A limited concentration range of diaphorin, a polyketide produced by a bacterial symbiont of the Asian citrus psyllid, promotes the in vitro gene expression with bacterial ribosomes.

Diaphorin is a polyketide produced by "Candidatus Profftella armatura" (Gammaproteobacteria: Burkholderiales), an obligate symbiont of a devastating agricultural pest, the Asian citrus psyllid Diaphorina citri (Hemiptera: Psyllidae). Physiological concentrations of diaphorin, which D. citri contains at levels as high as 2-20 mM, are inhibitory to various eukaryotes and Bacillus subtilis (Firmicutes: Bacilli) but promote the growth and metabolic activity of Escherichia coli (Gammaproteobacteria: Enterobacterales). Our previous study demonstrated that 5-mM diaphorin, which exhibits significant inhibitory and promoting effects on cultured B. subtilis and E. coli, respectively, inhibits in vitro gene expression utilizing purified B. subtilis and E. coli ribosomes. This suggested that the adverse effects of diaphorin on B. subtilis are partly due to its influence on gene expression. However, the result appeared inconsistent with the positive impact on E. coli. Moreover, the diaphorin concentration in bacterial cells, where genes are expressed in vivo, may be lower than in culture media. Therefore, the present study analyzed the effects of 50 and 500 µM of diaphorin on bacterial gene expression using the same analytical method. The result revealed that this concentration range of diaphorin, in contrast to 5-mM diaphorin, promotes the in vitro translation with the B. subtilis and E. coli ribosomes, suggesting that the positive effects of diaphorin on E. coli are due to its direct effects on translation. This study demonstrated for the first time that a pederin-type compound promotes gene expression, establishing a basis for utilizing its potential in pest management and industrial applications.IMPORTANCEThis study revealed that a limited concentration range of diaphorin, a secondary metabolite produced by a bacterial symbiont of an agricultural pest, promotes cell-free gene expression utilizing substrates and proteins purified from bacteria. The unique property of diaphorin, which is inhibitory to various eukaryotes and Bacillus subtilis but promotes the growth and metabolic activity of Escherichia coli, may affect the microbial flora of the pest insect, potentially influencing the transmission of devastating plant pathogens. Moreover, the activity may be exploited to improve the efficacy of industrial production by E. coli, which is often used to produce various important materials, including pharmaceuticals, enzymes, amino acids, and biofuels. This study elucidated a part of the mechanism by which the unique activity of diaphorin is expressed, constructing a foundation for applying the distinct property to pest management and industrial use.

Ultrastructure of the bacteriome and bacterial symbionts in the Asian citrus psyllid, Diaphorina citri.

IMPORTANCE: Omics analyses suggested a mutually indispensable tripartite association among the host D. citri and organelle-like bacteriome associates, Carsonella and Profftella, which are vertically transmitted through host generations. This relationship is based on the metabolic complementarity among these organisms, which is partly enabled by horizontal gene transfer between partners. However, little was known about the fine morphology of the symbionts and the bacteriome, the interface among these organisms. As a first step to address this issue, the present study performed transmission electron microscopy, which revealed previously unrecognized ultrastructures, including aggregations of ribosomes in Carsonella, numerous tubes and occasional protrusions of Profftella, apparently degrading Profftella, and host organelles with different abundance and morphology in distinct cell types. These findings provide insights into the behaviors of the symbionts and host cells to maintain the symbiotic relationship in D. citri.

Diaphorin, a polyketide produced by a bacterial endosymbiont of the Asian citrus psyllid, adversely affects the in vitro gene expression with ribosomes from Escherichia coli and Bacillus subtilis.

