Composition of bacterial communities associated with natural and laboratory populations of Asobara tabida infected with Wolbachia

Bacterial community and genome analysis of cytoplasmic incompatibility-inducing Wolbachia in American serpentine leafminer, Liriomyza trifolii

Liriomyza trifolii, an agricultural pest, is occasionally infected by Wolbachia. A Wolbachia strain present in Liriomyza trifolii is associated with cytoplasmic incompatibility (CI) effects, leading to the death of embryos resulting from incompatible crosses between antibiotic-treated or naturally Wolbachia-free strain females and Wolbachia-infected males. In this study, high-throughput sequencing of hypervariable rRNA genes was employed to characterize the bacterial community in Wolbachia-infected L. trifolii without antibiotic treatment. The analysis revealed that Wolbachia dominates the bacterial community in L. trifolii, with minor presence of Acinetobacter, Pseudomonas, and Limnobacter. To elucidate the genetic basis of the CI phenotype, metagenomic sequencing was also conducted to assemble the genome of the Wolbachia strain. The draft-genome of the Wolbachia strain wLtri was 1.35 Mbp with 34% GC content and contained 1,487 predicted genes. Notably, within the wLtri genome, there are three distinct types of cytoplasmic incompatibility factor (cif) genes: Type I, Type III, and Type V cifA;B. These genes are likely responsible for inducing the strong cytoplasmic incompatibility observed in L. trifolii.

Analyses of locomotion, wing morphology, and microbiome in Drosophila nigrosparsa after recovery from antibiotics

Antibiotics, such as tetracycline, have been frequently used to cure arthropods of Wolbachia endosymbionts. After the symbionts have been removed, the hosts must recover for some generations from the side effects of the antibiotics. However, most studies do not assess the direct and indirect longer-term effects of antibiotics used to remove Wolbachia, which may question the exact contribution of this endosymbiont to the effects observed. Here, we used the fly Drosophila nigrosparsa treated or not with tetracycline for three generations followed by two generations of recovery to investigate the effects of this antibiotic on the fly locomotion, wing morphology, and the gut microbiome. We found that antibiotic treatment did not affect fly locomotion two generations after being treated with the antibiotic. In addition, gut-microbiome restoration was tested as a more efficient solution to reduce the potential side effects of tetracycline on the microbiome. There was no significant difference in alpha diversity between gut restoration and other treatments, but the abundance of some bacterial taxa differed significantly between the gut-restoration treatment and the control. We conclude that in D. nigrosparsa the recovery period of two generations after being treated with the antibiotic is sufficient for locomotion, and suggest a general assessment of direct and indirect effects of antibiotics after a particular recovery time.

Diversity of midgut microbiota in laboratory-colonized and field-collected Aedes albopictus (Diptera: Culicidae): A preliminary study

Aedes (Ae.) albopictus is an important vector for many pathogens. Previous studies have revealed a role for midgut bacteria during pathogen infection in mosquitoes; however, studies of Ae. albopictus midgut bacteria are limited. We examined the diversity of midgut bacteria in female laboratory-colonized and field-collected Ae. albopictus. A total of 31 bacterial genera were identified representing 10 and 28 genera of laboratory-colonized and field-collected Ae. albopictus, respectively. The predominant bacterial genera in the laboratory-colonized Ae. albopictus were Staphylococcus and Micrococcus, whereas the bacterial diversity in the field-collected Ae. albopictus exhibited a higher proportion of Rhizobium and Agrobacterium as the dominant genera. However, only Staphylococcus showed a significant difference between laboratory-colonized and field-collected Ae. albopictus. The midgut bacterial species were identified from 30 laboratory-colonized Ae. albopictus mosquitoes. A total of 16 bacterial species were identified and the predominant bacterial species was Micrococcus luteus, followed by Staphylococcus epidermidis and Agrobacterium tumefaciens. Field mosquitoes were collected from the Sing Buri, Chumphon, and Yala Provinces of Thailand. The midgut bacterial species identified from the 10 Ae. albopictus collected from the Sing Buri Province included Bacillus subtilis, Staphylococcus haemolyticus, Staphylococcus hominis, and Serratia marcescens. Serratia marcescens was the only bacteria identified from this area. Midgut bacterial species were identified from 40 filed-collected Ae. albopictus from Chumphon Province. A total of 25 bacterial species were identified and the predominant species were Enterobacter cloacae, Micrococcus luteus, and Providencia rettgeri. Only 15 bacterial species were identified from the mosquitoes collected from Chumphon Province. A total of 18 bacterial species were identified from 30 Ae. albopictus collected from Yala Province and the predominant species were Rhizobium pusense and Agrobacterium tumefaciens. Only 12 bacterial species were found in mosquitoes collected from Yala Province. These findings indicate changes in the midgut bacteria population in Ae. albopictus from various locales, which may result from variability in the blood-meal source, diet, or habitat. A comprehensive survey of the midgut bacteria community prevalence in wild populations is critical for not only gaining a better understanding of the role of this bacterium in shaping the microbial community in Ae. albopictus, but also for informing current and future mosquito and disease control programs.

Torix Rickettsia are widespread in arthropods and reflect a neglected symbiosis

Background

Rickettsia are intracellular bacteria best known as the causative agents of human and animal diseases. Although these medically important Rickettsia are often transmitted via haematophagous arthropods, other Rickettsia, such as those in the Torix group, appear to reside exclusively in invertebrates and protists with no secondary vertebrate host. Importantly, little is known about the diversity or host range of Torix group Rickettsia.

