ABSTRACTMembers of the genusRickettsiellaare bacterial pathogens of insects and other arthropods. Recently, a novel facultative endosymbiont, “CandidatusRickettsiella viridis,” was described in the pea aphidAcyrthosiphon pisum, whose infection causes a striking host phenotype: red and green genetic color morphs exist in aphid populations, and upon infection with the symbiont, red aphids become green due to increased production of green polycyclic quinone pigments. Here we determined the complete genome sequence of the symbiont. The 1.6-Mb circular genome, harboring some 1,400 protein-coding genes, was similar to the genome of entomopathogenicRickettsiella grylli(1.6 Mb) but was smaller than the genomes of phylogenetically allied human pathogensCoxiella burnetii(2.0 Mb) andLegionella pneumophila(3.4 Mb). The symbiont’s metabolic pathways exhibited little complementarity to those of the coexisting primary symbiontBuchnera aphidicola, reflecting the facultative nature of the symbiont. The symbiont genome harbored neither polyketide synthase genes nor the evolutionarily allied fatty acid synthase genes that are suspected to catalyze the polycyclic quinone synthesis, indicating that the green pigments are produced not by the symbiont but by the host aphid. The symbiont genome retained many type IV secretion system genes and presumable effector protein genes, whose homologues inL. pneumophilawere reported to modulate a variety of the host's cellular processes for facilitating infection and virulence. These results suggest the possibility that the symbiont is involved in the green pigment production by affecting the host’s metabolism using the secretion machineries for delivering the effector molecules into the host cells.IMPORTANCEInsect body color is relevant to a variety of biological aspects such as species recognition, sexual selection, mimicry, aposematism, and crypsis. Hence, the bacterial endosymbiont “CandidatusRickettsiella viridis,” which alters aphid body color from red to green, is of ecological interest, given that different predators preferentially exploit either red- or green-colored aphids. Here we determined the complete 1.6-Mb genome of the symbiont and uncovered that, although the red-green color transition was ascribed to upregulated production of green polycyclic quinone pigments, the symbiont genome harbored few genes involved in the polycyclic quinone biosynthesis. Meanwhile, the symbiont genome contained type IV secretion system genes and presumable effector protein genes, whose homologues modulate eukaryotic cellular processes for facilitating infection and virulence in the pathogenLegionella pneumophila. We propose the hypothesis that the symbiont may upregulate the host’s production of polycyclic quinone pigments via cooption of secretion machineries and effector molecules for pathogenicity.
Legionella pneumophila targets autophagosomes and promotes host autophagy during late infection
