Data from: Consequences of symbiont co-infections for insect host phenotypes

1. Most animals host communities of symbiotic bacteria. In insects, these symbionts may have particularly intimate interactions with their hosts: many are intracellular and can play important roles in host ecology and evolution, including protection against natural enemies. 2. We investigated how interactions between different species or strains of endosymbiotic bacteria within an aphid host influence the outcome of symbiosis for both symbiont and host. 3. We first asked whether different combinations of facultative symbiont species or strains can exist in stable co-infections. We then investigated whether the benefits that facultative bacteria confer on their hosts (protection against natural enemies) are enhanced, reduced or unaltered by the presence of a co-infecting symbiont. We asked this both for co-infecting symbionts that confer different phenotypes on their hosts (protection against fungal pathogens vs. parasitoid wasps) and symbionts with overlapping functions. Finally, we investigated the additional survival costs to aphids of carrying multiple infections of symbiont species or strains, and compared symbiont titres in double and single infections. 4. We found that stable co-infections were possible between all of the combinations of facultative symbiont species (Regiella insecticola + Hamiltonella defensa, Regiella + Rickettsiella sp., Regiella + Spiroplasma sp.) and strains (Hamiltonella) that we studied. Where symbionts provided protection against different natural enemies, no alteration in protection was observed in the presence of co-infections. Where symbionts provided protection against the same natural enemy, the level of protection corresponded to the higher of the two symbionts present. In some instances, aphid hosts suffered additional survival costs when hosting double infections. In the case of Hamiltonella, however, infection with multiple strains of the same symbiont led to lower symbiont titres than single infections, and actually improved aphid survival. 5. We conclude that the long-term maintenance of symbiont co-infections in aphids is likely to be determined primarily by costs of co-infections and in some instances by redundancy of symbiont benefits.1. 绝大多数动物体内都定殖着共生细菌（symbiotic bacteria）群落。在昆虫中，这些共生体与宿主的相互作用往往更为紧密：多数共生体为胞内共生菌（endosymbiotic bacteria），可在宿主生态与演化过程中发挥关键作用，例如抵御外界天敌的侵害。 2. 本研究聚焦于蚜虫（aphid）宿主内不同物种或菌株的胞内共生菌之间的相互作用，探究其对共生体与宿主双方共生关系结局的影响。 3. 我们首先探究了不同兼性共生菌（facultative symbiont）物种或菌株的组合能否形成稳定的共感染（co-infection）状态。随后，我们分析了共感染共生体是否会改变兼性共生菌赋予宿主的生存优势——即抵御天敌的能力：这种改变可表现为效果增强、减弱或无变化。我们针对两类共感染共生体开展了上述探究：一类是对宿主赋予不同防御表型的共生体（如抵御真菌病原体（fungal pathogens）vs.抵御寄生蜂（parasitoid wasps）），另一类是功能存在重叠的共生体。最后，我们探究了蚜虫携带多种共生菌物种或菌株感染所额外带来的生存代价，并对比了双重感染与单一感染状态下共生菌滴度（symbiont titre）的水平差异。 4. 本研究发现，我们所测试的所有兼性共生菌物种组合（Regiella insecticola + Hamiltonella defensa、Regiella insecticola + Rickettsiella sp.、Regiella insecticola + Spiroplasma sp.）以及Hamiltonella菌株组合，均可形成稳定的共感染状态。当两类共生体分别针对不同天敌提供防御时，共感染并不会改变其防御效果；当两类共生体针对同一天敌提供防御时，宿主的防御能力将与二者中防御效果更强的共生体保持一致。在部分实验场景中，携带双重感染的蚜虫宿主会承受额外的生存代价。但针对Hamiltonella的实验中，同一共生体的多菌株感染相较于单一菌株感染，其共生菌滴度更低，反而能提升蚜虫的存活能力。 5. 综上，我们认为蚜虫体内共生体共感染的长期维持，主要取决于共感染所带来的生存代价，在部分场景中则取决于共生体防御收益的冗余性。