Results

This study describes the serendipitous discovery of Rickettsia amplicons in the Barcode of Life Data System (BOLD), a sequence database specifically designed for the curation of mitochondrial DNA barcodes. Of 184,585 barcode sequences analysed, Rickettsia is observed in ∼0.41% of barcode submissions and is more likely to be found than Wolbachia (0.17%). The Torix group of Rickettsia are shown to account for 95% of all unintended amplifications from the genus. A further targeted PCR screen of 1,612 individuals from 169 terrestrial and aquatic invertebrate species identified mostly Torix strains and supports the “aquatic hot spot” hypothesis for Torix infection. Furthermore, the analysis of 1,341 SRA deposits indicates that Torix infections represent a significant proportion of all Rickettsia symbioses found in arthropod genome projects.

Conclusions

This study supports a previous hypothesis that suggests that Torix Rickettsia are overrepresented in aquatic insects. In addition, multiple methods reveal further putative hot spots of Torix Rickettsia infection, including in phloem-feeding bugs, parasitoid wasps, spiders, and vectors of disease. The unknown host effects and transmission strategies of these endosymbionts make these newly discovered associations important to inform future directions of investigation involving the understudied Torix Rickettsia.

New Insights on the Zeugodacus cucurbitae (Coquillett) Bacteriome

Various factors, including the insect host, diet, and surrounding ecosystem can shape the structure of the bacterial communities of insects. We have employed next generation, high-throughput sequencing of the 16S rRNA to characterize the bacteriome of wild Zeugodacus (Bactrocera) cucurbitae (Coquillett) flies from three regions of Bangladesh. The tested populations developed distinct bacterial communities with differences in bacterial composition, suggesting that geography has an impact on the fly bacteriome. The dominant bacteria belonged to the families Enterobacteriaceae, Dysgomonadaceae and Orbaceae, with the genera Dysgonomonas, Orbus and Citrobacter showing the highest relative abundance across populations. Network analysis indicated variable interactions between operational taxonomic units (OTUs), with cases of mutual exclusion and copresence. Certain bacterial genera with high relative abundance were also characterized by a high degree of interactions. Interestingly, genera with a low relative abundance like Shimwellia, Gilliamella, and Chishuiella were among those that showed abundant interactions, suggesting that they are also important components of the bacterial community. Such knowledge could help us identify ideal wild populations for domestication in the context of the sterile insect technique or similar biotechnological methods. Further characterization of this bacterial diversity with transcriptomic and metabolic approaches, could also reveal their specific role in Z. cucurbitae physiology.

Evidence for strong environmental control on bacterial microbiomes of Antarctic springtails

Collembola are a key component of the soil biota globally, playing an important role in community and ecosystem dynamics. Equally significant are their associated microbiomes, that can contribute to key metabolic functions. In the present study, we investigated the bacterial community composition of four Antarctic springtail species to assess if and how the extreme Antarctic environment has shaped the collembolans’ microbiomes. Springtails were collected from two biogeographical regions, the maritime and the continental Antarctic. From each region, two endemic species, belonging to the genera Cryptopygus (Isotomidae, Entomobryomorpha) and Friesea (Neanuridae, Poduromorpha), were included. This experimental design allowed us to quantify the relative importance of ecological factors (different regions of occurrence) and/or phylogenetic divergence in the host (different Orders) in shaping the Collembola microbiome. The diversity and richness of springtail microbiomes was lower in the Antarctic taxa compared to published information from species from temperate regions. The microbiome composition was predominantly species-specific, with a limited core microbiome shared across the four species examined. While both geographic origin and host species influenced the associated microbiomes, the former was the prevalent driver, with closer similarity between springtails from the same bioregion than between those belonging to the same genus.

Population-specific effect of Wolbachia on the cost of fungal infection in spider mites

Many studies have revealed the ability of the endosymbiotic bacterium Wolbachia to protect its arthropod hosts against diverse pathogens. However, as Wolbachia may also increase the susceptibility of its host to infection, predicting the outcome of a particular Wolbachia-host-pathogen interaction remains elusive. Yet, understanding such interactions and their eco-evolutionary consequences is crucial for disease and pest control strategies. Moreover, how natural Wolbachia infections affect artificially introduced pathogens for biocontrol has never been studied. Tetranychus urticae spider mites are herbivorous crop pests, causing severe damage on numerous economically important crops. Due to the rapid evolution of pesticide resistance, biological control strategies using entomopathogenic fungi are being developed. However, although spider mites are infected with various Wolbachia strains worldwide, whether this endosymbiont protects them from fungi is as yet unknown. Here, we compared the survival of two populations, treated with antibiotics or naturally harboring different Wolbachia strains, after exposure to the fungal biocontrol agents Metarhizium brunneum and Beauveria bassiana. To control for potential effects of the bacterial community of spider mites, we also compared the susceptibility of two populations naturally uninfected by Wolbachia, treated with antibiotics or not. In one population, Wolbachia-infected mites had a better survival than uninfected ones in absence of fungi but not in their presence, whereas in the other population Wolbachia increased the mortality induced by B. bassiana. In one naturally Wolbachia-uninfected population, the antibiotic treatment increased the susceptibility of spider mites to M. brunneum, but it had no effect in the other treatments. These results suggest that natural Wolbachia infections may not hamper and may even improve the success of biological control using entomopathogenic fungi. However, they also draw caution on the generalization of such effects, given the complexity of within-host-pathogens interaction and the potential eco-evolutionary consequences of the use of biocontrol agents for Wolbachia-host associations.

