Psyllid endosymbionts exhibit patterns of co-speciation with hosts and destabilizing substitutions in ribosomal RNA

Rapid Loss of Nutritional Symbionts in an Endemic Hawaiian Herbivore Radiation Is Associated with Plant Galling Habit

Insect herbivores frequently cospeciate with symbionts that enable them to survive on nutritionally unbalanced diets. While ancient symbiont gain and loss events have been pivotal for insect diversification and feeding niche specialization, evidence of recent events is scarce. We examine the recent loss of nutritional symbionts (in as little as 1 MY) in sap-feeding Pariaconus, an endemic Hawaiian insect genus that has undergone adaptive radiation, evolving various galling and free-living ecologies on a single host-plant species, Metrosideros polymorpha within the last ∼5 MY. Using 16S rRNA sequencing, we investigated the bacterial microbiomes of 19 Pariaconus species and identified distinct symbiont profiles associated with specific host-plant ecologies. Phylogenetic analyses and metagenomic reconstructions revealed significant differences in microbial diversity and functions among psyllids with different host-plant ecologies. Within a few millions of years, Pariaconus species convergently evolved the closed-gall habit twice. This shift to enclosed galls coincided with the loss of the Morganella-like symbiont that provides the essential amino acid arginine to free-living and open-gall sister species. After the Pariaconus lineage left Kauai and colonized younger islands, both open- and closed-gall species lost the Dickeya-like symbiont. This symbiont is crucial for synthesizing essential amino acids (phenylalanine, tyrosine, and lysine) as well as B vitamins in free-living species. The recurrent loss of these symbionts in galling species reinforces evidence that galls are nutrient sinks and, combined with the rapidity of the evolutionary timeline, highlights the dynamic role of insect-symbiont relationships during the diversification of feeding ecologies. We propose new Candidatus names for the novel Morganella-like and Dickeya-like symbionts.

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.

Highly Reduced Complementary Genomes of Dual Bacterial Symbionts in the Mulberry Psyllid Anomoneura mori

The genomes of obligately host-restricted bacteria suffer from accumulating mildly deleterious mutations, resulting in marked size reductions. Psyllids (Hemiptera) are phloem sap-sucking insects with a specialized organ called the bacteriome, which typically harbors two vertically transmitted bacterial symbionts: the primary symbiont "Candidatus Carsonella ruddii" (Gammaproteobacteria) and a secondary symbiont that is phylogenetically diverse among psyllid lineages. The genomes of several Carsonella lineages were revealed to be markedly reduced (158-174‍ ‍kb), AT-rich (14.0-17.9% GC), and structurally conserved with similar gene inventories devoted to synthesizing essential amino acids that are scarce in the phloem sap. However, limited genomic information is currently available on secondary symbionts. Therefore, the present study investigated the genomes of the bacteriome-associated dual symbionts, Secondary_AM (Gammaproteobacteria) and Carsonella_AM, in the mulberry psyllid Anomoneura mori (Psyllidae). The results obtained revealed that the Secondary_AM genome is as small and AT-rich (229,822 bp, 17.3% GC) as those of Carsonella lineages, including Carsonella_AM (169,120 bp, 16.2% GC), implying that Secondary_AM is an evolutionarily ancient obligate mutualist, as is Carsonella. Phylogenomic ana-lyses showed that Secondary_AM is sister to "Candidatus Psyllophila symbiotica" of Cacopsylla spp. (Psyllidae), the genomes of which were recently reported (221-237‍ ‍kb, 17.3-18.6% GC). The Secondary_AM and Psyllophila genomes showed highly conserved synteny, sharing all genes for complementing the incomplete tryptophan biosynthetic pathway of Carsonella and those for synthesizing B vitamins. However, sulfur assimilation and carotenoid-synthesizing genes were only retained in Secondary_AM and Psyllophila, respectively, indicating ongoing gene silencing. Average nucleotide identity, gene ortholog similarity, genome-wide synteny, and substitution rates suggest that the Secondary_AM/Psyllophila genomes are more labile than Carsonella genomes.

Microbiome of Zoophytophagous Biological Control Agent Nesidiocoris tenuis

Many insects are associated with endosymbionts that influence the feeding, reproduction, and distribution of their hosts. Although the small green mirid, Nesidiocoris tenuis (Reuter) (Hemiptera: Miridae), a zoophytophagous predator that feeds on plants as well as arthropods, is a globally important biological control agent, its microbiome has not been sufficiently studied. In the present study, we assessed the microbiome variation in 96 N. tenuis individuals from 14 locations throughout Japan, based on amplicon sequencing of the 16S ribosomal RNA gene. Nine major bacteria associated with N. tenuis were identified: Rickettsia, two strains of Wolbachia, Spiroplasma, Providencia, Serratia, Pseudochrobactrum, Lactococcus, and Stenotrophomonas. Additionally, a diagnostic PCR analysis for three typical insect reproductive manipulators, Rickettsia, Wolbachia, and Spiroplasma, was performed on a larger sample size (n = 360) of N. tenuis individuals; the most prevalent symbiont was Rickettsia (69.7%), followed by Wolbachia (39.2%) and Spiroplasma (6.1%). Although some symbionts were co-infected, their prevalence did not exhibit any specific tendency, such as a high frequency in specific infection combinations. The infection frequency of Rickettsia was significantly correlated with latitude and temperature, while that of Wolbachia and Spiroplasma was significantly correlated with host plants. The predominance of these bacteria and the absence of obligate symbionts suggested that the N. tenuis microbiome is typical for predatory arthropods rather than sap-feeding insects. Rickettsia and Wolbachia were vertically transmitted rather than horizontally transmitted from the prey. The functional validation of each symbiont would be warranted to develop N. tenuis as a biological control agent.

Unveiling metabolic integration in psyllids and their nutritional endosymbionts through comparative transcriptomics analysis

Psyllids, a group of insects that feed on plant sap, have a symbiotic relationship with an endosymbiont called Carsonella. Carsonella synthesizes essential amino acids and vitamins for its psyllid host, but lacks certain genes required for this process, suggesting a compensatory role of psyllid host genes. To investigate this, gene expression was compared between two psyllid species, Bactericera cockerelli and Diaphorina citri, in specialized cells where Carsonella resides (bacteriomes). Collaborative psyllid genes, including horizontally transferred genes, showed patterns of conserved gene expression; however, species-specific patterns were also observed, suggesting differences in the nutritional metabolism between psyllid species. Also, the recycling of nitrogen in bacteriomes may primarily rely on glutamate dehydrogenase (GDH). Additionally, lineage-specific gene clusters were differentially expressed in B. cockerelli and D. citri bacteriomes and are highlighted here. These findings shed light on potential host adaptations for the regulation of this symbiosis due to host, microbiome, and environmental differences.

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.

Insect phylogeny structures the bacterial communities in the microbiome of psyllids (Hemiptera: Psylloidea) in Aotearoa New Zealand

The bacterial microbiome of psyllids has been studied for decades, with a strong focus on the primary and secondary endosymbionts capable of providing essential amino acids for the insects' diet and therefore playing a key role in the insects' ability to radiate on novel plant hosts. Here, we combine metabarcoding analysis of the bacterial communities hosted by psyllids with a multi-gene phylogenetic analysis of the insect hosts to determine what factors influence the bacterial diversity of the psyllids' microbiomes, especially in the context of the dispersal and evolutionary radiation of these insects in Aotearoa New Zealand. Using multi-gene phylogenetics with COI, 18S and EF-1α sequences from 102 psyllid species, we confirmed for the first time monophyly for all the six genera of native/endemic Aotearoa New Zealand psyllids, with indications that they derive from at least six dispersal events to the country. This also revealed that, after its ancestral arrival, the genus Powellia has radiated onto a larger and more diverse range of plants than either Psylla or Ctenarytaina, which is uncommon amongst monophyletic psyllids globally. DNA metabarcoding of the bacterial 16S gene here represents the largest dataset analysed to date from psyllids, including 246 individuals from 73 species. This provides novel evidence that bacterial diversity across psyllid species is strongly associated with psyllid phylogenetic structure, and to a lesser degree to their host plant association and geographic distribution. Furthermore, while the strongest co-phylogenetic signals were derived from the primary and secondary symbionts, a signal of phylosymbiosis was still retained among the remaining taxa of the bacterial microbiome, suggesting potential vertical transmission of bacterial lineages previously unknown to have symbiotic roles.

