Department of Zoology, University of Oxford, Oxford, UK.
J Anim Ecol. 2018 Mar;87(2):478-488. doi: 10.1111/1365-2656.12705. Epub 2017 Jul 3.
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. 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. 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. 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. 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.
大多数动物都有共生细菌群落。在昆虫中，这些共生体与它们的宿主可能有特别密切的相互作用：许多是细胞内共生体，可以在宿主的生态和进化中发挥重要作用，包括抵御天敌。我们研究了蚜虫宿主内不同种或株的内共生细菌之间的相互作用如何影响共生体和宿主的共生结果。我们首先询问了不同组合的兼性共生体物种或株是否可以在稳定的共感染中存在。然后，我们研究了兼性细菌赋予宿主的益处（抵御天敌）是否因共感染的共生体而增强、减少或不变。我们询问了共感染的共生体赋予宿主不同表型（抵御真菌病原体与寄生蜂）和功能重叠的共生体的情况。最后，我们调查了蚜虫携带多种共生体物种或株感染的额外生存成本，并比较了双感染和单感染的共生体滴度。我们发现，在所研究的兼性共生体物种（Regiella insecticola + Hamiltonella defensa、Regiella + Rickettsiella sp.、Regiella + Spiroplasma sp.）和株（Hamiltonella）的所有组合之间都可以形成稳定的共感染。当共生体提供针对不同天敌的保护时，共感染的存在不会改变保护水平。当共生体提供针对同一天敌的保护时，保护水平对应于存在的两种共生体中的较高水平。在某些情况下，蚜虫宿主在双重感染时会遭受额外的生存成本。然而，在 Hamiltonella 的情况下，感染同一共生体的多个株系会导致共生体滴度低于单感染，实际上还改善了蚜虫的生存。我们得出结论，蚜虫中共生体共感染的长期维持很可能主要取决于共感染的成本，在某些情况下还取决于共生体益处的冗余。