Microbial Symbionts of Parasitoids

Parasitoids depend on other insects for the development of their offspring. Their eggs are laid in or on a host insect that is consumed during juvenile development. Parasitoids harbor a diversity of microbial symbionts including viruses, bacteria, and fungi. In contrast to symbionts of herbivorous and hematophagous insects, parasitoid symbionts do not provide nutrients. Instead, they are involved in parasitoid reproduction, suppression of host immune responses, and manipulation of the behavior of herbivorous hosts. Moreover, recent research has shown that parasitoid symbionts such as polydnaviruses may also influence plant-mediated interactions among members of plant-associated communities at different trophic levels, such as herbivores, parasitoids, and hyperparasitoids. This implies that these symbionts have a much more extended phenotype than previously thought. This review focuses on the effects of parasitoid symbionts on direct and indirect species interactions and the consequences for community ecology.

Tracing insect pests: is there new potential in molecular techniques?

Insects are amongst the greatest pests of agriculture, horticulture and forestry worldwide, inflicting damage and economic costs both directly and by transmitting plant viruses. Many kinds of insects are now resistant or cross-resistant to pesticides. Tracking studies have become very important for combatting insect pests and for better understanding their biology (eg insect population dynamics, movements, feeding behaviour and other ecological interactions). A wide variety of tracing approaches have been used including discriminative, tracer and molecular methods. The perfect technique for insect tracking is the technique that harmonizes with insects' 'normal' biology. Furthermore, the technique should be environmentally safe, cost-effective and easy to use. This paper reviews the current techniques used for insect traceability, documents the advantages and drawbacks of each method, and puts special focus on molecular techniques, including PCR-denaturing gradient gel electrophoresis as a new and promising traceability tool that could provide insects with a unique biological barcode and thus make it possible to trace their movements.

Bacterial and fungal gut communities of Agrilus mali at different developmental stages and fed different diets

Agrilus mali (Coleoptera: Buprestidae) is an invasive wood borer pest that has caused considerable damage to the Xinjiang wild fruit forest. In this study, we investigated the bacterial and fungal intestinal microbial communities of A. mali during different developmental stages, including larvae, pupae and newly eclosed adults or fed different diets (leaves of Malus halliana and Malus pumila) using Illumina MiSeq high-throughput sequencing technology. The results showed that microbial alpha diversity first increased and then decreased during the developmental stages, with the most dominant bacteria and fungi exhibiting the dynamic patterns "Decrease", "Increase" and "Fluctuation". With respect to the different diets, the bacterial communities were similar between the newly eclosed adults and adults fed M. pumila leaves, while the structure of the fungal communities showed great differences between newly eclosed adults and adults fed different diets. Through a co-correlation network analysis, we observed complex microbial interactions among bacterial and fungal taxa that were associated with potential diverse functions and intricate biological processes in the intestinal microbiota of A. mali. Overall, the results of this study demonstrated that the invasive insect A. mali harbours diverse, dynamic, and presumably multifunctional microbial communities, an understanding of which could improve our ability to develop more effective management approaches to control A. mali.

A change in the bacterial community of spider mites decreases fecundity on multiple host plants

Bacterial symbionts may influence the fitness of their herbivore hosts, but such effects have been poorly studied across most invertebrate groups. The spider mite, Tetranychus truncatus, is a polyphagous agricultural pest harboring various bacterial symbionts whose function is largely unknown. Here, by using a high‐throughput 16S rRNA amplicon sequencing approach, we characterized the bacterial diversity and community composition of spider mites fed on five host plants after communities were modified following tetracycline exposure. We demonstrated that spider mite bacterial diversity and community composition were significantly affected by host plants and antibiotics. In particular, the abundance of the maternally inherited endosymbionts Wolbachia and Spiroplasma significantly differed among spider mites that were reared on different plant species and were completely removed by antibiotics. There was an overall tendency for daily fecundity to be lower in the mites with reduced bacterial diversity following the antibiotic treatment. Our data suggest that host plants and antibiotics can shape spider mite bacterial communities and that bacterial symbionts improve mite performance.

Bacterial microbiota of Aedes aegypti mosquito larvae is altered by intoxication with Bacillus thuringiensis israelensis

Background

Insect microbiota is a dynamic microbial community that can actively participate in defense against pathogens. Bacillus thuringiensis (Bt) is a natural entomopathogen widely used as a bioinsecticide for pest control. Although Bt’s mode of action has been extensively studied, whether the presence of microbiota is mandatory for Bt to effectively kill the insect is still under debate. An association between a higher tolerance and a modified microbiota was already evidenced but a critical point remained to be solved: is the modified microbiota a cause or a consequence of a higher tolerance to Bt?