Diaphorin is a polyketide produced by "Candidatus Profftella armatura" (Gammaproteobacteria), an obligate mutualist of an important agricultural pest, the Asian citrus psyllid Diaphorina citri (Hemiptera). Our previous study demonstrated that diaphorin, at physiological concentrations in D. citri, inhibits the growth and cell division of Bacillus subtilis (Firmicutes) but promotes the growth and metabolic activity of Escherichia coli (Gammaproteobacteria). This unique property of diaphorin may aid microbial mutualism in D. citri, potentially affecting the transmission of "Candidatus Liberibacter spp." (Alphaproteobacteria), the pathogens of the most destructive citrus disease Huanglongbing. Moreover, this property may be exploited to promote microbes' efficiency in producing industrial materials. However, the mechanism underlying this activity is unknown. Diaphorin belongs to the family of pederin-type compounds, which inhibit protein synthesis in eukaryotes by binding to eukaryotic ribosomes. Therefore, as a first step to assess diaphorin's direct influence on bacterial gene expression, this study examined the effect of diaphorin on the in vitro translation using ribosomes of B. subtilis and E. coli, quantifying the production of the green fluorescent protein. The results showed that the gene expression involving B. subtilis and E. coli ribosomes along with five millimolar diaphorin was 29.6% and 13.1%, respectively, less active than the control. This suggests that the diaphorin's adverse effects on B. subtilis are attributed to, at least partly, its inhibitory effects on gene expression. Moreover, as ingredients of the translation system were common other than ribosomes, the greater inhibitory effects observed with the B. subtilis ribosome imply that the ribosome is among the potential targets of diaphorin. On the other hand, the results also imply that diaphorin's positive effects on E. coli are due to targets other than the core machinery of transcription and translation. This study demonstrated for the first time that a pederin congener affects bacterial gene expression.

16S rRNA Gene Sequencing of Six Psyllid Species of the Family Carsidaridae Identified Various Bacteria Including Symbiopectobacterium.

Psyllids (Hemiptera: Sternorrhyncha: Psylloidea) are plant sap-sucking insects that are closely associated with various microbes. To obtain a more detailed understanding of the ecological and evolutionary behaviors of microbes in Psylloidea, the bacterial populations of six psyllid species, belonging to the family Carsidaridae, were analyzed using high-throughput amplicon sequencing of the 16S rRNA gene. The majority of the secondary symbionts identified in the present study were gammaproteobacteria, particularly those of the order Enterobacterales, including Arsenophonus and Sodalis, which are lineages found in a wide variety of insect hosts. Additionally, Symbiopectobacterium, another Enterobacterales lineage, which has recently been recognized and increasingly shown to be vertically transmitted and mutualistic in various invertebrates, was identified for the first time in Psylloidea. This lineage is closely related to Pectobacterium spp., which are plant pathogens, but forms a distinct clade exhibiting no pathogenicity to plants. Non-Enterobacterales gammaproteobacteria found in the present study were Acinetobacter, Pseudomonas (both Pseudomonadales), Delftia, Comamonas (both Burkholderiales), and Xanthomonas (Xanthomonadales), a putative plant pathogen. Regarding alphaproteobacteria, three Wolbachia (Rickettsiales) lineages belonging to supergroup B, the major group in insect lineages, were detected in four psyllid species. In addition, a Wolbachia lineage of supergroup O, a minor group recently found for the first time in Psylloidea, was detected in one psyllid species. These results suggest the pervasive transfer of bacterial symbionts among animals and plants, providing deeper insights into the evolution of the interactions among these organisms.

High-resolution Microbiome Analyses of Nine Psyllid Species of the Family Triozidae Identified Previously Unrecognized but Major Bacterial Populations, including Liberibacter and Wolbachia of Supergroup O.

