Fukatsu, Takema

ABSTRACT Members of the genus Rickettsiella are bacterial pathogens of insects and other arthropods. Recently, a novel facultative endosymbiont, “ Candidatus Rickettsiella viridis,” was described in the pea aphid Acyrthosiphon 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 entomopathogenic Rickettsiella grylli (1.6 Mb) but was smaller than the genomes of phylogenetically allied human pathogens Coxiella burnetii (2.0 Mb) and Legionella pneumophila (3.4 Mb). The symbiont’s metabolic pathways exhibited little complementarity to those of the coexisting primary symbiont Buchnera 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 in L. pneumophila were 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. IMPORTANCE Insect body color is relevant to a variety of biological aspects such as species recognition, sexual selection, mimicry, aposematism, and crypsis. Hence, the bacterial endosymbiont “ Candidatus Rickettsiella 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 pathogen Legionella 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.

ABSTRACT Huanglongbing, or citrus greening, is a devastating disease of citrus plants recently spreading worldwide, which is caused by an uncultivable bacterial pathogen, “ Candidatus Liberibacter asiaticus,” and vectored by a phloem-sucking insect, Diaphorina citri . We investigated the infection density dynamics of “ Ca . Liberibacter asiaticus” in field populations of D. citri with experiments using field-collected insects to address how “ Ca . Liberibacter asiaticus” infection density in the vector insect is relevant to pathogen transmission to citrus plants. Of 500 insects continuously collected from “ Ca . Liberibacter asiaticus”-infected citrus trees with pathological symptoms in the spring and autumn of 2009, 497 (99.4%) were “ Ca . Liberibacter asiaticus” positive. The infections were systemic across head-thorax and abdomen, ranging from 10 3 to 10 7 bacteria per insect. In spring, the infection densities were low in March, at ∼10 3 bacteria per insect, increasing up to 10 6 to 10 7 bacteria per insect in April and May, and decreasing to 10 5 to 10 6 bacteria per insect in late May, whereas the infection densities were constantly ∼10 6 to 10 7 bacteria per insect in autumn. Statistical analysis suggested that several factors, such as insect sex, host trees, and collection dates, may be correlated with “ Ca . Liberibacter asiaticus” infection densities in field D. citri populations. Inoculation experiments with citrus seedlings using field-collected “ Ca . Liberibacter asiaticus”-infected insects suggested that (i) “ Ca . Liberibacter asiaticus”-transmitting insects tend to exhibit higher infection densities than do nontransmitting insects, (ii) a threshold level (∼10 6 bacteria per insect) of “ Ca . Liberibacter asiaticus” density in D. citri is required for successful transmission to citrus plants, and (iii) D. citri attaining the threshold infection level transmits “ Ca . Liberibacter asiaticus” to citrus plants in a stochastic manner. These findings provide valuable insights into understanding, predicting, and controlling this notorious citrus pathogen.

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.

ABSTRACT “ Candidatus Midichloria mitochondrii” is an intramitochondrial bacterium of the order Rickettsiales associated with the sheep tick Ixodes ricinus . Bacteria phylogenetically related to “ Ca . Midichloria mitochondrii” (midichloria and like organisms [MALOs]) have been shown to be associated with a wide range of hosts, from amoebae to a variety of animals, including humans. Despite numerous studies focused on specific members of the MALO group, no comprehensive phylogenetic and statistical analyses have so far been performed on the group as a whole. Here, we present a multidisciplinary investigation based on 16S rRNA gene sequences using both phylogenetic and statistical methods, thereby analyzing MALOs in the overall framework of the Rickettsiales . This study revealed that (i) MALOs form a monophyletic group; (ii) the MALO group is structured into distinct subgroups, verifying current genera as significant evolutionary units and identifying several subclades that could represent novel genera; (iii) the MALO group ranks at the level of described Rickettsiales families, leading to the proposal of the novel family “ Candidatus Midichloriaceae.” In addition, based on the phylogenetic trees generated, we present an evolutionary scenario to interpret the distribution and life history transitions of these microorganisms associated with highly divergent eukaryotic hosts: we suggest that aquatic/environmental protista have acted as evolutionary reservoirs for members of this novel family, from which one or more lineages with the capacity of infecting metazoa have evolved.

Abstract Bat flies of the family Nycteribiidae are known for their extreme morphological and physiological traits specialized for ectoparasitic blood-feeding lifestyle on bats, including lack of wings, reduced head and eyes, adenotrophic viviparity with a highly developed uterus and milk glands, as well as association with endosymbiotic bacteria. We investigated Japanese nycteribiid bat flies representing 4 genera, 8 species and 27 populations for their bacterial endosymbionts. From all the nycteribiid species examined, a distinct clade of gammaproteobacteria was consistently detected, which was allied to endosymbionts of other insects such as Riesia spp. of primate lice and Arsenophonus spp. of diverse insects. In adult insects, the endosymbiont was localized in specific bacteriocytes in the abdomen, suggesting an intimate host–symbiont association. In adult females, the endosymbiont was also found in the cavity of milk gland tubules, which suggests uterine vertical transmission of the endosymbiont to larvae through milk gland secretion. In adult females of Penicillidia jenynsii, we discovered a previously unknown type of symbiotic organ in the Nycteribiidae: a pair of large bacteriomes located inside the swellings on the fifth abdominal ventral plate. The endosymbiont genes consistently exhibited adenine/thymine biased nucleotide compositions and accelerated rates of molecular evolution. The endosymbiont genome was estimated to be highly reduced, ∼0.76 Mb in size. The endosymbiont phylogeny perfectly mirrored the host insect phylogeny, indicating strict vertical transmission and host–symbiont co-speciation in the evolutionary course of the Nycteribiidae. The designation ‘Candidatus Aschnera chinzeii’ is proposed for the endosymbiont clade.

ABSTRACTHere we investigated the bacterial endosymbionts of weevils of the genusCurculio. From all four species ofCurculioweevils examined, a novel group of bacterial gene sequences were consistently identified. Molecular phylogenetic analyses demonstrated that the sequences formed a distinct clade in theGammaproteobacteria, which was not related to previously known groups of weevil endosymbionts such asNardonellaspp. andSodalis-allied symbionts. In situ hybridization revealed that the bacterium was intracellularly harbored in a bacteriome associated with larval midgut. In adult females, the bacterium was localized in the germalia at the tip of each overiole, suggesting vertical transmission via ovarial passage. Diagnostic PCR surveys detected high prevalence of the bacterial infection in natural host populations. Electron microscopy identified the reduced cell wall of the bacterial cells, and the bacterial genes exhibited AT-biased nucleotide composition and accelerated molecular evolution, which are suggestive of a long-lasting endosymbiotic association. On the basis of these results, we conclude that the novel endocellular bacteria represent the primary symbiont ofCurculioweevils and proposed the designation “CandidatusCurculioniphilus buchneri.” In addition to “Ca.Curculioniphilus,” we identifiedSodalis-allied gammaproteobacterial endosymbionts from the chestnut weevil,Curculio sikkimensis, which exhibited partial infection frequencies in host insect populations and neither AT-biased nucleotide composition nor accelerated molecular evolution. We suggest that suchSodalis-allied secondary symbionts in weevils might provide a potential source for symbiont replacements, as has occurred in an ancestor ofSitophilusgrain weevils.