Methods

In this study we focused on the mosquito species Aedes aegypti, as no work has been performed on Diptera on this topic to date, and on B. thuringiensis israelensis (Bti), which is used worldwide for mosquito control. To avoid using antibiotics to cure bacterial microbiota, mosquito larvae were exposed to an hourly increasing dose of Bti during 25 hours to separate the most susceptible larvae dying quickly from more tolerant individuals, with longer survival.

Results

Denaturing gradient gel electrophoresis (DGGE) fingerprinting revealed that mosquito larval bacterial microbiota was strongly affected by Bti infection after only a few hours of exposure. Bacterial microbiota from the most tolerant larvae showed the lowest diversity but the highest inter-individual differences. The proportion of Bti in the host tissue was reduced in the most tolerant larvae as compared to the most susceptible ones, suggesting an active control of Bti infection by the host.

Conclusions

Here we show that a modified microbiota is associated with a higher tolerance of mosquitoes to Bti, but that it is rather a consequence of Bti infection than the cause of the higher tolerance. This study paves the way to future investigations aiming at unraveling the role of host immunity, inter-species bacterial competition and kinetics of host colonization by Bti that could be at the basis of the phenotype observed in this study.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-2741-8) contains supplementary material, which is available to authorized users.

Life inside a gall: closeness does not favour horizontal transmission of Rickettsia between a gall wasp and its parasitoid

The incidence of horizontal transmission as a route for spreading symbiont infections is still being debated, but a common view is that horizontal transfers require intimate between-species relationships. Here we study a system that meets ideal requirements for horizontal transmission: the gall wasp Leptocybe invasa and its parasitoid Quadrastichus mendeli (Hymenoptera: Eulophidae). These wasps belong to the same subfamily, spend most of their lives inside the same minute gall and are both infected by Rickettsia, a maternally inherited endosymbiotic bacteria that infects several arthropods, sometimes manipulating their reproduction, like inducing thelytokous parthenogenesis in L. invasa. Despite intimate contact, close phylogenetic relationship and the parasitoid's host specificity, we show that host and parasitoid do not share the same Rickettsia. We provide indirect evidence that Rickettsia infecting Q. mendeli may be inducing thelytokous parthenogenesis, as the symbiont is densely present in the reproductive apparatus and is vertically transmitted. Phylogenetic analyses based on 16S and gltA placed this symbiont in the leech group. The confirmed and presumed parthenogenesis-inducing Rickettsia discovered so far only infect eulophid wasps, and belong to three different groups, suggesting multiple independent evolution of the parthenogenesis inducing phenotype. We also show some degree of cospeciation between Rickettsia and their eulophid hosts.

Torix group Rickettsia are widespread in Culicoides biting midges (Diptera: Ceratopogonidae), reach high frequency and carry unique genomic features

There is increasing interest in the heritable bacteria of invertebrate vectors of disease as they present novel targets for control initiatives. Previous studies on biting midges (Culicoides spp.), known to transmit several RNA viruses of veterinary importance, have revealed infections with the endosymbiotic bacteria, Wolbachia and Cardinium. However, rickettsial symbionts in these vectors are underexplored. Here, we present the genome of a previously uncharacterized Rickettsia endosymbiont from Culicoides newsteadi (RiCNE). This genome presents unique features potentially associated with host invasion and adaptation, including genes for the complete non-oxidative phase of the pentose phosphate pathway, and others predicted to mediate lipopolysaccharides and cell wall modification. Screening of 414 Culicoides individuals from 29 Palearctic or Afrotropical species revealed that Rickettsia represent a widespread but previously overlooked association, reaching high frequencies in midge populations and present in 38% of the species tested. Sequence typing clusters the Rickettsia within the Torix group of the genus, a group known to infect several aquatic and hematophagous taxa. FISH analysis indicated the presence of Rickettsia bacteria in ovary tissue, indicating their maternal inheritance. Given the importance of biting midges as vectors, a key area of future research is to establish the impact of this endosymbiont on vector competence.

Relative Abundance and Strain Diversity in the Bacterial Endosymbiont Community of a Sap-Feeding Insect Across Its Native and Introduced Geographic Range

Most insects are associated with bacterial symbionts. The bacterial diversity and community composition within hosts may play an important role in shaping insect population biology, ecology and evolution. We focussed on the bacterial microbiome of the Australian fig homotomid Mycopsylla fici (Hemiptera: Psylloidea), which can cause defoliation of its only host tree, Ficus macrophylla. This sap-feeding insect is native to mainland Australia and Lord Howe Island (LHI) but also occurs where its host has been planted, notably in New Zealand. By using a high-throughput 16S rDNA amplicon sequencing approach, we compared the bacterial diversity and community composition in individual adult males of four host populations, Sydney, Brisbane, LHI and Auckland. We also compared males, females and nymphs of the Sydney population. The microbiome of M. fici was simple and consisted mostly of the following three maternally inherited endosymbiont species: the primary endosymbiont Carsonella, a secondary (S-) endosymbiont and Wolbachia. However, the relative abundance of their sequence reads varied between host populations, except for similarities between Sydney and Auckland. In addition, insects from Sydney and Auckland had identical bacterial strains supporting the hypothesis that Sydney is the source population for Auckland. In contrast, mainland and LHI populations harboured the same S-endosymbiont, co-diverged Carsonella but different Wolbachia strains. Besides detecting endosymbiont-specific patterns of either co-evolution or horizontal acquisition, our study highlights that relative abundance of maternally inherited endosymbionts should also be taken into account when studying bacterial communities across host populations, as variations in bacterial density may impact host biology and ecology.

