Selective elimination of aphid endosymbionts: effects of antibiotic dose and host genotype, and fitness consequences.

Multiple endosymbionts commonly coexist in the same host insects. In order to gain an understanding of the biological roles of the individual symbionts in such complex systems, experimental techniques for enabling the selective removal of a specific symbiont from the host are of great importance. By using the pea aphid-Buchnera-Serratia endosymbiotic system as a model, the efficacy, generality, and fitness consequences of selective elimination techniques at various antibiotic doses and under a variety of host genotypes were investigated. In all the disymbiotic aphid strains examined, the facultative symbiont Serratia was selectively eliminated by ampicillin treatment in a dose-dependent manner, suggesting a generality of the elimination technique irrespective of host genotype. However, fitness consequences of the Serratia elimination differed between the aphid strains, indicating substantial effects of host genotype. In all the disymbiotic aphid strains, the obligate symbiont Buchnera was selectively eliminated by rifampicin treatment irrespective of the antibiotic dose. However, the survival and reproduction of the Buchnera-free aphids varied in a dose-dependent manner, and the dose dependence was strikingly different between the aphid genotypes. These results provide a basis for the development of new protocols for manipulating insect endosymbiotic microbiota.

Rickettsia symbiont in the pea aphid Acyrthosiphon pisum: novel cellular tropism, effect on host fitness, and interaction with the essential symbiont Buchnera.

In natural populations of the pea aphid Acyrthosiphon pisum, a facultative bacterial symbiont of the genus Rickettsia has been detected at considerable infection frequencies worldwide. We investigated the effects of the Rickettsia symbiont on the host aphid and also on the coexisting essential symbiont Buchnera. In situ hybridization revealed that the Rickettsia symbiont was specifically localized in two types of host cells specialized for endosymbiosis: secondary mycetocytes and sheath cells. Electron microscopy identified bacterial rods, about 2 mum long and 0.5 mum thick, in sheath cells of Rickettsia-infected aphids. Virus-like particles were sometimes observed in association with the bacterial cells. By an antibiotic treatment, we generated Rickettsia-infected and Rickettsia-eliminated aphid strains with an identical genetic background. Comparison of these strains revealed that Rickettsia infection negatively affected some components of the host fitness. Quantitative PCR analysis of the bacterial population dynamics identified a remarkable interaction between the coexisting symbionts: Buchnera population was significantly suppressed in the presence of Rickettsia, particularly at the young adult stage, when the aphid most actively reproduces. On the basis of these results, we discussed the possible mechanisms that enable the prevalence of Rickettsia infection in natural host populations in spite of the negative fitness effects observed in the laboratory.