Data from: Conditional reduction of predation risk associated with a facultative symbiont in an insect

Symbionts are widespread among eukaryotes and their impacts on the ecology and evolution of their hosts are meaningful. Most insects harbour obligate and facultative symbiotic bacteria that can influence their phenotype. In the pea aphid Acyrthosiphon pisum, an astounding symbiotic-mediated phenotype has been recently observed: when infected with the symbiotic bacteria Rickettsiella viridis, young red aphid larvae become greener at adulthood and even darker green when co-infected with Rickettsiella viridis and Hamiltonella defensa. As body colour affects the susceptibility towards natural enemies in aphids, the influence of the colour change due to these facultative symbionts on the host survival in presence of predators was tested. Our results suggested that the Rickettsiella viridis infection may impact positively host survival by reducing predation risk. Due to results from uninfected aphids (i.e., more green ones attacked), the main assumption is that this symbiotic infection would deter the predatory ladybird feeding by reducing the profitability of their hosts rather than decreasing host detection through body colour change. Aphids co-infected with Rickettsiella viridis and Hamiltonella defensa were, however, more exposed to predation suggesting an ecological cost associated with multiple infections. The underlying mechanisms and ecological consequences of these symbiotic effects are discussed.

共生体（symbionts）在真核生物中广泛分布，其对宿主的生态学与进化过程具有重要影响。多数昆虫携带有专性与兼性共生细菌，此类细菌可调控宿主的表型（phenotype）。在豌豆蚜（Acyrthosiphon pisum）中，近期研究发现了一种由共生体介导的惊人表型：当被共生细菌绿色立克次体菌（Rickettsiella viridis）感染时，幼年红色蚜虫个体在成虫阶段体色会变得更绿；若同时感染绿色立克次体菌与汉密尔顿氏防御菌（Hamiltonella defensa），其成虫体色会呈更深的绿色。由于蚜虫的体色会影响其对天敌的易感性，本研究针对这类兼性共生体介导的体色变化，探究了其在捕食者存在的条件下对宿主存活的影响。研究结果显示，绿色立克次体菌感染可通过降低捕食风险，对宿主存活产生正向影响。结合未感染蚜虫的试验结果（即更多绿色个体被捕食），本研究的核心假设为：此类共生感染并非通过改变体色降低宿主被发现的概率，而是通过降低宿主作为猎物的可利用价值，来阻止捕食性瓢虫的取食行为。但同时被绿色立克次体菌与汉密尔顿氏防御菌感染的蚜虫，其被捕食风险更高，这表明多重共生感染存在生态成本。本研究对这类共生效应的潜在机制与生态后果进行了讨论。