Metagenomic analysis of microbial community of a parasitoid wasp Megaphragma amalphitanum

The vast majority of multicellular organisms coexist with bacterial symbionts that may play various roles during their life cycle. Parasitoid wasp Megaphragma amalphitanum (Hymenoptera: Trichogrammatidae) belongs to the smallest known insects whose size is comparable with some bacteria. Using 16S rRNA gene sequencing and Whole Genome Sequencing (WGS), we described microbiota diversity for this arthropod and its potential impact on their lifecycle. Metagenomic sequences were deposited to SRA database which is available at NCBI with accession number SRX2363723 and SRX2363724. We found that small body size and limited lifespan do not lead to a significant reduction of bacterial symbionts diversity. At the same time, we show here a specific feature of microbiota composition in M. amalphitanum – the absence of the Rickettsiaceae family representatives that are known to cause sex-ratio distortion in arthropods and well represented in other populations of parasitoid wasps.

The microbiome of Folsomia candida: an assessment of bacterial diversity in a Wolbachia-containing animal

The springtail Folsomia candida is an important model organism for soil ecology, ecotoxicology and ecogenomics. The decomposer activities of soil invertebrates like Folsomia depend on their relationship with microbial communities including gut symbionts. In this paper, we apply high-throughput sequencing to provide a detailed characterization of the bacterial community associated with parthenogenetic F. candida. First, we evaluated a method to suppress the amplification of DNA from the endosymbiont Wolbachia, to prevent it from interfering with the identification of less abundant operational taxonomic units (OTUs). The suppression treatment applied was effective against Wolbachia and did not interfere with the detection of the most abundant OTUs (59 OTUs, contributing over 87% of the reads). However, this method did affect the inferred community composition. Significant differences were subsequently observed in the composition of bacterial communities associated with two different strains of F. candida. A total of 832 OTUs were found, of which 45% were only present in one strain and 17% only in the other. Among the 20 most abundant OTUs, 16 were shared between strains. Denaturing gradient gel electrophoresis and clone libraries, although unable to capture the full diversity of the bacterial community, provided results that supported the NGS data.

Microbial communities associated with the larval gut and eggs of the Western corn rootworm

BACKGROUND

The western corn rootworm (WCR) is one of the economically most important pests of maize. A better understanding of microbial communities associated with guts and eggs of the WCR is required in order to develop new pest control strategies, and to assess the potential role of the WCR in the dissemination of microorganisms, e.g., mycotoxin-producing fungi.

METHODOLOGY/PRINCIPAL FINDINGS

Total community (TC) DNA was extracted from maize rhizosphere, WCR eggs, and guts of larvae feeding on maize roots grown in three different soil types. Denaturing gradient gel electrophoresis (DGGE) and sequencing of 16S rRNA gene and ITS fragments, PCR-amplified from TC DNA, were used to investigate the fungal and bacterial communities, respectively. Microorganisms in the WCR gut were not influenced by the soil type. Dominant fungal populations in the gut were affiliated to Fusarium spp., while Wolbachia was the most abundant bacterial genus. Identical ribosomal sequences from gut and egg samples confirmed a transovarial transmission of Wolbachia sp. Betaproteobacterial DGGE indicated a stable association of Herbaspirillum sp. with the WCR gut. Dominant egg-associated microorganisms were the bacterium Wolbachia sp. and the fungus Mortierella gamsii.

CONCLUSION/SIGNIFICANCE

The soil type-independent composition of the microbial communities in the WCR gut and the dominance of only a few microbial populations suggested either a highly selective environment in the gut lumen or a high abundance of intracellular microorganisms in the gut epithelium. The dominance of Fusarium species in the guts indicated WCR larvae as vectors of mycotoxin-producing fungi. The stable association of Herbaspirillum sp. with WCR gut systems and the absence of corresponding sequences in WCR eggs suggested that this bacterium was postnatally acquired from the environment. The present study provided new insights into the microbial communities associated with larval guts and eggs of the WCR. However, their biological role remains to be explored.

Influence of changing plant food sources on the gut microbiota of saltmarsh detritivores

Changes in agricultural land-use of saltmarshes along the German North Sea coast have favoured the succession of the marsh grass Elytrigia atherica over the long-established Spartina anglica. Consequently, E. atherica represents a potential food source of increasing importance for plant-feeding soil detritivores. Considering the importance of this ecological guild for decomposition processes and nutrient cycling, we focussed on two sympatric saltmarsh soil macrodetritivores and their associated gut microbiota to investigate how the digestive processes of these species may be affected by changing plant food sources. Using genetic fingerprints of partial 16S rRNA gene sequences, we analysed composition and diversity of the bacterial gut community in a diplopod and an amphipod crustacean in relation to different feeding regimes representing the natural vegetation changes. Effects of syntopy on the host-specific gut microbiota were also taken into account by feeding the two detritivore species either independently or on the same plant sample. Bacterial community composition was influenced by both the host species and the available plant food sources, but the latter had a stronger effect on microbial community structure. Furthermore, bacterial diversity was highest after feeding on a mixture of both plant species, regardless of the host species. The gut microbiota of these two detritivores can thus be expected to change along with the on-going succession at the plant community level in this environment. Cloning and sequencing of bacterial 16S rRNA gene fragments further indicated a host-related effect since the two detritivores differed in terms of predominant bacterial taxa: diplopods harboured mainly representatives of the phyla Bacteroidetes and Gammaproteobacteria. In contrast, the genus Vibrio was found for the amphipod host across all feeding conditions.

