ABSTRACT
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.