Bacterial Endosymbionts of Bactericera maculipennis and Three Mitochondrial Haplotypes of B. cockerelli (Hemiptera: Psylloidea: Triozidae)

Insects harbor bacterial endosymbionts that provide their hosts with nutritional benefit or with protection against natural enemies, plant defenses, insecticides, or abiotic stresses. We used directed sequencing of 16S rDNA to identify and compare endosymbionts of Bactericera maculipennis (Crawford) and the western, central, and northwestern haplotypes of B. cockerelli (Šulc) (Hemiptera: Psylloidea: Triozidae). Both species are native to North America, are known to harbor the plant pathogen 'Candidatus Liberibacter solanacearum' and develop on shared host plants within the Convolvulaceae. The Old-World species Heterotrioza chenopodii (Reuter) (Psylloidea: Triozidae), now found in North America, was included as an outgroup. 16S sequencing confirmed that both Bactericera species harbor 'Candidatus Liberibacter solanacearum' and revealed that both species harbor unique strains of Wolbachia and Sodalis. However, the presence of Wolbachia and Sodalis varied among haplotypes of B. cockerelli. The central and western haplotypes harbored the same strains of Wolbachia, which was confirmed by Sanger sequencing of the wsp and ftsZ genes. Wolbachia was also detected in very low abundance from the northwestern haplotype by high-throughput sequencing of 16S but was not detected from this haplotype by PCR screening. The northwestern and central haplotypes also harbored Sodalis, which was not detected in the western haplotype. Heterotrioza chenopodii harbored an entirely different community of potential endosymbionts compared with the Bactericera spp. that included Rickettsia and an unidentified bacterium in the Enterobacteriaceae. Results of this study provide a foundation for further research on the interactions between psyllids and their bacterial endosymbionts.

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.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-021-02429-2.

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.

Uncovering Symbionts Across the Psyllid Tree of Life and the Discovery of a New Liberibacter Species, "Candidatus" Liberibacter capsica

Sap-feeding insects in the order Hemiptera associate with obligate endosymbionts that are required for survival and facultative endosymbionts that can potentially modify resistance to stress, enemies, development, and reproduction. In the superfamily Psylloidea, the jumping plant lice (psyllids), less is known about the diversity and prevalence of their endosymbionts compared to other sap-feeding pests such as aphids (Aphididae). To address this knowledge gap, using 16S rRNA sequencing we identify symbionts across divergent psyllid host lineages from around the world. Taking advantage of a new comprehensive phylogenomic analyses of Psylloidea, we included psyllid samples from 44 species of 35 genera of five families, collected from 11 international locations for this study. Across psyllid lineages, a total of 91 OTUs were recovered, predominantly of the Enterobacteriaceae (68%). The diversity of endosymbionts harbored by each psyllid species was low with an average of approximately 3 OTUs. Two clades of endosymbionts (clade 1 and 2), belonging to Enterobacteriaceae, were identified that appear to be long term endosymbionts of the psyllid families Triozidae and Psyllidae, respectively. We also conducted high throughput metagenomic sequencing on three Ca. Liberibacter infected psyllid species (Russelliana capsici, Trichochermes walkeri, and Macrohomotoma gladiata), initially identified from 16S rRNA sequencing, to obtain more genomic information on these putative Liberibacter plant pathogens. The phylogenomic analyses from these data identified a new Ca. Liberibacter species, Candidatus Liberibacter capsica, that is a potential pathogen of solanaceous crops. This new species shares a distant ancestor with Ca. L. americanus, which occurs in the same range as R. capsici in South America. We also detected the first association between a psyllid specializing on woody hosts and the Liberibacter species Ca. L. psyllaurous, which is a globally distributed pathogen of herbaceous crop hosts in the Solanaceae. Finally, we detected a potential association between a psyllid pest of figs (M. gladiata) and a Ca. Liberibacter related to Ca. L. asiaticus, which causes severe disease in citrus. Our findings reveal a wider diversity of associations between facultative symbionts and psyllids than previously reported and suggest numerous avenues for future work to clarify novel associations of ecological, evolutionary, and pathogenic interest.

Speciation across the Tree of Life

Much of what we know about speciation comes from detailed studies of well-known model systems. Although there have been several important syntheses on speciation, few (if any) have explicitly compared speciation among major groups across the Tree of Life. Here, we synthesize and compare what is known about key aspects of speciation across taxa, including bacteria, protists, fungi, plants, and major animal groups. We focus on three main questions. Is allopatric speciation predominant across groups? How common is ecological divergence of sister species (a requirement for ecological speciation), and on what niche axes do species diverge in each group? What are the reproductive isolating barriers in each group? Our review suggests the following patterns. (i) Based on our survey and projected species numbers, the most frequent speciation process across the Tree of Life may be co-speciation between endosymbiotic bacteria and their insect hosts. (ii) Allopatric speciation appears to be present in all major groups, and may be the most common mode in both animals and plants, based on non-overlapping ranges of sister species. (iii) Full sympatry of sister species is also widespread, and may be more common in fungi than allopatry. (iv) Full sympatry of sister species is more common in some marine animals than in terrestrial and freshwater ones. (v) Ecological divergence of sister species is widespread in all groups, including ~70% of surveyed species pairs of plants and insects. (vi) Major axes of ecological divergence involve species interactions (e.g. host-switching) and habitat divergence. (vii) Prezygotic isolation appears to be generally more widespread and important than postzygotic isolation. (viii) Rates of diversification (and presumably speciation) are strikingly different across groups, with the fastest rates in plants, and successively slower rates in animals, fungi, and protists, with the slowest rates in prokaryotes. Overall, our study represents an initial step towards understanding general patterns in speciation across all organisms.

Whitefly Endosymbionts: Biology, Evolution, and Plant Virus Interactions

Whiteflies (Hemiptera: Aleyrodidae) are sap-feeding global agricultural pests. These piercing-sucking insects have coevolved with intracellular endosymbiotic bacteria that help to supplement their nutrient-poor plant sap diets with essential amino acids and carotenoids. These obligate, primary endosymbionts have been incorporated into specialized organs called bacteriomes where they sometimes coexist with facultative, secondary endosymbionts. All whitefly species harbor the primary endosymbiont Candidatus Portiera aleyrodidarum and have a variable number of secondary endosymbionts. The secondary endosymbiont complement harbored by the cryptic whitefly species Bemisia tabaci is particularly complex with various assemblages of seven different genera identified to date. In this review, we discuss whitefly associated primary and secondary endosymbionts. We focus on those associated with the notorious B. tabaci species complex with emphasis on their biological characteristics and diversity. We also discuss their interactions with phytopathogenic begomoviruses (family Geminiviridae), which are transmitted exclusively by B. tabaci in a persistent-circulative manner. Unraveling the complex interactions of these endosymbionts with their insect hosts and plant viruses could lead to advancements in whitefly and whitefly transmitted virus management.