Psyllids (Hemiptera: Sternorrhyncha: Psylloidea) are plant sap-sucking insects that include important agricultural pests. To obtain insights into the ecological and evolutionary behaviors of microbes, including plant pathogens, in Psylloidea, high-resolution ana-lyses of the microbiomes of nine psyllid species belonging to the family Triozidae were performed using high-throughput amplicon sequencing of the 16S rRNA gene. Analyses identified various bacterial populations, showing that all nine psyllids have at least one secondary symbiont, along with the primary symbiont "Candidatus Carsonella ruddii" (Gammaproteobacteria: Oceanospirillales: Halomonadaceae). The majority of the secondary symbionts were gammaproteobacteria, particularly those of the order Enterobacterales, which included Arsenophonus and Serratia symbiotica, a bacterium formerly recognized only as a secondary symbiont of aphids (Hemiptera: Sternorrhyncha: Aphidoidea). The non-Enterobacterales gammaproteobacteria identified in the present study were Diplorickettsia (Diplorickettsiales: Diplorickettsiaceae), a potential human pathogen, and Carnimonas (Oceanospirillales: Halomonadaceae), a lineage detected for the first time in Psylloidea. Regarding alphaproteobacteria, the potential plant pathogen "Ca. Liberibacter europaeus" (Rhizobiales: Rhizobiaceae) was detected for the first time in Epitrioza yasumatsui, which feeds on the Japanese silverberry Elaeagnus umbellata (Elaeagnaceae), an aggressive invasive plant in the United States and Europe. Besides the detection of Wolbachia (Rickettsiales: Anaplasmataceae) of supergroup B in three psyllid species, a lineage belonging to supergroup O was identified for the first time in Psylloidea. These results suggest the rampant transfer of bacterial symbionts among animals and plants, thereby providing deeper insights into the evolution of interkingdom interactions among multicellular organisms and bacteria, which will facilitate the control of pest psyllids.

Diaphorin, a Polyketide Produced by a Bacterial Symbiont of the Asian Citrus Psyllid, Inhibits the Growth and Cell Division of Bacillus subtilis but Promotes the Growth and Metabolic Activity of Escherichia coli.

Diaphorin is a polyketide produced by "Candidatus Profftella armatura" (Gammaproteobacteria: Burkholderiales), an obligate symbiont of a notorious agricultural pest, the Asian citrus psyllid Diaphorina citri (Hemiptera: Psyllidae). Diaphorin belongs to the pederin family of bioactive agents found in various host-symbiont systems, including beetles, lichens, and sponges, harboring phylogenetically diverse bacterial producers. Previous studies showed that diaphorin, which is present in D. citri at concentrations of 2 to 20 mM, has inhibitory effects on various eukaryotes, including the natural enemies of D. citri. However, little is known about its effects on prokaryotic organisms. To address this issue, the present study assessed the biological activities of diaphorin on two model prokaryotes, Escherichia coli (Gammaproteobacteria: Enterobacterales) and Bacillus subtilis (Firmicutes: Bacilli). Their growth and morphological features were analyzed using spectrophotometry, optical microscopy followed by image analysis, and transmission electron microscopy. The metabolic activity of E. coli was further assessed using the ß-galactosidase assay. The results revealed that physiological concentrations of diaphorin inhibit the growth and cell division of B. subtilis but promote the growth and metabolic activity of E. coli. This finding implies that diaphorin functions as a defensive agent of the holobiont (host plus symbionts) against some bacterial lineages but is metabolically beneficial for others, which potentially include obligate symbionts of D. citri. IMPORTANCE Certain secondary metabolites, including antibiotics, evolve to mediate interactions among organisms. These molecules have distinct spectra for microorganisms and are often more effective against Gram-positive bacteria than Gram-negative ones. However, it is rare that a single molecule has completely opposite activities on distinct bacterial lineages. The present study revealed that a secondary metabolite synthesized by an organelle-like bacterial symbiont of psyllids inhibits the growth of Gram-positive Bacillus subtilis but promotes the growth of Gram-negative Escherichia coli. This finding not only provides insights into the evolution of microbiomes in animal hosts but also may potentially be exploited to promote the effectiveness of industrial material production by microorganisms.

Extreme Polyploidy of Carsonella, an Organelle-Like Bacterium with a Drastically Reduced Genome.

Polyploidy is the state of having multiple copies of the genome within a nucleus or a cell, which has repeatedly evolved across the domains of life. Whereas most bacteria are monoploid, some bacterial species and endosymbiotic organelles that are derived from bacteria are stably polyploid. In the present study, using absolute quantitative PCR, we assessed the ploidy of Candidatus Carsonella ruddii (Gammaproteobacteria, Oceanospirillales), the obligate symbiont of the hackberry petiole gall psyllid, Pachypsylla venusta (Hemiptera, Psylloidea). The genome of this symbiont is one of the smallest known for cellular organisms, at 160 kb. The analysis revealed that Carsonella within a single bacteriocyte has ∼6 × 104 copies of the genome, indicating that some Carsonella cells can contain thousands or even tens of thousands of genomic copies per cell. The basis of polyploidy of Carsonella is unknown, but it potentially plays a role in the repair of DNA damage through homologous recombination. IMPORTANCE Mitochondria and plastids are endosymbiotic organelles in eukaryotic cells and are derived from free-living bacteria. They have many highly reduced genomes from which numerous genes have been transferred to the host nucleus. Similar, but more recently established, symbiotic systems are observed in some insect lineages. Although the genomic sequence data of such bacterial symbionts are rapidly accumulating, little is known about their ploidy. The present study revealed that a bacterium with a drastically reduced genome is an extreme polyploid, which is reminiscent of the case of organelles.

