Expression of host S-adenosylmethionine decarboxylase gene and polyamine composition in aphid bacteriocytes

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.

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.

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.

Differences in gene expression of the ciliate Paramecium caudatum caused by endonuclear symbiosis with Holospora obtusa, revealed using differential display reverse transcribed PCR

We identified six genes of Paramecium caudatum, which differentially expressed in Holospora obtusa-bearing and H. obtusa-free cells using differential display reverse transcribed PCR (DDRT-PCR). Northern blot analyses revealed that two of the genes, CA10-3 and CA20-2, were expressed extensively in the H. obtusa-free cell, while the other four, AS16-1, CS14, CS21 and CA17-1, were expressed more in the H. obtusa-bearing cell. Putative amino acid sequences of CA10-3, AS16-1 and CA17-1 showed high homologies with known genes related to intracellular signaling, transcription and aerobic metabolism. CS14 and CS21 also showed homologies with some genes whose products are still functionally unknown, but CA20-2 encoded a novel protein. We show in this study that H. obtusa alters multiple gene expression of the host after establishing endosymbiosis.

Extraordinary proliferation of microorganisms in aposymbiotic pea aphids, Acyrthosiphon pisum

Aposymbiotic pea aphids, which were deprived of their intracellular symbiotic bacterium, Buchnera, exhibit growth retardation and no fecundity. High performance liquid chromatographic (HPLC) analysis revealed that these aposymbiotic aphids, when reared on broad bean plants, accumulated a large amount of histamine. To assess the possibility of extraordinary proliferation of microorganisms other than Buchnera, we enumerated eubacteria and fungi in aphids using the real-time quantitative PCR method that targets genes encoding small-subunit rRNAs. The result showed that these microorganisms were extremely abundant in the aposymbiotic aphids reared on plants. Microbial communities in aposymbiotic aphids were further profiled by phylogenetic analysis of small-subunit rDNAs. Of 172 nonchimeric sequences of fungal 18S rDNAs, 138 (80.2%) belonged to the phylum Ascomycota. Among them, 21 clustered within a monophyletic group consisting of insect-pathogenic fungi and yeast-like symbionts of homopteran insects. Thirty-one (18.0%), two (1.2%), and one (0.6%) clones were clustered within the Basidiomycota, Zygomycota, and Oomycota, respectively. Of 167 nonchimeric sequences of eubacterial 16S rDNAs, 84 (50.3%) belonged to the gamma-subdivision of Proteobacteria to which most primary endosymbionts of insects and prolific histamine producers belong. Forty (24.0%), 25 (15.0%), 10 (6.0%), and five (3.0%) clones were clustered within alpha-Proteobacteria, Cytophaga-Flavobacterium-Bacteroides (CFB) group, Actinobacteria, and beta-Proteobacteria, respectively. Three had no phylogenetic association with known taxonomic divisions. None of the sequences studied in this study coincided exactly with those deposited in GenBank.