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，此类共生体与宿主的互作往往更为紧密：诸多共生体为胞内共生，并可在宿主的生态与演化进程中发挥关键作用，例如抵御天敌侵扰。 2. 本研究聚焦于蚜虫宿主内不同物种或菌株的胞内共生细菌（endosymbiotic bacteria）之间的互作，如何影响共生体与宿主双方的共生结局。 3. 我们首先探究了兼性共生体（facultative symbiont）的不同物种或菌株组合是否可形成稳定的共感染状态；随后我们进一步研究，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（即抵御天敌的能力），该改变可表现为增强、减弱或无变化。我们针对两类共感染共生体开展了此类探究：一类是对宿主赋予不同表型的共生体（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体物种或菌株感染时产生的额外生存代价，并对比了双重感染与单一感染状态下的共生体滴度（symbiont titres）。 4. 本研究发现，我们所测试的所有兼性共生体物种组合（昆虫雷杰菌（Regiella insecticola）+ 防御汉密尔顿菌（Hamiltonella defensa）、昆虫雷杰菌 + 立克次体属某物种（Rickettsiella sp.）、昆虫雷杰菌 + 螺原体属某物种（Spiroplasma sp.））以及汉密尔顿菌菌株组合，均可形成稳定的共感染状态。当两类共生体分别抵御不同天敌时，共感染并未改变宿主的防护效果；当两类共生体抵御同一类天敌时，宿主的防护效果与两者中防护能力更强的共生体保持一致。部分情况下，携带双重感染的蚜虫宿主会承受额外的生存代价。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一菌株感染，其共生体滴度更低，反而可提升蚜虫的存活率。 5. 本研究最终得出结论：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，在部分情况下则取决于共生体防护优势的冗余性。