失效，不过部分蚜虫基因型与汉密尔顿菌株的组合在温和升温条件下的防御稳定性更强。我们同时发现，单一的X型共生菌株既无法对抗埃氏蚜茧蜂的寄生，也不能在高温环境下挽救丧失的汉密尔顿菌防御功能。与之相对，蚜虫的内源抗性在所有温度梯度下均能发挥作用，这表明不同的防御模式对环境变化的耐受能力存在差异。通过对野外采集的豌豆蚜种群进行调查，我们发现北美各采样点的汉密尔顿菌携带频率与当地日均温呈负相关，这与温暖气候下共生体防御效能下降的理论预期相符。基于上述发现，我们提出全球气温上升可能会引发防御性互利共生关系的广泛瓦解，这一前景对人类健康与生态系统健康均具有潜在影响。