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.

Characterization of the bacterial communities of psyllids associated with Rutaceae in Bhutan by high throughput sequencing

Background

Several plant-pathogenic bacteria are transmitted by insect vector species that often also act as hosts. In this interface, these bacteria encounter plant endophytic, insect endosymbiotic and other microbes. Here, we used high throughput sequencing to examine the bacterial communities of five different psyllids associated with citrus and related plants of Rutaceae in Bhutan: Diaphorina citri, Diaphorina communis, Cornopsylla rotundiconis, Cacopsylla heterogena and an unidentified Cacopsylla sp.

Results

The microbiomes of the psyllids largely comprised their obligate P-endosymbiont ‘Candidatus Carsonella ruddii’, and one or two S-endosymbionts that are fixed and specific to each lineage. In addition, all contained Wolbachia strains; the Bhutanese accessions of D. citri were dominated by a Wolbachia strain first found in American isolates of D. citri, while D. communis accessions were dominated by the Wolbachia strain, wDi, first detected in D. citri from China. The S-endosymbionts from the five psyllids grouped with those from other psyllid taxa; all D. citri and D. communis individuals contained sequences matching ‘Candidatus Profftella armatura’ that has previously only been reported from other Diaphorina species, and the remaining psyllid species contained OTUs related to unclassified Enterobacteriaceae. The plant pathogenic ‘Candidatus Liberibacter asiaticus’ was found in D. citri but not in D. communis. Furthermore, an unidentified ‘Candidatus Liberibacter sp.’ occurred at low abundance in both Co. rotundiconis and the unidentified Cacopsylla sp. sampled from Zanthoxylum sp.; the status of this new liberibacter as a plant pathogen and its potential plant hosts are currently unknown. The bacterial communities of Co. rotundiconis also contained a range of OTUs with similarities to bacteria previously found in samples taken from various environmental sources.

Conclusions

The bacterial microbiota detected in these Bhutanese psyllids support the trends that have been seen in previous studies: psyllids have microbiomes largely comprising their obligate P-endosymbiont and one or two S-endosymbionts. In addition, the association with plant pathogens has been demonstrated, with the detection of liberibacters in a known host, D. citri, and identification of a putative new species of liberibacter in Co. rotundiconis and Cacopsylla sp.

An Intranuclear Sodalis-Like Symbiont and Spiroplasma Coinfect the Carrot Psyllid, Bactericera trigonica (Hemiptera, Psylloidea)

Endosymbionts harbored inside insects play critical roles in the biology of their insect host and can influence the transmission of pathogens by insect vectors. Bactericera trigonica infests umbelliferous plants and transmits the bacterial plant pathogen Candidatus Liberibacter solanacearum (CLso), causing carrot yellows disease. To characterize the bacterial diversity of B. trigonica, as a first step, we used PCR-restriction fragment length polymorphism (PCR-RFLP) and denaturing gradient gel electrophoresis (DGGE) analyses of 16S rDNA to identify Sodalis and Spiroplasma endosymbionts. The prevalence of both symbionts in field-collected psyllid populations was determined: Sodalis was detected in 100% of field populations, while Spiroplasma was present in 82.5% of individuals. Phylogenetic analysis using 16S rDNA revealed that Sodalis infecting B. trigonica was more closely related to symbionts infecting weevils, stink bugs and tsetse flies than to those from psyllid species. Using fluorescent in situ hybridization and immunostaining, Sodalis was found to be localized inside the nuclei of the midgut cells and bacteriocytes. Spiroplasma was restricted to the cytoplasm of the midgut cells. We further show that a recently reported Bactericera trigonica densovirus (BtDNV), a densovirus infecting B. trigonica was detected in 100% of psyllids and has reduced titers inside CLso-infected psyllids by more than two-fold compared to CLso uninfected psyllids. The findings of this study will help to increase our understanding of psyllid-endosymbiont interactions.

Insight into the microbial world of Bemisia tabaci cryptic species complex and its relationships with its host

The 37 currently recognized Bemisia tabaci cryptic species are economically important species and contain both primary and secondary endosymbionts, but their diversity has never been mapped systematically across the group. To achieve this, PacBio sequencing of full-length bacterial 16S rRNA gene amplicons was carried out on 21 globally collected species in the B. tabaci complex, and two samples from B. afer were used here as outgroups. The microbial diversity was first explored across the major lineages of the whole group and 15 new putative bacterial sequences were observed. Extensive comparison of our results with previous endosymbiont diversity surveys which used PCR or multiplex 454 pyrosequencing platforms showed that the bacterial diversity was underestimated. To validate these new putative bacteria, one of them (Halomonas) was first confirmed to be present in MED B. tabaci using Hiseq2500 and FISH technologies. These results confirmed PacBio is a reliable and informative venue to reveal the bacterial diversity of insects. In addition, many new secondary endosymbiotic strains of Rickettsia and Arsenophonus were found, increasing the known diversity in these groups. For the previously described primary endosymbionts, one Portiera Operational Taxonomic Units (OTU) was shared by all B. tabaci species. The congruence of the B. tabaci-host and Portiera phylogenetic trees provides strong support for the hypothesis that primary endosymbionts co-speciated with their hosts. Likewise, a comparison of bacterial alpha diversities, Principal Coordinate Analysis, indistinct endosymbiotic communities harbored by different species and the co-divergence analyses suggest a lack of association between overall microbial diversity with cryptic species, further indicate that the secondary endosymbiont-mediated speciation is unlikely to have occurred in the B. tabaci species group.

Partnering With a Pest: Genomes of Hemlock Woolly Adelgid Symbionts Reveal Atypical Nutritional Provisioning Patterns in Dual-Obligate Bacteria

Nutritional bacterial symbionts enhance the diets of sap-feeding insects with amino acids and vitamins missing from their diets. In many lineages, an ancestral senior symbiont is joined by a younger junior symbiont. To date, an emergent pattern is that senior symbionts supply a majority of amino acids, and junior symbionts supply a minority. Similar to other hemipterans, adelgids harbor obligate symbionts, but have higher diversity of bacterial associates, suggesting a history of symbiont turnover. The metabolic roles of dual symbionts in adelgids and their contributions to the consortium are largely unexplored. Here, we investigate the symbionts of Adelges tsugae, the hemlock woolly adelgid (HWA), an invasive species introduced from Japan to the eastern United States, where it kills hemlock trees. The response of hemlocks to HWA feeding has aspects of a defensive reaction against pathogens, and some have speculated that symbionts may be involved. We sequenced the genomes of "Ca. Annandia adelgestsuga" and "Ca. Pseudomonas adelgestsugas" symbionts to detail their metabolic capabilities, infer ages of relationship, and search for effectors of plant defenses. We also tested the relationship of "Ca. Annandia" to symbionts of other insects. We find that both symbionts provide nutrients, but in more balanced proportions than dual symbionts of other hemipterans. The lesser contributions of the senior "Ca. Annandia" support our hypothesis for symbiont replacements in adelgids. Phylogenomic results were ambiguous regarding the position of "Ca. Annandia". We found no obvious effectors of plant defenses related to insect virulence, but hypothetical proteins in symbionts are unknown players.