Microbiome analyses of 12 psyllid species of the family Psyllidae identified various bacteria including Fukatsuia and Serratia symbiotica, known as secondary symbionts of aphids.

BACKGROUND: Psyllids (Hemiptera: Psylloidea) comprise a group of plant sap-sucking insects that includes important agricultural pests. They have close associations not only with plant pathogens, but also with various microbes, including obligate mutualists and facultative symbionts. Recent studies are revealing that interactions among such bacterial populations are important for psyllid biology and host plant pathology. In the present study, to obtain further insight into the ecological and evolutionary behaviors of bacteria in Psylloidea, we analyzed the microbiomes of 12 psyllid species belonging to the family Psyllidae (11 from Psyllinae and one from Macrocorsinae), using high-throughput amplicon sequencing of the 16S rRNA gene. RESULTS: The analysis showed that all 12 psyllids have the primary symbiont, Candidatus Carsonella ruddii (Gammaproteobacteria: Oceanospirillales), and at least one secondary symbiont. The majority of the secondary symbionts were gammaproteobacteria, especially those of the family Enterobacteriaceae (order: Enterobacteriales). Among them, symbionts belonging to "endosymbionts3", which is a genus-level monophyletic group assigned by the SILVA rRNA database, were the most prevalent and were found in 9 of 11 Psyllinae species. Ca. Fukatsuia symbiotica and Serratia symbiotica, which were recognized only as secondary symbionts of aphids, were also identified. In addition to other Enterobacteriaceae bacteria, including Arsenophonus, Sodalis, and "endosymbionts2", which is another genus-level clade, Pseudomonas (Pseudomonadales: Pseudomonadaceae) and Diplorickettsia (Diplorickettsiales: Diplorickettsiaceae) were identified. Regarding Alphaproteobacteria, the potential plant pathogen Ca. Liberibacter europaeus (Rhizobiales: Rhizobiaceae) was detected for the first time in Anomoneura mori (Psyllinae), a mulberry pest. Wolbachia (Rickettsiales: Anaplasmataceae) and Rickettsia (Rickettsiales: Rickettsiaceae), plausible host reproduction manipulators that are potential tools to control pest insects, were also detected. CONCLUSIONS: The present study identified various bacterial symbionts including previously unexpected lineages in psyllids, suggesting considerable interspecific transfer of arthropod symbionts. The findings provide deeper insights into the evolution of interactions among insects, bacteria, and plants, which may be exploited to facilitate the control of pest psyllids in the future.

Comparative Genomics Underlines Multiple Roles of Profftella, an Obligate Symbiont of Psyllids: Providing Toxins, Vitamins, and Carotenoids.

