Data from: Defensive symbionts mediate species coexistence in phytophagous insects

1. Competition of two species for the same resource is expected to result in competitive exclusion of the inferior competitor. In natural communities, however, other antagonists and symbionts moderate competition. Thus we have to go beyond studying pairwise interactions. 2. Natural enemies may facilitate coexistence if they affect the superior competitor more strongly, or they can hinder coexistence via apparent competition. Less well studied is the role of symbionts, which may influence species coexistence in conjunction with enemies. 3. Eukaryotes commonly harbor microbial endosymbionts that provide protection against natural enemies, but are costly in their absence. Such defensive symbionts could thus mediate coexistence of species competing for the same resource, both in the presence and in the absence of enemies, but as yet there is little evidence for this claim. 4. We addressed this proposed role of defensive symbionts in replicated simple communities consisting of two aphid species sharing the same host plant and the same natural enemy, a parasitoid wasp. Both, one, or neither species were infected with a resistance-conferring symbiont, and they competed in the absence as well as the presence of parasitoids. 5. The symbiont had significant effects in the absence of parasitoids by lowering competitive ability especially in one species, but the effects were more dramatic in the presence of parasitoids. With both species protected by the symbiont, parasitoid densities remained low and both aphid species persisted. When neither species was protected, parasitoids drove both species to extinction. Surprisingly, the same outcome was observed when only one species was protected. The susceptible species supported high densities of parasitoids that also killed the resistant aphids via mechanisms other than parasitism, presumably by disturbing them to the point of starvation. This is an intriguing form of apparent competition. 6. Our results demonstrate an important role of defensive symbionts in insect communities through modifying species interactions. This highlights the need for experimental data when studying species coexistence in competitive networks. Furthermore, the observation that a susceptible host can negatively affect a resistant host via a shared parasitoid is an instructive insight for biological control.1. 两种物种争夺同一资源时，理论上会导致竞争劣势者被竞争排斥（competitive exclusion）。但在自然群落中，其他拮抗生物与共生体会调节种间竞争，因此我们不能仅局限于对双物种互作的研究。 2. 天敌对物种共存的影响存在双向性：若天敌对优势竞争者的抑制作用更强，则可促进物种共存；反之，也可能通过表观竞争（apparent competition）阻碍共存。目前针对共生体在该过程中所扮演角色的研究尚且不足，而共生体往往会与天敌共同影响物种共存格局。 3. 真核生物通常携带能够抵御天敌的微生物内共生体（endosymbionts），但这类共生体在无天敌环境下会给宿主带来适合度成本。据此推测，这类防御型共生体可在天敌存在与否的两种情境下，介导争夺同一资源的物种实现共存，但目前相关实证证据仍较为匮乏。 4. 本研究针对这一假说展开验证：我们构建了重复的简单群落模型，群落包含两种共享同一宿主植物与同一种天敌——寄生蜂（parasitoid wasp）的蚜虫物种。实验设置了三种感染情境：仅其中一种蚜虫携带赋予抗性功能的共生体、两种蚜虫均携带该共生体，以及两种蚜虫均未携带；同时分别在无寄生蜂与有寄生蜂的两种环境下开展种间竞争实验。 5. 在无寄生蜂的情境下，共生体对其中一种蚜虫的竞争能力产生了显著抑制作用，而该效应在有寄生蜂的情境下更为显著。当两种蚜虫均受共生体保护时，寄生蜂种群密度维持在较低水平，两种蚜虫均得以存续；若两种蚜虫均未获得保护，则寄生蜂会导致二者均走向灭绝。令人意外的是，仅有一种蚜虫获得共生体保护时，也出现了相同的结局：易感蚜虫种群会支撑起高密度的寄生蜂种群，而这些寄生蜂会通过非寄生的其他机制杀死抗性蚜虫——推测是通过持续干扰导致其饥饿死亡，这是一种颇具研究价值的表观竞争形式。 6. 本研究结果证实，防御型共生体可通过重塑物种互作关系，在昆虫群落中发挥重要的调控作用。这一发现凸显了在竞争网络中研究物种共存时，获取实验数据的必要性。此外，易感宿主可通过共享的寄生蜂对抗性宿主产生负面影响这一观测结果，也为生物防治（biological control）领域提供了极具启发性的研究视角。