Data from: Cheaper isn’t always worse: more protective isolates of a defensive symbiont are less costly to the aphid host

Defences against parasites are typically associated with costs to the host that contribute to the maintenance of variation in resistance. This also applies to the defence provided by the facultative bacterial endosymbiont Hamiltonella defensa, which protects its aphid hosts against parasitoid wasps while imposing life-history costs. To investigate the cost–benefit relationship within protected hosts, we introduced multiple isolates of H. defensa to the same genetic backgrounds of black bean aphids, Aphis fabae, and we quantified the protection against their parasitoid Lysiphlebus fabarum as well as the costs to the host (reduced lifespan and reproduction) in the absence of parasitoids. Surprisingly, we observed the opposite of a trade-off. Strongly protective isolates of H. defensa reduced lifespan and lifetime reproduction of unparasitized aphids to a lesser extent than weakly protective isolates. This finding has important implications for the evolution of defensive symbiosis and highlights the need for a better understanding of how strain variation in protective symbionts is maintained.

宿主对抗寄生虫的防御策略通常会使宿主付出相应代价，而这类代价是维持宿主抗性变异的关键因素。这一规律同样适用于兼性细菌内共生体（facultative bacterial endosymbiont）汉氏防御共生菌（Hamiltonella defensa）所提供的防御功能：该共生菌可保护其蚜虫宿主免受寄生蜂的寄生，但同时会给宿主带来生活史层面的适应性代价。为探究受保护宿主体内的成本-收益关联，我们将多株H. defensa分离株接种到遗传背景一致的黑蚜（Aphis fabae）体内，随后定量测定了宿主对抗其寄生蜂天敌（Lysiphlebus fabarum）的防护效果，同时记录了无寄生蜂胁迫时宿主的生存与繁殖代价，即寿命缩短与繁殖量下降。出乎意料的是，我们观测到了与经典生存-繁殖权衡理论相悖的结果：防护能力较强的H. defensa分离株，对未被寄生的蚜虫的寿命和终身繁殖力的削弱程度反而弱于防护能力较弱的分离株。这一发现对防御性共生体系的演化研究具有重要启示，同时也凸显了我们亟需更深入地阐明保护性共生菌的菌株变异是如何得以维持的。