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. 两种物种对同一资源的竞争，理论上会导致劣势竞争者被竞争排斥（competitive exclusion）。但在自然群落中，其他天敌与共生体会对竞争关系起到调节作用，因此我们不能仅局限于两两物种互作的研究。
2. 若天敌对优势竞争者的抑制作用更强，可促进物种共存；反之，也可能通过表观竞争（apparent competition）阻碍物种共存。目前针对共生体作用的研究尚少，而共生体可与天敌协同影响物种共存。
3. 真核生物通常携带可抵御天敌的微生物内共生体（microbial endosymbionts），但在无天敌环境中，维持这类共生体需付出一定代价。这类防御型共生体或可介导共享同一资源的物种共存，无论天敌存在与否，但目前相关实证证据仍较为匮乏。
4. 本研究针对该假说开展实验：我们构建了重复的简单群落，群落包含两种蚜虫，二者共享同一寄主植物与同一种天敌——寄生蜂（parasitoid wasp）。实验设置了三种感染情境：仅其中一种蚜虫携带抗性功能共生体（resistance-conferring symbiont）、两种均携带该共生体、两种均不携带，并分别在有无寄生蜂的条件下开展竞争实验。
5. 在无寄生蜂的条件下，共生体对蚜虫的竞争能力产生了显著影响，尤其显著降低了其中一种蚜虫的竞争能力；而在有寄生蜂的条件下，共生体的作用更为突出。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；若两种均未受保护，寄生蜂会导致二者均走向灭绝。令人意外的是，仅有一种蚜虫受保护时，竟出现了相同结局：易感蚜虫维持了高密度的寄生蜂种群，而寄生蜂通过非寄生的其他机制杀死了抗蚜个体，推测是通过干扰使其达到饥饿致死的程度。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改变物种间的互作关系，在昆虫群落中发挥重要作用。这凸显了在竞争网络中研究物种共存时，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发现，为生物防治（biological control）提供了极具启发性的见解。