Phenotypic effect of "Candidatus Rickettsiella viridis," a facultative symbiont of the pea aphid (Acyrthosiphon pisum), and its interaction with a coexisting symbiont.

A gammaproteobacterial facultative symbiont of the genus Rickettsiella was recently identified in the pea aphid, Acyrthosiphon pisum. Infection with this symbiont altered the color of the aphid body from red to green, potentially affecting the host's ecological characteristics, such as attractiveness to different natural enemies. In European populations of A. pisum, the majority of Rickettsiella-infected aphids also harbor another facultative symbiont, of the genus Hamiltonella. We investigated this Rickettsiella symbiont for its interactions with the coinfecting Hamiltonella symbiont, its phenotypic effects on A. pisum with and without Hamiltonella coinfection, and its infection prevalence in A. pisum populations. Histological analyses revealed that coinfecting Rickettsiella and Hamiltonella exhibited overlapping localizations in secondary bacteriocytes, sheath cells, and hemolymph, while Rickettsiella-specific localization was found in oenocytes. Rickettsiella infections consistently altered hosts' body color from red to green, where the greenish hue was affected by both host and symbiont genotypes. Rickettsiella-Hamiltonella coinfections also changed red aphids to green; this greenish hue tended to be enhanced by Hamiltonella coinfection. With different host genotypes, Rickettsiella infection exhibited either weakly beneficial or nearly neutral effects on host fitness, whereas Hamiltonella infection and Rickettsiella-Hamiltonella coinfection had negative effects. Despite considerable frequencies of Rickettsiella infection in European and North American A. pisum populations, no Rickettsiella infection was detected among 1,093 insects collected from 14 sites in Japan. On the basis of these results, we discuss possible mechanisms for the interaction of Rickettsiella with other facultative symbionts, their effects on their hosts' phenotypes, and their persistence in natural host populations. We propose the designation "Candidatus Rickettsiella viridis" for the symbiont.