<i>Rickettsia</i> Symbiont in the Pea Aphid <i>Acyrthosiphon pisum</i> : Novel Cellular Tropism, Effect on Host Fitness, and Interaction with the Essential Symbiont <i>Buchnera</i>

TLDR

Rickettsia symbionts are commonly found in natural pea aphid populations worldwide. The study examined how Rickettsia affects pea aphid fitness and its interaction with the essential symbiont Buchnera. Antibiotic treatment produced matched aphid strains differing only in Rickettsia presence to assess effects. Rickettsia localizes to secondary mycetocytes and sheath cells, forms ~2 µm rods, sometimes associates with virus‑like particles, reduces host fitness, and suppresses Buchnera populations—especially in young adults—suggesting mechanisms that sustain its prevalence despite fitness costs.

Abstract

ABSTRACT 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 μm long and 0.5 μm 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.