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.

包括温度在内的环境因子，对物种间互作（包括互利共生与拮抗作用）具有显著影响。多数昆虫物种均携带有可垂直传播的细菌共生体，其中不少共生体能够帮助宿主抵御天敌。然而，诸多共生体及其代谢产物具有热敏感性，因此其功能效力会随温度波动发生变化。在豌豆蚜（Acyrthosiphon pisum）中，细菌共生体汉密尔顿氏菌（Hamiltonella defensa）及其伴随的APSE噬菌体（APSE bacteriophages），可帮助宿主抵御该蚜虫的主要寄生蜂——麦蚜茧蜂（Aphidius ervi）。本研究探究了温度对宿主内源防御以及共生体介导的抗寄生蜂防御的影响。此外，本研究还考察了X型共生体（X-type symbiont）的防御特性——该类细菌被推测在与汉密尔顿氏菌（Hamiltonella defensa）共存时，可调控蚜虫的防御反应。研究结果表明，汉密尔顿氏菌介导的防御在高温环境下会失效，但部分蚜虫基因型与汉密尔顿氏菌菌株的组合，在适度升温条件下的防御稳定性更强。同时本研究发现，单一X型共生体菌株既无法抵御麦蚜茧蜂的寄生，也不能在高温环境下恢复汉密尔顿氏菌丧失的防御能力。与之相对，蚜虫的内源防御在所有温度梯度下均能发挥作用，这表明两种不同的防御模式对环境变化的耐受能力并不均等。通过对野外采集的豌豆蚜样本进行调查，本研究发现北美各地理区域中汉密尔顿氏菌的携带频率与日均温呈负相关关系，这符合"温暖气候下共生体的益处会降低"的理论预期。基于上述研究结果，本研究提出：全球气温持续上升可能会引发防御型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重要影响。