The Asian citrus psyllid Diaphorina citri (Insecta: Hemiptera: Psylloidea), a serious pest of citrus species worldwide, harbors vertically transmitted intracellular mutualists, Candidatus Profftella armatura (Profftella_DC, Gammaproteobacteria: Burkholderiales) and Candidatus Carsonella ruddii (Carsonella_DC, Gammaproteobacteria: Oceanospirillales). Whereas Carsonella_DC is a typical nutritional symbiont, Profftella_DC is a unique defensive symbiont with organelle-like features, including intracellular localization within the host, perfect infection in host populations, vertical transmission over evolutionary time, and drastic genome reduction down to much less than 1 Mb. Large parts of the 460-kb genome of Profftella_DC are devoted to genes for synthesizing a polyketide toxin; diaphorin. To better understand the evolution of this unusual symbiont, the present study analyzed the genome of Profftella_Dco, a sister lineage to Profftella_DC, using Diaphorina cf. continua, a host psyllid congeneric with D. citri. The genome of coresiding Carsonella (Carsonella_Dco) was also analyzed. The analysis revealed nearly perfect synteny conservation in these genomes with their counterparts from D. citri. The substitution rate analysis further demonstrated genomic stability of Profftella which is comparable to that of Carsonella. Profftella_Dco and Profftella_DC shared all genes for the biosynthesis of diaphorin, hemolysin, riboflavin, biotin, and carotenoids, underlining multiple roles of Profftella, which may contribute to stabilizing symbiotic relationships with the host. However, acyl carrier proteins were extensively amplified in polyketide synthases DipP and DipT for diaphorin synthesis in Profftella_Dco. This level of acyl carrier protein augmentation, unprecedented in modular polyketide synthases of any known organism, is not thought to influence the polyketide structure but may improve the synthesis efficiency.

16S rRNA Sequencing Detected Profftella, Liberibacter, Wolbachia, and Diplorickettsia from Relatives of the Asian Citrus Psyllid.

The Asian citrus psyllid Diaphorina citri (Hemiptera: Psylloidea) is a serious pest of citrus species worldwide because it transmits Candidatus Liberibacter spp. (Alphaproteobacteria: Rhizobiales), the causative agents of the incurable citrus disease, huanglongbing or greening disease. Diaphorina citri possesses a specialized organ called a bacteriome, which harbors vertically transmitted intracellular mutualists, Ca. Carsonella ruddii (Gammaproteobacteria: Oceanospirillales) and Ca. Profftella armatura (Gammaproteobacteria: Betaproteobacteriales). Whereas Carsonella is a typical nutritional symbiont, Profftella is an unprecedented type of toxin-producing defensive symbiont, unusually sharing organelle-like features with nutritional symbionts. Additionally, many D. citri strains are infected with Wolbachia, which manipulate reproduction in various arthropod hosts. In the present study, in an effort to obtain insights into the evolution of symbioses between Diaphorina and bacteria, microbiomes of psyllids closely related to D. citri were investigated. Bacterial populations of Diaphorina cf. continua and Diaphorina lycii were analyzed using Illumina sequencing of 16S rRNA gene amplicons and compared with data obtained from D. citri. The analysis revealed that all three Diaphorina spp. harbor Profftella as well as Carsonella lineages, implying that Profftella is widespread within the genus Diaphorina. Moreover, the analysis identified Ca. Liberibacter europaeus and Diplorickettsia sp. (Gammaproteobacteria: Diplorickettsiales) in D. cf. continua, and a total of four Wolbachia (Alphaproteobacteria: Rickettsiales) lineages in the three psyllid species. These results provide deeper insights into the interactions among insects, bacteria, and plants, which would eventually help to better manage horticulture.

Concentration and distribution of diaphorin, and expression of diaphorin synthesis genes during Asian citrus psyllid development.

The Asian citrus psyllid Diaphorina citri Kuwayama (Hemiptera: Sternorrhyncha: Psylloidea: Liviidae) is an important pest of citrus species worldwide because it transmits Candidatus Liberibacter spp. (Alphaproteobacteria); the causative agents of an incurable citrus disease known as huanglongbing or greening disease. D. citri possesses a vertically transmitted intracellular symbiont, Candidatus Profftella armatura (Betaproteobacteria), which produces diaphorin; a polyketide that is toxic to various eukaryotic organisms. Our previous study demonstrated that the total amount and average concentration of diaphorin in adult D. citri, when homogeneous distribution is assumed within the insect, are sufficient to exert inhibitory effects on fungi and insects, including the Asian lady beetle Harmonia axyridis (Coleoptera: Coccinellidae); one of the major predators of D. citri. However, diaphorin may be localized to a limited body area within D. citri, and its concentration may change during development. In the present study, to better understand the physiological and ecological function of diaphorin, we assessed the distribution of diaphorin within the D. citri body and analyzed concentrations of diaphorin in various developmental stages. Expression of genes involved in diaphorin synthesis was also analyzed. The results demonstrated that diaphorin is distributed widely in the D. citri body, which appears to be a prerequisite for effective deterrence of natural enemies. The concentration of diaphorin was shown to change significantly during the development of D. citri. It was highest in mature adults, followed by embryos and teneral adults, and lowest in nymphs. The lowest concentrations of diaphorin observed in nymphs are still presumed to be effective in deterring invasive natural enemies, including parasites, parasitoids, and entomopathogenic fungi. Quantitative RT-PCR indicated that amounts of transcripts for diaphorin synthesis genes dipP and dipT were at a minimum in embryos, increased during the nymphal period, and reached a maximum level just after adult eclosion. The alteration pattern of the amounts of transcripts for diaphorin synthesis genes appeared to partially disagree with that of the concentration of diaphorin. The present study provides new insights into the function of diaphorin, which is essential for further investigations that aim to improve the efficacy of D. citri biological control.