Microbial community of predatory bugs of the genus Macrolophus (Hemiptera: Miridae)

BACKGROUND

The predatory mirids of the genus Macrolophus are key natural enemies of various economically important agricultural pests. Both M. caliginosus and M. pygmaeus are commercially available for the augmentative biological control of arthropod pests in European greenhouses. The latter species is known to be infected with Wolbachia -inducing cytoplasmic incompatibility in its host- but the presence of other endosymbionts has not been demonstrated. In the present study, the microbial diversity was examined in various populations of M. caliginosus and M. pygmaeus by 16S rRNA sequencing and denaturing gradient gel electrophoresis.

RESULTS

Besides Wolbachia, a co-infection of 2 Rickettsia species was detected in all M. pygmaeus populations. Based on a concatenated alignment of the 16S rRNA gene, the gltA gene and the coxA gene, the first is phylogenetically related to Rickettsia bellii, whereas the other is closely related to Rickettsia limoniae. All M. caliginosus populations were infected with the same Wolbachia and limoniae-like Rickettsia strain as M. pygmaeus, but did not harbour the bellii-like Rickettsia strain. Interestingly, individuals with a single infection were not found. A PCR assay on the ovaries of M. pygmaeus and M. caliginosus indicated that all endosymbionts are vertically transmitted. The presence of Wolbachia and Rickettsia in oocytes was confirmed by a fluorescence in situ hybridisation. A bio-assay comparing an infected and an uninfected M. pygmaeus population suggested that the endosymbionts had minor effects on nymphal development of their insect host and did not influence its fecundity.

CONCLUSION

Two species of the palaearctic mirid genus Macrolophus are infected with multiple endosymbionts, including Wolbachia and Rickettsia. Independent of the origin, all tested populations of both M. pygmaeus and M. caliginosus were infected with three and two endosymbionts, respectively. There was no indication that infection with endosymbiotic bacteria had a fitness cost in terms of development and fecundity of the predators.

Influence of Wolbachia on host gene expression in an obligatory symbiosis

BACKGROUND

Wolbachia are intracellular bacteria known to be facultative reproductive parasites of numerous arthropod hosts. Apart from these reproductive manipulations, recent findings indicate that Wolbachia may also modify the host's physiology, notably its immune function. In the parasitoid wasp, Asobara tabida, Wolbachia is necessary for oogenesis completion, and aposymbiotic females are unable to produce viable offspring. The absence of egg production is also associated with an increase in programmed cell death in the ovaries of aposymbiotic females, suggesting that a mechanism that ensures the maintenance of Wolbachia in the wasp could also be responsible for this dependence. In order to decipher the general mechanisms underlying host-Wolbachia interactions and the origin of the dependence, we developed transcriptomic approaches to compare gene expression in symbiotic and aposymbiotic individuals.

RESULTS

As no genetic data were available on A. tabida, we constructed several Expressed Sequence Tags (EST) libraries, and obtained 12,551 unigenes from this species. Gene expression was compared between symbiotic and aposymbiotic ovaries through in silico analysis and in vitro subtraction (SSH). As pleiotropic functions involved in immunity and development could play a major role in the establishment of dependence, the expression of genes involved in oogenesis, programmed cell death (PCD) and immunity (broad sense) was analyzed by quantitative RT-PCR. We showed that Wolbachia might interfere with these numerous biological processes, in particular some related to oxidative stress regulation. We also showed that Wolbachia may interact with immune gene expression to ensure its persistence within the host.

CONCLUSIONS

This study allowed us to constitute the first major dataset of the transcriptome of A. tabida, a species that is a model system for both host/Wolbachia and host/parasitoid interactions. More specifically, our results highlighted that symbiont infection may interfere with numerous pivotal processes at the individual level, suggesting that the impact of Wolbachia should also be investigated beyond reproductive manipulations.

Bacterial communities of diverse Drosophila species: ecological context of a host-microbe model system

Drosophila melanogaster is emerging as an important model of non-pathogenic host-microbe interactions. The genetic and experimental tractability of Drosophila has led to significant gains in our understanding of animal-microbial symbiosis. However, the full implications of these results cannot be appreciated without the knowledge of the microbial communities associated with natural Drosophila populations. In particular, it is not clear whether laboratory cultures can serve as an accurate model of host-microbe interactions that occur in the wild, or those that have occurred over evolutionary time. To fill this gap, we characterized natural bacterial communities associated with 14 species of Drosophila and related genera collected from distant geographic locations. To represent the ecological diversity of Drosophilids, examined species included fruit-, flower-, mushroom-, and cactus-feeders. In parallel, wild host populations were compared to laboratory strains, and controlled experiments were performed to assess the importance of host species and diet in shaping bacterial microbiome composition. We find that Drosophilid flies have taxonomically restricted bacterial communities, with 85% of the natural bacterial microbiome composed of only four bacterial families. The dominant bacterial taxa are widespread and found in many different host species despite the taxonomic, ecological, and geographic diversity of their hosts. Both natural surveys and laboratory experiments indicate that host diet plays a major role in shaping the Drosophila bacterial microbiome. Despite this, the internal bacterial microbiome represents only a highly reduced subset of the external bacterial communities, suggesting that the host exercises some level of control over the bacteria that inhabit its digestive tract. Finally, we show that laboratory strains provide only a limited model of natural host-microbe interactions. Bacterial taxa used in experimental studies are rare or absent in wild Drosophila populations, while the most abundant associates of natural Drosophila populations are rare in the lab.