The evolutionary development of plant-feeding insects and their nutritional endosymbionts

Herbivorous insects have evolved diverse mechanisms enabling them to feed on plants with suboptimal nutrient availability. Low nutrient availability negatively impacts insect herbivore development and fitness. To overcome this obstacle numerous insect lineages have evolved intimate associations with nutritional endosymbionts. This is especially true for insects that specialize on nitrogen-poor substrates, as these insects are highly dependent on intracellular symbionts to provide nitrogen lacking in their insect host's diet. Emerging evidence in these systems suggest that the symbiont's and/or the insect's biosynthetic pathways are dynamically regulated throughout the insect's development to potentially cope with the insect's changing nutritional demands. In this review, we evaluate the evolutionary development of symbiotic insect cells (bacteriocytes) by comparing and contrasting genes and mechanisms involved in maintaining and regulating the nutritional symbiosis throughout insect development in a diversity of insect herbivore-endosymbiont associations. With new advances in genome sequencing and functional genomics, we evaluate to what extent nutritional symbioses are shaped by (i) the regulation of symbiont titer, (ii) the regulation of insect symbiosis genes, and (iii) the regulation of symbiont genes. We discuss how important these mechanisms are for the biosynthesis of essential amino acids and vitamins across insect life stages in divergent insect-symbiont systems. We conclude by suggesting future directions of research to further elucidate the evolutionary development of bacteriocytes and the impact of these nutritional symbioses on insect-plant interactions.

Genetic diversity of Diaphorina citri and its endosymbionts across east and south-east Asia

BACKGROUND

Diaphorina citri is the vector of 'Candidatus Liberibacter asiaticus', the most widespread pathogen associated huanglongbing, the most serious disease of citrus. To enhance our understanding of the distribution and origin of the psyllid, we investigated the genetic diversity and population structures of 24 populations in Asia and one from Florida based on the mtCOI gene. Simultaneously, genetic diversity and population structures of the primary endosymbiont (P-endosymbiont) 'Candidatus Carsonella ruddii' and secondary endosymbiont (S-endosymbiont) 'Candidatus Profftella armatura' of D. citri were determined with the housekeeping genes.

RESULT

AMOVA analysis indicated that populations of D. citri and its endosymbionts in east and south-east Asia were genetically distinct from populations in Pakistan and Florida. Furthermore, P-endosymbiont populations displayed a strong geographical structure across east and south-east Asia, while low genetic diversity indicated the absence of genetic structure among the populations of D. citri and its S-endosymbiont across these regions.

CONCLUSION

The 'Ca. C. ruddii' is more diverse and structured than the D. citri and the 'Ca. P. armatura' across east and south-east Asia. Multiple introductions of the psyllid have occurred in China. Management application for controlling the pest is proposed based on the genetic information of D. citri and its endosymbionts. © 2017 Society of Chemical Industry.

Dynamic Acquisition and Loss of Dual-Obligate Symbionts in the Plant-Sap-Feeding Adelgidae (Hemiptera: Sternorrhyncha: Aphidoidea)

Sap-sucking insects typically engage in obligate relationships with symbiotic bacteria that play nutritional roles in synthesizing nutrients unavailable or in scarce supply from the plant-sap diets of their hosts. Adelgids are sap-sucking insects with complex life cycles that involve alternation between conifer tree species. While all adelgid species feed on spruce during the sexual phase of their life cycle, each adelgid species belongs to a major lineage that feeds on a distinct genus of conifers as their alternate host. Previous work on adelgid symbionts had discovered pairs of symbionts within each host species, and unusual diversity across the insect family, but left several open questions regarding the status of bacterial associates. Here, we explored the consistency of symbionts within and across adelgid lineages, and sought evidence for facultative vs. obligate symbiont status. Representative species were surveyed for symbionts using 16S ribosomal DNA gene sequencing, confirming that different symbiont pairs were consistently present within each major adelgid lineage. Several approaches were used to establish whether symbionts exhibited characteristics of long-term, obligate mutualists. Patterns of symbiont presence across adelgid species and diversification with host insects suggested obligate relationships. Fluorescent in situ hybridization and electron microscopy localized symbionts to bacteriocyte cells within the bacteriome of each species (with one previously known exception), and detection of symbionts in eggs indicated their vertical transmission. Common characteristics of long-term obligate symbionts, such as nucleotide compositional bias and pleomorphic symbiont cell shape were also observed. Superimposing microbial symbionts on the adelgid phylogeny revealed a dynamic pattern of symbiont gains and losses over a relatively short period of time compared to other symbionts associated with sap-sucking insects, with each adelgid species possessing an older, “senior” symbiont and a younger “junior” symbiont. A hypothesis relating adelgid life cycles to relaxed constraints on symbionts is proposed, with the degradation of senior symbionts and repeated acquisition of more junior symbionts creating opportunities for repeated colonization of new alternate-conifer hosts by adelgids.

Symbionts in waiting: the dynamics of incipient endosymbiont complementation and replacement in minimal bacterial communities of psyllids

BACKGROUND

Obligate bacterial primary (P-) endosymbionts that are maternally inherited and codiverge with hosts are widespread across insect lineages with nutritionally restricted diets. Secondary (S-) endosymbionts are mostly facultative, but in some hosts, they complement P-endosymbiont function and therefore become obligate. Phylogenetic evidence exists for host switching and replacement of S-endosymbionts. The community dynamics that precede endosymbiont replacement and complementation have been little studied across host species, yet they are fundamental to the evolution of endosymbiosis.

RESULTS

We performed bacterial 16S rRNA gene amplicon sequencing of 25 psyllid species (Hemiptera, Psylloidea) across different developmental stages and ecological niches by focusing on the characterisation of the bacteria other than the universally present P-endosymbiont Carsonella (Gammaproteobacteria). Most species harboured only one dominant representative of diverse gammaproteobacterial S-endosymbionts that was consistently detected across all host individuals and populations (Arsenophonus in eight species, Sodalis or Sodalis-like bacteria in four species, unclassified Enterobacteriaceae in eight species). The identity of this dominant obligate S-endosymbiont varied across closely related host species. Unexpectedly, five psyllid species had two or three co-occurring endosymbiont species other than Carsonella within all host individuals, including a Rickettsiella-like bacterium (Gammaproteobacteria) in one psyllid species. Based on standard and quantitative PCR, all psyllids carried Carsonella, at higher titres than their dominant S-endosymbionts. Some psyllids also had Alphaproteobacteria (Lariskella, Rickettsia, Wolbachia) at varying prevalence. Incidence of other bacteria, including known plant pathogens, was low. Ecological niche of gall-forming, lerp-forming and free-living psyllid species did not impact endosymbiont communities. Two flush-feeding psyllid species had population-specific differences, and this was attributable to the higher endosymbiont diversity in native ranges and the absence of some endosymbionts in invasive ranges.

CONCLUSIONS

Our data support the hypothesis of strict vertical transmission of minimal core communities of bacteria in psyllids. We also found evidence for S-endosymbiont replacement across closely related psyllid species. Multiple dominant S-endosymbionts present in some host species, including at low titre, constitute potential examples of incipient endosymbiont complementation or replacement. Our multiple comparisons of deep-sequenced minimal insect bacterial communities exposed the dynamics involved in shaping insect endosymbiosis.

Bacterial Endosymbionts of the Psyllid Cacopsylla pyricola (Hemiptera: Psyllidae) in the Pacific Northwestern United States

Insects often have facultative associations with bacterial endosymbionts, which can alter the insects' susceptibility to parasitism, pathogens, plant defenses, and certain classes of insecticides. We collected pear psylla, Cacopsylla pyricola (Förster) (Hemiptera: Psyllidae), from pear orchards in Washington and Oregon, and surveyed them for the presence of bacterial endosymbionts. Adult psyllids were collected on multiple dates to allow us to assay specimens of both the summer ("summerform") and the overwintering ("winterform") morphotypes. Two endosymbionts, Arsenophonus and Phytoplasma pyri, were detected in psyllids of both morphotypes in both states. A separate survey revealed similar associations present in psyllids collected in 1987. Arsenophonus was present in 80-100% of psyllids in all growing regions. A slightly lower proportion of summerform than winterform psyllids harbored the bacterium. Arsenophonus was present in the bacteriomes and developing oocytes of most psyllids, indicating that this endosymbiont is transovarially transmitted. This bacterium was also observed in the salivary glands and midguts of some psyllids. Phytoplasma pyri was present in a greater proportion of pear psylla from orchards near Yakima, WA, than from other regions, and was present in a higher proportion of winterforms than summerforms. We did not detect Wolbachia, Profftella, or Liberibacter europaeus, which are associated with other psyllid pests, including other species of Cacopsylla. Our study is the first to survey North American populations of C. pyricola for endosymbionts, and provides a foundation for further research on how bacterial associations may influence the ecology and management of this pest.