Diaphorin, a polyketide produced by a bacterial symbiont of the Asian citrus psyllid, kills various human cancer cells.

Diaphorin is a polyketide produced by Candidatus Profftella armatura (Betaproteobacteria), an organelle-like defensive symbiont harbored by a plant sap-sucking insect, Asian citrus psyllid Diaphorina citri (Hemiptera: Liviidae). Diaphorin belongs to the pederin family, a group of compounds that share much of their core structure with that of pederin, which is characterized by two dihydropyran rings bridged by an N-acyl aminal. Most members of this family have potent antitumor activity, making them promising anticancer drug candidates. The present study assessed the therapeutic potential of diaphorin for its antitumor activity against 39 human cancer cell lines including those from breast, brain, colon, lung, skin, ovary, kidney, stomach, and prostate. The results showed that diaphorin had inhibitory activity against all 39 cancer cell lines tested. The GI50, TGI, and LC50 values ranged from 0.28 µM- 2.4 µM, 1.6 µM -11 µM, and 7.5 µM-> 100 µM, respectively. These values are among the highest in the pederin family, indicating that the anticancer activity of diaphorin is milder than those of other pederin congeners. The inhibitory effects of diaphorin significantly differed among the distinct cancer types. The maximum difference was about 10-fold, which was similar to those of most other pederin congeners.

Diaphorin, a polyketide synthesized by an intracellular symbiont of the Asian citrus psyllid, is potentially harmful for biological control agents.

The Asian citrus psyllid Diaphorina citri Kuwayama (Hemiptera: Sternorrhyncha: Psylloidea: Liviidae) is an important pest of citrus species worldwide because it transmits Candidatus Liberibacter spp. (Alphaproteobacteria), the causative agents of an incurable citrus disease known as huanglongbing or greening disease. Diaphorina citri possesses a vertically-transmitted intracellular symbiont, Candidatus Profftella armatura (Betaproteobacteria), which produces diaphorin, a polyketide that is significantly toxic to mammalian cells. Diaphorin is an analog of pederin, a defensive polyketide in the body fluid of Paederus rove beetles (Coleoptera: Staphylinidae) that deters predators. In the present study, as a first step to assess the possibility that diaphorin is toxic to biological control agents, we assayed diaphorin activities against insects and fungi. The target cells and organisms were (a) the Sf9 cell line derived from the fall armyworm moth Spodoptera frugiperda (Lepidoptera: Noctuidae), (b) the pea aphid Acyrthosiphon pisum (Hemiptera: Sternorrhyncha: Aphidoidea: Aphididae), a phloem sap-sucking insect that is closely related to psyllids, (c) the Asian lady beetle Harmonia axyridis (Coleoptera: Coccinellidae), one of the major predators of D. citri, and (d) the budding yeast Saccharomyces cerevisiae (Ascomycota: Saccharomycetes) as a model of fungal pathogens. For a comparison, we also evaluated pederin activities. The results of our analyses revealed the following: (1) Diaphorin and pederin are significantly toxic to the tested insects and yeast; (2) Their toxicities vary widely among the target cells and organisms; (3) Diaphorin is generally less toxic than pederin; (4) The toxicities of diaphorin and pederin are considerably different in the Sf9 insect cell line and S. cerevisiae, but similar in A. pisum and H. axyridis; and (5) The amount of diaphorin contained in D. citri is toxic to all of the tested cells and organisms, suggesting that this polyketide is potentially harmful for biological control agents.

