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. 绝大多数动物体内均定殖有共生细菌（symbiotic bacteria）群落。在昆虫中，这些共生体（symbiont）与宿主的互作关系尤为紧密：其中多数为胞内共生菌，可在宿主的生态与演化进程中发挥关键作用，例如帮助宿主抵御天敌。 2. 本研究针对蚜虫宿主体内不同物种或菌株的胞内共生细菌（endosymbiotic bacteria）之间的互作机制展开探究，旨在明确其对共生体与宿主双方共生结局的影响。 3. 我们首先探究了兼性共生菌（facultative symbiont）的不同物种或菌株组合能否稳定存在于共感染状态中。随后，我们研究了共感染共生体的存在是否会改变兼性细菌赋予宿主的生存优势（抵御天敌）——具体包括该优势是增强、减弱还是维持不变。我们分别针对两类共感染共生体开展了相关研究：一类是对宿主赋予不同防御表型的共生体（分别抵御真菌病原体与寄生蜂），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产生的存活代价，并对比了共感染与单感染状态下的共生菌滴度（symbiont titre）。 4. 本研究发现，本次探究所涉及的所有兼性共生菌物种组合（昆虫雷氏菌+防御汉密尔顿菌、昆虫雷氏菌+立克次体属某物种、昆虫雷氏菌+螺原体属某物种）以及菌株组合（防御汉密尔顿菌菌株），均可形成稳定共感染。当共生体分别抵御不同天敌时，共感染并未对其防御效果产生显著改变；当共生体抵御同一类天敌时，宿主的防御水平与两种共生体中防御能力更强的一方保持一致。在部分情形下，蚜虫宿主在遭受双重感染时会承担额外的存活代价。但就防御汉密尔顿菌而言，同一共生体的多菌株感染相较于单感染会降低共生菌滴度，反而能够提升蚜虫的存活能力。 5. 综上，蚜虫体内共生体共感染的长期维持，主要可能由共感染所带来的代价所决定，在部分情形下则取决于共生体防御优势的冗余性。