Phylogeographic analyses of bacterial endosymbionts in fig homotomids (Hemiptera: Psylloidea) reveal codiversification of both primary and secondary endosymbionts

While obligate primary (P-) endosymbionts usually cospeciate with their insect hosts, less is known about codiversification of secondary (S-) endosymbionts that are generally considered facultative. Typically, insects of the superfamily Psylloidea harbour one P- (Carsonella) and at least one S-endosymbiont, thought to compensate for Carsonella genome reduction. Most codiversification studies have used phylogenies of psyllids and their endosymbionts across and within host families or genera, but few have explored patterns within species. We focussed on P- and S-endosymbionts of three Mycopsylla (Homotomidae) species to explore whether they have congruent phylogenies and within-species geographic structures. The P-endosymbiont Carsonella, a S-endosymbiont and Wolbachia all had 100% prevalence, while Arsenophonus was only found in one species at low prevalence. Congruent phylogenies of Mycopsylla and P-endosymbionts across populations and species support strict cospeciation. S-endosymbiont phylogenies were also congruent across host species but low genetic variation in the S-endosymbiont was not correlated with host phylogeography, possibly due to a shorter evolutionary association. Between species, Wolbachia and Mycopsylla phylogenies were incongruent, probably due to horizontal transmission events. Our study is the first to explore endosymbionts of Mycopsylla and further supports the codivergence of Psylloidea hosts and P-endosymbionts, with obligate host interactions for both P- and S-endosymbionts.

Development of ovary structures in the last larval and adult stages of psyllids (Insecta, Hemiptera, Sternorrhyncha: Psylloidea)

The development and organization of the ovaries of ten species from four Psylloidea families (Psyllidae, Triozidae, Aphalaridae and Liviidae) have been investigated. The ovaries of the last larval stage (i.e. fifth instar) of all examined species are filled with numerous clusters of cystocytes which undergo synchronous incomplete mitotic division. Cystocytes of the given cluster are arranged into a rosette with polyfusome in the centre. These clusters are associated with single somatic cells. At the end of the fifth instar, the clusters begin to separate from each other, forming spherical ovarioles which are surrounded by a single layer of somatic cells. In the ovarioles of very young females all cystocytes enter the prophase of meiosis and differentiate shortly thereafter into oocytes and trophocytes (nurse cells). Meanwhile, somatic cells differentiate into cells of the inner epithelial sheath surrounding the trophocytes and into the prefollicular cells that encompass the oocytes. During this final differentiation, the trophocytes lose their cell membranes and become syncytial. Oocytes remain cellular and most of them (termed arrested oocytes) do not grow. In the ovarioles of older females, one oocyte encompassed by its follicle cells starts growing, still connected to the syncytial tropharium by a nutritive cord. After the short phase of previtellogenesis alone, the oocyte enters its vitellogenic the growth phase in the vitellarium. At that time, the second oocyte may enter the vitellarium and start its previtellogenic growth. In the light of the obtained results, the phylogeny of psyllids, as well as phylogenetic relationships between taxa of Hemiptera: Sternorrhyncha are discussed.

Global genetic variation in the Asian citrus psyllid, Diaphorina citri (Hemiptera: Liviidae) and the endosymbiont Wolbachia: links between Iran and the USA detected

BACKGROUND

The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), is one of the most serious pests of citrus in the world, because it transmits the pathogen that causes citrus greening disease. To determine genetic variation among geographic populations of D. citri, microsatellite markers, mitochondrial gene cytochrome oxidase I (mtCOI) and the Wolbachia-Diaphorina, wDi, gene wsp sequence data were used to characterize Iranian and Pakistani populations. Also, a Bayesian phylogenetic technique was utilized to elucidate the relationships among the sequences data in this study and all mtCOI and wsp sequence data available in GenBank and the Wolbachia database.

RESULTS

Microsatellite markers revealed significant genetic differentiation among Iranian populations, as well as between Iranian and Pakistani populations (FST  = 0.0428, p < 0.01). Within Iran, the Sistan-Baluchestan population is significantly different from the Hormozgan (Fareghan) and Fars populations. By contrast, mtCOI data revealed two polymorphic sites separating the sequences from Iran and Pakistan. Global phylogenetic analyses showed that D. citri populations in Iran, India, Saudi Arabia, Brazil, Mexico, Florida and Texas (USA) are similar. Wolbachia, wDi, wsp sequences were similar among Iranian populations, but different between Iranian and Pakistani populations.

CONCLUSION

The South West Asia (SWA) group is the most likely source of the introduced Iranian populations of D. citri. This assertion is also supported by the sequence similarity of the Wolbachia, wDi, strains from the Florida, USA and Iranian D. citri. These results should be considered when looking for biological controls in either country.

Evolutionary relationships of Pemphigus and allied genera (Hemiptera: Aphididae: Eriosomatinae) and their primary endosymbiont, Buchnera aphidicola

Aphids harbor primary endosymbionts, Buchnera aphidicola, in specialized cells within their body cavities. Aphids and Buchnera have strict mutualistic relationships in nutrition exchange. This ancient association has received much attention from researchers who are interested in endosymbiotic evolution. Previous studies have found parallel phylogenetic relationships between non-galling aphids and Buchnera at lower taxonomic levels (genus, species). To understand whether relatively isolated habitats such as galls have effect on the parallel relationships between aphids and Buchnera, the present paper investigated the phylogenetic relationships of gall aphids from Pemphigus and allied genera, which induce pseudo-galls or galls on Populus spp. (poplar) and Buchnera. The molecular phylogenies inferred from three aphid genes (COI, COII and EF-1α) and two Buchnera genes (gnd, 16S rRNA gene) indicated significant congruence between aphids and Buchnera at generic as well as interspecific levels. Interestingly, both aphid and Buchnera phylogenies supported three main clades corresponding to the galling locations of aphids, namely leaf, the joint of leaf blade and petiole, and branch of the host plant. The results suggest phylogenetic conservatism of gall characters, which indicates gall characters are more strongly affected by aphid phylogeny, rather than host plants.