Behavior of bacteriome symbionts during transovarial transmission and development of the Asian citrus psyllid.

The Asian citrus psyllid Diaphorina citri Kuwayama (Hemiptera: Liviidae) is a serious pest worldwide, transmitting Candidatus Liberibacter spp. (Alphaproteobacteria), the causative agents of a devastating citrus disease known as huanglongbing or greening disease. In a symbiotic organ called the bacteriome, D. citri possesses an organelle-like defensive symbiont, Candidatus Profftella armatura (Betaproteobacteria), and a nutritional symbiont, Ca. Carsonella ruddii (Gammaproteobacteria). Drastically reduced symbiont genomes and metabolic complementarity among the symbionts and D. citri indicate their mutually indispensable association. Moreover, horizontal gene transfer between the Profftella and Liberibacter lineages suggests ecological and evolutionary interactions between the bacteriome symbiont and the HLB pathogen. Using fluorescence in situ hybridization, we examined the behavior of Profftella and Carsonella during transovarial transmission and the development of D. citri. In the bacteriomes of sexually-mature female adults, symbionts transformed from an extremely elongated tubular form into spherical or short-rod forms, which migrated toward the ovary. The symbionts then formed mosaic masses, which entered at the posterior pole of the vitellogenic oocytes. After anatrepsis, Carsonella and Profftella migrated to the central and peripheral parts of the mass, respectively. Following the appearance of host nuclei, the mass cellularized, segregating Carsonella and Profftella in the central syncytium and peripheral uninucleate bacteriocytes, respectively. Subsequently, the uninucleate bacteriocytes harboring Profftella assembled at the posterior pole, while the syncytium, containing Carsonella, sat on the anterior side facing the germ band initiating katatrepsis. During dorsal closure, the syncytium was divided into uninuclear bacteriocytes, which surrounded the mass of bacteriocytes containing Profftella. Once fully surrounded, the bacteriocyte mass containing Profftella was fused into a syncytium. Prior to hatching, a pair of wing-like protrusions arose from both lateral sides of the bacteriome, which continued to grow throughout the nymphal stages. These findings provide a foundation for better understanding the intricate relationship between D. citri and its microbiota.

Horizontal gene transfers in insects.

Horizontal gene transfer is the transfer of genetic material across species boundaries. Although horizontal gene transfers are relatively rare in animals, the recent rapid accumulation of genomic data has identified increasing amounts of exogenous DNA inserts in insect genomes. Most of the horizontally acquired sequences appear to be non-functional; however, there is growing evidence that some genes are truly expressed and confer novel functions on the recipient insects. These include previously unavailable metabolic properties including digesting food, degrading toxins, providing resistance to pathogens, and facilitating an obligate mutualistic relationship with intracellular bacteria. A recent analysis revealed that an aphid gene of bacterial origin encodes a protein that is transported into the obligate symbiont, paralleling the evolution of endosymbiotic organelles.

[New insights from infection-specific gene expression network].

Most of our current knowledge about the gene regulation of pathogen comes from studies with in vitro conditions that mimic their host environments, revealing many types of virulence genes and their regulatory network. Recent advances in DNA sequencing and techniques for transcriptome analysis allow us to identify pathogenic genes specifically expressed in vivo. Analyses for gene expression of pathogens in response to the host environment, including immune response and change of bacterial flora during infection, provide clues to understanding the underlying events to establish the infectious diseases. Here, we would like to introduce next epoch-making ideas and concepts to understand the real picture of microbial infection through the recent works of gene regulation in host environments.

Parallel histories of horizontal gene transfer facilitated extreme reduction of endosymbiont genomes in sap-feeding insects.