Bacterial diversity of field-caught mosquitoes, Aedes albopictus and Aedes aegypti, from different geographic regions of Madagascar

Symbiotic bacteria are known to play important roles in the biology of insects, but the current knowledge of bacterial communities associated with mosquitoes is very limited and consequently their contribution to host behaviors is mostly unknown. In this study, we explored the composition and diversity of mosquito-associated bacteria in relation with mosquitoes' habitats. Wild Aedes albopictus and Aedes aegypti were collected in three different geographic regions of Madagascar. Culturing methods and denaturing gradient gel electrophoresis (DGGE) and sequencing of the rrs amplicons revealed that Proteobacteria and Firmicutes were the major phyla. Isolated bacterial genera were dominated by Bacillus, followed by Acinetobacter, Agrobacterium and Enterobacter. Common DGGE bands belonged to Acinetobacter, Asaia, Delftia, Pseudomonas, Enterobacteriaceae and an uncultured Gammaproteobacterium. Double infection by maternally inherited Wolbachia pipientis prevailed in 98% of males (n=272) and 99% of females (n=413); few individuals were found to be monoinfected with Wolbachia wAlbB strain. Bacterial diversity (Shannon-Weaver and Simpson indices) differed significantly per habitat whereas evenness (Pielou index) was similar. Overall, the bacterial composition and diversity were influenced both by the sex of individuals and by the environment inhabited by the mosquitoes; the latter might be related to both the vegetation and the animal host populations that Aedes used as food sources.

Acetic acid bacteria, newly emerging symbionts of insects

Recent research in microbe-insect symbiosis has shown that acetic acid bacteria (AAB) establish symbiotic relationships with several insects of the orders Diptera, Hymenoptera, Hemiptera, and Homoptera, all relying on sugar-based diets, such as nectars, fruit sugars, or phloem sap. To date, the fruit flies Drosophila melanogaster and Bactrocera oleae, mosquitoes of the genera Anopheles and Aedes, the honey bee Apis mellifera, the leafhopper Scaphoideus titanus, and the mealybug Saccharicoccus sacchari have been found to be associated with the bacterial genera Acetobacter, Gluconacetobacter, Gluconobacter, Asaia, and Saccharibacter and the novel genus Commensalibacter. AAB establish symbiotic associations with the insect midgut, a niche characterized by the availability of diet-derived carbohydrates and oxygen and by an acidic pH, selective factors that support AAB growth. AAB have been shown to actively colonize different insect tissues and organs, such as the epithelia of male and female reproductive organs, the Malpighian tubules, and the salivary glands. This complex topology of the symbiosis indicates that AAB possess the keys for passing through body barriers, allowing them to migrate to different organs of the host. Recently, AAB involvement in the regulation of innate immune system homeostasis of Drosophila has been shown, indicating a functional role in host survival. All of these lines of evidence indicate that AAB can play different roles in insect biology, not being restricted to the feeding habit of the host. The close association of AAB and their insect hosts has been confirmed by the demonstration of multiple modes of transmission between individuals and to their progeny that include vertical and horizontal transmission routes, comprising a venereal one. Taken together, the data indicate that AAB represent novel secondary symbionts of insects.

Wolbachia modulates Chikungunya replication in Aedes albopictus

The Aedes albopictus mosquito has been involved as the principal vector of recent major outbreaks due to the chikungunya virus (CHIKV). The species is naturally infected by two strains of Wolbachia (wAlbA and wAlbB). Wolbachia infections are thought to have spread by manipulating the reproduction of their hosts; cytoplasmic incompatibility is the mechanism used by Wolbachia to invade natural populations of many insects including Ae. albopictus. Here, we report a study on the effects of removing Wolbachia from Ae. albopictus on CHIKV replication and examine the consequences of CHIKV infection on some life-history traits (survival and reproduction) of Wolbachia-free Ae. albopictus. We found that Wolbachia-free mosquitoes maintained a highly heterogeneous CHIKV replication compared to Wolbachia-infected individuals. In Wolbachia-infected Ae. albopictus, the regular increase of CHIKV followed by a steady viral load from day 4 post-infection onwards was concomitant with a decline in Wolbachia density. This profile was also detected when examining the two key organs for viral transmission, the midgut and the salivary glands. Moreover, Wolbachia-free Ae. albopictus was not altered in life-history traits such as survival, oviposition and hatching characteristics whether infected or not with CHIKV. We found that Wolbachia is not essential for viral replication, its presence could lead to optimize replication from day 4 post-infection onwards, coinciding with a decrease in Wolbachia density. Wolbachia may regulate viral replication in Ae. albopictus, with consequences on survival and reproduction.