Shoot feeding as a nutrient acquisition strategy in free-living psylloids

Shoot feeding by sucking insects is accepted as an adaptation to feeding where plant nutrients are most concentrated and/or of higher quality. Psylloids are an important hemipteran taxon, most of which are free-living and comprise many shoot feeding species, whose nutritional ecology has been largely ignored. I conducted a longitudinal study of Ctenarytaina eucalypti (Maskell) and C. bipartita Burckhardt et al. (Aphalaridae) feeding on eucalypts to document how within-plant (ontogenic) variation in nutritional quality, in particular of free amino acids, determines host suitability and hence the distribution and abundance of nymphs. Nymphs were most abundant within developing apical buds but were not more abundant on branchlets of greater vigour (indicated by rate of extension). Nymphs could be found up to two (C. bipartita) to three (C. eucalypti) alternate leaf pairs distant from apical buds but infrequently and in low numbers; they were never found on older, fully expanded leaves. The position of a leaf on a branchlet (indicative of age) determined its nutritional quality. Younger leaves had higher water contents, lower chlorophyll contents and differed in amino acid (essential and non-essential) composition compared to older leaves. The abundance of C. eucalypti nymphs on expanding leaves and in buds was positively correlated with the concentrations of methionine, valine and threonine in E. globulus leaves at the same or comparable position on a branchlet. The abundance of C. bipartita nymphs was positively correlated with foliar leucine concentrations. Shoot feeding by these two psyllids facilitates access to more concentrated, better quality plant nutrients but may not entirely explain the adaptive significance of their behaviour. The humid microclimate created by the architecture of the hosts’ apical buds protects eggs and nymphs from desiccation and is suggested to have had a significant influence on the evolution of host utilisation strategies of psyllids within this genus.

Genome evolution: a bacterium with a Napoleon complex

New work on an important agricultural pest reveals an unexpected toxin-producing defensive bacterial symbiont. Surprisingly, the symbiont's genome is highly reduced, with genes devoted to polyketide synthesis making up a large fraction of its coding capacity.

Novel Rickettsiella bacterium in the leafhopper Orosius albicinctus (Hemiptera: Cicadellidae)

Bacteria in the genus Rickettsiella (Coxiellaceae), which are mainly known as arthropod pathogens, are emerging as excellent models to study transitions between mutualism and pathogenicity. The current report characterizes a novel Rickettsiella found in the leafhopper Orosius albicinctus (Hemiptera: Cicadellidae), a major vector of phytoplasma diseases in Europe and Asia. Denaturing gradient gel electrophoresis (DGGE) and pyrosequencing were used to survey the main symbionts of O. albicinctus, revealing the obligate symbionts Sulcia and Nasuia, and the facultative symbionts Arsenophonus and Wolbachia, in addition to Rickettsiella. The leafhopper Rickettsiella is allied with bacteria found in ticks. Screening O. albicinctus from the field showed that Rickettsiella is highly prevalent, with over 60% of individuals infected. A stable Rickettsiella infection was maintained in a leafhopper laboratory colony for at least 10 generations, and fluorescence microscopy localized bacteria to accessory glands of the female reproductive tract, suggesting that the bacterium is vertically transmitted. Future studies will be needed to examine how Rickettsiella affects host fitess and its ability to vector phytopathogens.

Evolutionary replacement of obligate symbionts in an ancient and diverse insect lineage

Many insect groups depend on ancient obligate symbioses with bacteria that undergo long-term genomic degradation due to inactivation and loss of ancestral genes. Sap-feeding insects in the hemipteran suborder Auchenorrhyncha show complex symbioses with at least two obligate bacterial symbionts, inhabiting specialized host cells (bacteriocytes). We explored the symbiotic relationships of the spittlebugs (Auchenorrhyncha: Cercopoidea) using phylogenetic and microscopy methods. Results show that most spittlebugs contain the symbionts Sulcia muelleri (Bacteroidetes) and Zinderia insecticola (Betaproteobacteria) with each restricted to its own bacteriocyte type. However, the ancestral Zinderia symbiont has been replaced with a novel symbiont closely related to Sodalis glossinidius (Enterobacteriaceae) in members of the ecologically successful spittlebug tribe Philaenini. At least one spittlebug species retains Sulcia and Zinderia, but also has acquired a Sodalis-like symbiont, possibly representing a transitional stage in the evolutionary succession of symbioses. Phylogenetic analyses including symbionts of other Auchenorrhyncha lineages suggest that Zinderia, like Sulcia, descends from an ancestral symbiont present in the common ancestor of Auchenorrhyncha. This betaproteobacterial symbiont has been repeatedly replaced by other symbionts, such as the Sodalis-like symbiont of spittlebugs. Symbiont replacement may offer a route for hosts to escape dependence on an ancient, degraded and potentially inefficient symbiont.

Diversity of proteobacterial endosymbionts in hemlock woolly adelgid (Adelges tsugae) (Hemiptera: Adelgidae) from its native and introduced range

Knowledge of intraspecific variation in symbioses may aid in understanding the ecology of widespread insects in different parts of their range. We investigated bacterial symbionts of Adelges tsugae, a pest of hemlocks in eastern North America introduced from Asia. Amplification, cloning, and sequencing of bacterial 16S rDNA, in situ hybridizations, and electron microscopy revealed that A. tsugae harbours up to five bacterial phylotypes, according to population. Three Gammaproteobacteria species are maternally transmitted. The first, designated 'Ca. Pseudomonas adelgestsugas' resides in the haemocoel, and was detected in all populations except Taiwan. The second phylotype, 'Ca. Serratia symbiotica', resides in bacteriocytes of populations on Tsuga sieboldii in Japan and in E. North America. The third phylotype, designated 'Ca. Annandia adelgestsuga', clustered within a lineage of several insect endosymbionts that included Buchnera aphidicola. It was detected in bacteriocytes in all populations, and in salivary glands of first instars. Two Betaproteobacteria phylotypes were detected in some Japanese T. sieboldii and eastern North America populations, and were observed only in salivary glands with no evidence of maternal transmission. Our results support the ideas that symbiont gain and loss has been volatile in adelgids, and that symbionts may help to trace the source of invasive species.

Genome reduction and co-evolution between the primary and secondary bacterial symbionts of psyllids

Genome reduction in obligately intracellular bacteria is one of the most well-established patterns in the field of molecular evolution. In the extreme, many sap-feeding insects harbor nutritional symbionts with genomes that are so reduced that it is not clear how they perform basic cellular functions. For example, the primary symbiont of psyllids (Carsonella) maintains one of the smallest and most AT-rich bacterial genomes ever identified and has surprisingly lost many genes that are thought to be essential for its role in provisioning its host with amino acids. However, our understanding of this extreme case of genome reduction is limited, as genomic data for Carsonella are available from only a single host species, and little is known about the functional role of "secondary" bacterial symbionts in psyllids. To address these limitations, we analyzed complete Carsonella genomes from pairs of congeneric hosts in three divergent genera within the Psyllidae (Ctenarytaina, Heteropsylla, and Pachypsylla) as well as complete secondary symbiont genomes from two of these host species (Ctenarytaina eucalypti and Heteropsylla cubana). Although the Carsonella genomes are generally conserved in size, structure, and GC content and exhibit genome-wide signatures of purifying selection, we found that gene loss has remained active since the divergence of the host species and had a particularly large impact on the amino acid biosynthesis pathways that define the symbiotic role of Carsonella. In some cases, the presence of additional bacterial symbionts may compensate for gene loss in Carsonella, as functional gene content indicates a high degree of metabolic complementarity between co-occurring symbionts. The genomes of the secondary symbionts also show signatures of long-term evolution as vertically transmitted, intracellular bacteria, including more extensive genome reduction than typically observed in facultative symbionts. Therefore, a history of co-evolution with secondary bacterial symbionts can partially explain the ongoing genome reduction in Carsonella. However, the absence of these secondary symbionts in other host lineages indicates that the relationships are dynamic and that other mechanisms, such as changes in host diet or functional coordination with the host genome, must also be at play.