Bacteria confined to intracellular environments experience extensive genome reduction. In extreme cases, insect endosymbionts have evolved genomes that are so gene-poor that they blur the distinction between bacteria and endosymbiotically derived organelles such as mitochondria and plastids. To understand the host's role in this extreme gene loss, we analyzed gene content and expression in the nuclear genome of the psyllid Pachypsylla venusta, a sap-feeding insect that harbors an ancient endosymbiont (Carsonella) with one of the most reduced bacterial genomes ever identified. Carsonella retains many genes required for synthesis of essential amino acids that are scarce in plant sap, but most of these biosynthetic pathways have been disrupted by gene loss. Host genes that are upregulated in psyllid cells housing Carsonella appear to compensate for endosymbiont gene losses, resulting in highly integrated metabolic pathways that mirror those observed in other sap-feeding insects. The host contribution to these pathways is mediated by a combination of native eukaryotic genes and bacterial genes that were horizontally transferred from multiple donor lineages early in the evolution of psyllids, including one gene that appears to have been directly acquired from Carsonella. By comparing the psyllid genome to a recent analysis of mealybugs, we found that a remarkably similar set of functional pathways have been shaped by independent transfers of bacterial genes to the two hosts. These results show that horizontal gene transfer is an important and recurring mechanism driving coevolution between insects and their bacterial endosymbionts and highlight interesting similarities and contrasts with the evolutionary history of mitochondria and plastids.

Horizontal gene acquisition of Liberibacter plant pathogens from a bacteriome-confined endosymbiont of their psyllid vector.

he Asian citrus psyllid Diaphorina citri is a notorious agricultural pest that transmits the phloem-inhabiting alphaproteobacterial 'Candidatus Liberibacter asiaticus' and allied plant pathogens, which cause the devastating citrus disease called Huanglongbing or greening disease. D. citri harbors two distinct bacterial mutualists in the symbiotic organ called bacteriome: the betaproteobacterium 'Candidatus Profftella armatura' in the syncytial cytoplasm at the center of the bacteriome, and the gammaproteobacterium 'Candidatus Carsonella ruddii' in uninucleate bacteriocytes. Here we report that a putative amino acid transporter LysE of Profftella forms a highly supported clade with proteins of L. asiaticus, L. americanus, and L. solanacearum. L. crescens, the most basal Liberibacter lineage currently known, lacked the corresponding gene. The Profftella-Liberibacter subclade of LysE formed a clade with proteins from betaproteobacteria of the order Burkholderiales, to which Profftella belongs. This phylogenetic pattern favors the hypothesis that the Liberibacter lineage acquired the gene from the Profftella lineage via horizontal gene transfer (HGT) after L. crescens diverged from other Liberibacter lineages. K A/K S analyses further supported the hypothesis that the genes encoded in the Liberibacter genomes are functional. These findings highlight the possible evolutionary importance of HGT between plant pathogens and their insect vector's symbionts that are confined in the symbiotic organ and seemingly sequestered from external microbial populations.

Defensive bacteriome symbiont with a drastically reduced genome.

Diverse insect species harbor symbiotic bacteria, which play important roles such as provisioning nutrients and providing defense against natural enemies [1-6]. Whereas nutritional symbioses are often indispensable for both partners, defensive symbioses tend to be of a facultative nature [1-12]. The Asian citrus psyllid Diaphorina citri is a notorious agricultural pest that transmits Liberibacter spp. (Alphaproteobacteria), causing the devastating citrus greening disease or Huanglongbing [13, 14]. In a symbiotic organ called the bacteriome, D. citri harbors two distinct intracellular symbionts: a putative nutrition provider, Carsonella_DC (Gammaproteobacteria), and an unnamed betaproteobacterium with unknown function [15], for which we propose the name "Candidatus Profftella armatura." Here we report that Profftella is a defensive symbiont presumably of an obligate nature with an extremely streamlined genome. The genomes of Profftella and Carsonella_DC were drastically reduced to 464,857 bp and 174,014 bp, respectively, suggesting their ancient and mutually indispensible association with the host. Strikingly, 15% of the small Profftella genome encoded horizontally acquired genes for synthesizing a novel polyketide toxin. The toxin was extracted, pharmacologically and structurally characterized, and designated diaphorin. The presence of Profftella and its diaphorin-biosynthetic genes was perfectly conserved in the world's D. citri populations.