Rickettsia symbionts cause parthenogenetic reproduction in the parasitoid wasp Pnigalio soemius (Hymenoptera: Eulophidae)

Bacteria in the genus Rickettsia are intracellular symbionts of disparate groups of organisms. Some Rickettsia strains infect vertebrate animals and plants, where they cause diseases, but most strains are vertically inherited symbionts of invertebrates. In insects Rickettsia symbionts are known to have diverse effects on hosts ranging from influencing host fitness to manipulating reproduction. Here we provide evidence that a Rickettsia symbiont causes thelytokous parthenogenesis (in which mothers produce only daughters from unfertilized eggs) in a parasitoid wasp, Pnigalio soemius (Hymenoptera: Eulophidae). Feeding antibiotics to thelytokous female wasps resulted in production of progeny that were almost all males. Cloning and sequencing of a fragment of the 16S rRNA gene amplified with universal primers, diagnostic PCR screening of symbiont lineages associated with manipulation of reproduction, and fluorescence in situ hybridization (FISH) revealed that Rickettsia is always associated with thelytokous P. soemius and that no other bacteria that manipulate reproduction are present. Molecular analyses and FISH showed that Rickettsia is distributed in the reproductive tissues and is transovarially transmitted from mothers to offspring. Comparison of antibiotic-treated females and untreated females showed that infection had no cost. Phylogenetic analyses of 16S rRNA and gltA gene sequences placed the symbiont of P. soemius in the bellii group and indicated that there have been two separate origins of the parthenogenesis-inducing phenotype in the genus Rickettsia. A possible route for evolution of induction of parthenogenesis in the two distantly related Rickettsia lineages is discussed.

Transinfection and growth discrepancy of Drosophila Wolbachia strain wMel in cell lines of the mosquito Aedes albopictus

AIM

The Wolbachia strain wMel can protect Drosophila melanogaster against pathogenic RNA viruses. To analyse the potential of this inhibitory effect against arboviruses vectorized by these mosquitoes, we here first transinfected the Aedes albopictus Aa23 and C6/36 cell lines with the Wolbachia strain wMel and then monitored their infection dynamics.

METHODS AND RESULTS

Wolbachia strain wMel was transferred into A. albopictus Aa23 and C6/36 cell lines using the shell vial technique. The presence of the bacterium in the transinfected cells was monitored by quantitative PCR and fluorescence in situ hybridization. Bacteria could be detected in the cytoplasm of both the Aa23 and C6/36 cell lines. However, the dynamics and stability of the bacterial infection differed depending on the initial cell background. The Aa23 cell line, which had been treated with a tetracycline antibiotic 2 years previously to eliminate its natural Wolbachia wAlbB-infecting strain, lost the introduced Wolbachia wMel strain after 12 passages postinfection. In contrast, the C6/36 cell line, which had originally been aposymbiotic, displayed a stable infection with Wolbachia wMel. The bacterial density in C6/36 was greater than that of the A. albopictus RML12 cell line from which the wMel strain had originated.

CONCLUSIONS

Transient or persistent transinfection of A. albopictus Aa23 and C6/36 cell lines with Wolbachia wMel strain was achieved. The results indicate the influence of the genetic background of mosquito cells in maintaining Wolbachia originating from a distant dipteral host.

SIGNIFICANCE AND IMPACT OF THE STUDY

The cell model built here can now be used to investigate the viral inhibitory effect of the Wolbachia wMel strain against arboviruses such as dengue and chikungunya, which are transmitted by the mosquito A. albopictus.

Persistent Wolbachia and cultivable bacteria infection in the reproductive and somatic tissues of the mosquito vector Aedes albopictus

Background

Commensal and symbiotic microbes have a considerable impact on the behavior of many arthropod hosts, including hematophagous species that transmit pathogens causing infectious diseases to human and animals. Little is known about the bacteria associated with mosquitoes other than the vectorized pathogens. This study investigated Wolbachia and cultivable bacteria that persist through generations in Ae. albopictus organs known to host transmitted arboviruses, such as dengue and chikungunya.

Methodology/Principal Findings

We used culturing, diagnostic and quantitative PCR, as well as in situ hybridization, to detect and locate bacteria in whole individual mosquitoes and in dissected tissues. Wolbachia, cultivable bacteria of the genera Acinetobacter, Comamonas, Delftia and Pseudomonas co-occurred and persisted in the bodies of both males and females of Ae. albopictus initially collected in La Réunion during the chikungunya outbreak, and maintained as colonies in insectaries. In dissected tissues, Wolbachia and the cultivable Acinetobacter can be detected in the salivary glands. The other bacteria are commonly found in the gut. Quantitative PCR estimates suggest that Wolbachia densities are highest in ovaries, lower than those of Acinetobacter in the gut, and approximately equal to those of Acinetobacter in the salivary glands. Hybridization using specific fluorescent probes successfully localized Wolbachia in all germ cells, including the oocytes, and in the salivary glands, whereas the Acinetobacter hybridizing signal was mostly located in the foregut and in the anterior midgut.

Conclusions/Significance

Our results show that Proteobacteria are distributed in the somatic and reproductive tissues of mosquito where transmissible pathogens reside and replicate. This location may portend the coexistence of symbionts and pathogens, and thus the possibility that competition or cooperation phenomena may occur in the mosquito vector Ae. albopictus. Improved understanding of the vectorial system, including the role of bacteria in the vector's biology and competence, could have major implications for understanding viral emergences and for disease control.