'Candidatus Liberibacter europaeus' sp. nov. that is associated with and transmitted by the psyllid Cacopsylla pyri apparently behaves as an endophyte rather than a pathogen

'Candidatus Liberibacter spp.' cause serious plant diseases. 'Candidatus Liberibacter asiaticus', 'Ca. L. americanus' and 'Ca. L. africanus' are the aetiological agents of citrus greening (Huanglongbing) in Asia, America and Africa. 'Candidatus Liberibacter solanacearum' causes diseases in Solanaceae in America and New Zealand. All four species are vectored by psyllid insects of different genera. Here, we show that the pear psyllid pest Cacopsylla pyri (L.) hosts a novel liberibacter species that we named 'Ca. Liberibacter europaeus'. It can bloom to high titres in the psyllid host, with more than 10(9) 16S rRNA gene copies per individual. Fluorescent in situ hybridization experiments showed that 'Ca. L. europaeus' is present in the host midgut lumen, salivary glands and Malpighian tubules. 'Candidatus L. europaeus' has a relatively high prevalence (> 51%) in C. pyri from different areas in the Piedmont and Valle d'Aosta regions in Italy and can be transmitted to pear plants in experimental transmission trials. However, even though high titres of the bacterium (more than 10(8) 16S rRNA gene copies g(-1) of pear plant tissue) could be detected, in the pear tissues no specific disease symptoms could be observed in the infected plants over a 6-month period. Despite liberibacters representing potential quarantine organisms, 'Ca. L. europaeus', first described in Italy and Europe, apparently behaves as an endophyte rather than a pathogen.

A novel obligate intracellular gamma-proteobacterium associated with ixodid ticks, Diplorickettsia massiliensis, Gen. Nov., Sp. Nov

BACKGROUND

Obligate intracellular bacteria of arthropods often exhibit a significant role in either human health or arthropod ecology.

METHODOLOGY/PRINCIPAL FINDINGS

An obligate intracellular gamma-proteobacterium was isolated from the actively questing hard tick Ixodes ricinus using mammalian and amphibian cell lines. Transmission electron microscopy revealed a unique morphology of the bacterium, including intravacuolar localization of bacteria grouped predominantly in pairs and internal structures composed of electron-dense crystal-like structures and regular multilayer sheath-like structures. The isolate 20B was characterized to determine its taxonomic position using a polyphasic approach. Comparative 16S rRNA gene sequence analysis showed that this strain belongs to the family Coxiellaceae, order Legionellales of Gamma-proteobacteria, and the closest relatives are different Rickettsiella spp. The level of 16S rRNA gene sequence similarity between strain 20B and other recognized species of the family was below 94.5%. Partial sequences of the rpoB, parC and ftsY genes confirmed the phylogenetic position of the new isolate. The G+C content estimated on the basis of whole genome analysis of strain 20B was 37.88%. On the basis of its phenotypic and genotypic properties, together with phylogenetic distinctiveness, we propose that strain 20B to be classified in the new genus Diplorickettsia as the type strain of a novel species named Diplorickettsia massiliensis sp. nov.

CONCLUSIONS/SIGNIFICANCE

Considering the source of its isolation (hard tick, often biting humans) the role of this bacterium in the pathology of humans, animals and ticks should be further investigated.

Genome size of Pachypsylla venusta (Hemiptera: Psyllidae) and the ploidy of its bacteriocyte, the symbiotic host cell that harbors intracellular mutualistic bacteria with the smallest cellular genome

Psyllids harbor the primary symbiont, Carsonella ruddii (gamma-Proteobacteria), within the cytoplasm of specialized cells called bacteriocytes. Carsonella has the smallest known cellular genome (160 kb), lacking numerous genes that appear to be essential for bacterial life. This raises the question regarding the genetic mechanisms of the host which supports the survival of Carsonella. Our preceding analyses have indicated that some of the genes that are encoded in the psyllid genome and which are highly expressed in the bacteriocyte are of bacterial origin. This implies that psyllids acquired genes from bacteria by lateral gene transfer (LGT) and are using these genes to maintain the primary symbiont, Carsonella. To reveal the complete picture of LGT from symbiotic bacteria to the genome of psyllids, whole genome analysis of psyllids is essential. In order to assess the feasibility of whole genome analysis of the host psyllid, the genome size of the hackberry petiole gall psyllid, Pachypsylla venusta, was estimated. Feulgen image analysis densitometry and flow cytometry demonstrated that the haploid genome size of P. venusta is 0.74 pg (724 Mb), verifying the feasibility of whole genome analysis. Feulgen image analysis densitometry further revealed that bacteriocytes of P. venusta are invariably 16-ploid. This higher ploidy may be essential to facilitate the symbiotic relationship with bacteria, as it appears to be a feature common to insect bacteriocytes. These results provide a foundation for genomics-based research into host-symbiont interactions.

One nutritional symbiosis begat another: phylogenetic evidence that the ant tribe Camponotini acquired Blochmannia by tending sap-feeding insects

Background

Bacterial endosymbiosis has a recurring significance in the evolution of insects. An estimated 10-20% of insect species depend on bacterial associates for their nutrition and reproductive viability. Members of the ant tribe Camponotini, the focus of this study, possess a stable, intracellular bacterial mutualist. The bacterium, Blochmannia, was first discovered in Camponotus and has since been documented in a distinct subgenus of Camponotus, Colobopsis, and in the related genus Polyrhachis. However, the distribution of Blochmannia throughout the Camponotini remains in question. Documenting the true host range of this bacterial mutualist is an important first step toward understanding the various ecological contexts in which it has evolved, and toward identifying its closest bacterial relatives. In this study, we performed a molecular screen, based on PCR amplification of 16S rDNA, to identify bacterial associates of diverse Camponotini species.

Results

Phylogenetic analyses of 16S rDNA gave four important insights: (i) Blochmannia occurs in a broad range of Camponotini genera including Calomyrmex, Echinopla, and Opisthopsis, and did not occur in outgroups related to this tribe (e.g., Notostigma). This suggests that the mutualism originated in the ancestor of the tribe Camponotini. (ii) The known bacteriocyte-associated symbionts of ants, in Formica, Plagiolepis, and the Camponotini, arose independently. (iii) Blochmannia is nestled within a diverse clade of endosymbionts of sap-feeding hemipteran insects, such as mealybugs, aphids, and psyllids. In our analyses, a group of secondary symbionts of mealybugs are the closest relatives of Blochmannia. (iv) Blochmannia has cospeciated with its known hosts, although deep divergences at the genus level remain uncertain.

Conclusions

The Blochmannia mutualism occurs in Calomyrmex, Echinopla, and Opisthopsis, in addition to Camponotus, and probably originated in the ancestral lineage leading to the Camponotini. This significant expansion of its known host range implies that the mutualism is more ancient and ecologically diverse than previously documented. Blochmannia is most closely related to endosymbionts of sap-feeding hemipterans, which ants tend for their carbohydrate-rich honeydew. Based on phylogenetic results, we propose Camponotini might have originally acquired this bacterial mutualist through a nutritional symbiosis with other insects.

Arsenophonus, an emerging clade of intracellular symbionts with a broad host distribution

BACKGROUND

The genus Arsenophonus is a group of symbiotic, mainly insect-associated bacteria with rapidly increasing number of records. It is known from a broad spectrum of hosts and symbiotic relationships varying from parasitic son-killers to coevolving mutualists.The present study extends the currently known diversity with 34 samples retrieved mainly from hippoboscid (Diptera: Hippoboscidae) and nycteribiid (Diptera: Nycteribiidae) hosts, and investigates phylogenetic relationships within the genus.

RESULTS

The analysis of 110 Arsenophonus sequences (incl. Riesia and Phlomobacter), provides a robust monophyletic clade, characterized by unique molecular synapomorphies. On the other hand, unstable inner topology indicates that complete understanding of Arsenophonus evolution cannot be achieved with 16S rDNA. Moreover, taxonomically restricted Sampling matrices prove sensitivity of the phylogenetic signal to sampling; in some cases, Arsenophonus monophyly is disrupted by other symbiotic bacteria. Two contrasting coevolutionary patterns occur throughout the tree: parallel host-symbiont evolution and the haphazard association of the symbionts with distant hosts. A further conspicuous feature of the topology is the occurrence of monophyletic symbiont lineages associated with monophyletic groups of hosts without a co-speciation pattern. We suggest that part of this incongruence could be caused by methodological artifacts, such as intragenomic variability.

CONCLUSION

The sample of currently available molecular data presents the genus Arsenophonus as one of the richest and most widespread clusters of insect symbiotic bacteria. The analysis of its phylogenetic lineages indicates a complex evolution and apparent ecological versatility with switches between entirely different life styles. Due to these properties, the genus should play an important role in the studies of evolutionary trends in insect intracellular symbionts. However, under the current practice, relying exclusively on 16S rRNA sequences, the phylogenetic analyses are sensitive to various methodological artifacts that may even lead to description of new Arsenophonus lineages as independent genera (e.g. Riesia and Phlomobacter). The resolution of the evolutionary questions encountered within the Arsenophonus clade will thus require identification of new molecular markers suitable for the low-level phylogenetics.

Specific detection and quantification of the phytopathogenic agent 'Candidatus Phytoplasma prunorum'

'Candidatus Phytoplasma prunorum' is a wall-less bacterium associated with European stone fruit yellows (ESFY), a severe disease of Prunus spp. (mainly apricot and Japanese plum trees). It can be spread by one insect vector, Cacopsylla pruni, and by the trade of infected material. The availability of PCR-based methods allowing a sensitive and specific detection of 'Ca. P. prunorum' is crucial for this phytoplasma because, at present, it is uncultured and cannot be detected serologically. We developed a PCR test which, in contrast to the existing detection tools, provides a fast, specific and sensitive detection of 'Ca. P. prunorum' in plants and insects. For studies requiring an absolute quantification of the phytoplasma titer, the same primers were used to develop a real-time PCR assay, including a standard for C. pruni. The sensitivity of these molecular tools was compared by serial dilutions and their specificity was assessed both in silico and experimentally for reference strains and field samples of the closely related phytoplasma 'Ca. P. prunorum', 'Ca. P. pyri' (pear decline agent) and 'Ca. P. mali' (apple proliferation agent), as well as for representative strains of the 'Ca. Phytoplasma' genus.

Genomics and evolution of heritable bacterial symbionts

Insect heritable symbionts have proven to be ubiquitous, based on molecular screening of various insect lineages. Recently, molecular and experimental approaches have yielded an immensely richer understanding of their diverse biological roles, resulting in a burgeoning research literature. Increasingly, commonalities and intermediates are being discovered between categories of symbionts once considered distinct: obligate mutualists that provision nutrients, facultative mutualists that provide protection against enemies or stress, and symbionts such as Wolbachia that manipulate reproductive systems. Among the most far-reaching impacts of widespread heritable symbiosis is that it may promote speciation by increasing reproductive and ecological isolation of host populations, and it effectively provides a means for transfer of genetic information among host lineages. In addition, insect symbionts provide some of the extremes of cellular genomes, including the smallest and the fastest evolving, raising new questions about the limits of evolution of life.

An effect of 16S rRNA intercistronic variability on coevolutionary analysis in symbiotic bacteria: molecular phylogeny of Arsenophonus triatominarum

The genes of ribosomal RNA are the most popular and frequently used markers for bacterial phylogeny and reconstruction of insect-symbiont coevolution. In primary symbionts, such as Buchnera and Wigglesworthia, genome economization leads to the establishment of a single copy of these sequences. In phylogenetic studies, they provide sufficient information and yield phylogenetic trees congruent with host evolution. In contrast, other symbiotic lineages (e.g., the genus Arsenophonus) carry a higher number of rRNA copies in their genomes, which may have serious consequences for phylogenetic inference. In this study, we show that in Arsenophonus triatominarum the degree of heterogeneity can affect reconstruction of phylogenetic relationships and mask possible coevolution between the symbiont and its host. Phylogenetic arrangement of individual rRNA copies was used, together with a calculation of their divergence time, to demonstrate that the incongruent 16S rDNA trees and low nucleotide diversity in the secondary symbiont could be reconciled with the coevolutionary scenario.

Genetic and electron-microscopic characterization of Rickettsiella tipulae, an intracellular bacterial pathogen of the crane fly, Tipula paludosa

Rickettsiella tipulae is an intracellular bacterial pathogen of larvae of the crane fly, Tipula paludosa (Diptera: Tipulidae) and has previously been claimed to represent an independent species within the genus Rickettsiella. Recently, this taxon has been reorganized and transferred as a whole from the alpha-proteobacterial order Rickettsiales to the gamma-proteobacterial order Legionellales. Here we present the electron-microscopic identification of this rickettsial pathogen together with the first DNA sequence information for R. tipulae. The results of our 16S rDNA-based phylogenetic analysis demonstrate that the transfer to the order Legionellales is justified for R. tipulae. However, there is no phylogenetic basis to consider R. tipulae an independent species, but instead conclusive evidence substantiating its species level co-assignment with Rickettsiella melolonthae. Furthermore, implications of our results for a possible reorganization of the internal structure of the genus Rickettsiella are discussed.

Frequency of secondary symbiont infection in an invasive psyllid relates to parasitism pressure on a geographic scale in California

Two endosymbionts, an obligate primary symbiont and a facultative secondary symbiont, are harbored within the invasive red gum (eucalyptus) lerp psyllid, Glycaspis brimblecombei, in California. An extensive survey of diversity and frequency of G. brimblecombei's secondary symbiont in multiple populations throughout the state of California was conducted using PCR detection, restriction enzymes, cloning, and sequencing. A total of 380 G. brimblecombei individuals in 19 populations were screened for secondary symbionts. Based on molecular screening results, only one type of secondary symbiont was present in G. brimblecombei populations in California. Overall, 40% of the 380 psyllids screened were infected with the secondary symbiont. Interestingly, secondary symbiont infection frequencies in G. brimblecombei populations varied dramatically from 0 to 75% and were significantly related to parasitism pressure by Psyllaphaegus bliteus, a solitary endoparasitoid of the psyllid.

Molecular evidence for polyphyletic origin of the primary symbionts of sucking lice (phthiraptera, anoplura)

Based on 16S rDNA analyses, the primary symbionts of sucking lice were found to form a polyphyletic assemblage of several distant lineages that have arisen several times within Enterobacteriaceae and at least once within Legionellaceae. Another independent lineage of endosymbiotic enterobacteria inhabits a sister group of the sucking lice, Rhynchophthirina. The inspection of 16S rDNA supports the symbiotic nature of the investigated bacteria; they display a typical trait of degenerative processes, an increased AT content (Adenine-Thymine content) in comparison with free-living bacteria. The calculation of divergence time between the closest anopluran and rhynchophthirine symbionts further support their independent origin. The results shown here, together with evidence from other groups, indicate that the significance of primary symbionts for blood-feeding insects should be reconsidered.

The co-evolutionary genetics of ecological communities

Co-evolution has produced many intriguing adaptations and made significant contributions to biodiversity through the co-adaptive radiations of interacting groups, such as pollinating insects and flowering plants or hosts and endosymbionts. New methods from molecular genetics and comparative genomics, in conjunction with advances in evolutionary genetic theory, are for the first time providing tools for detecting, investigating and understanding the genetic bases of the co-adaptive process and co-speciation. Advances in the emerging field of community genetics, which integrates genetics and community ecology, could revolutionize how co-evolution is studied, how genes are functionally annotated and how conservation geneticists implement preservation strategies.