The bacterial symbiont Wolbachia induces resistance to RNA viral infections in Drosophila melanogaster

Drosophila symbionts in infection: when a friend becomes an enemy

The insect microbiome is comprised of extracellular microbial communities that colonize the host surfaces and endosymbionts that reside inside host cells and tissues. Both of these communities participate in essential aspects of host biology, including the immune response and interactions with pathogens. In recent years, our knowledge about the role of the insect microbiome in infection has increased tremendously. While many studies have highlighted the microbiome's protective effect against various natural enemies of insects, unexpected discoveries have shown that some members of the microbiota can facilitate pathogenic infections. Here, we summarize studies in the fruit fly, Drosophila melanogaster, that have substantially progressed our understanding of host-pathogen-microbiome interactions during infection. We summarize studies on the protective mechanisms of Drosophila gut microbiota, highlight examples of microbiome exploitation by pathogens, and detail the mechanisms of endosymbiont-mediated host protection. In addition, we delve into a previously neglected topic in Drosophila microbiome research-the crosstalk between endosymbionts and gut microbiota. Finally, we address how endosymbionts and gut microbiota remain resilient to host immune responses and stably colonize the host during infection. By examining how the microbiome is influenced by and reciprocally affects infection outcomes, this review provides timely and cohesive coverage of the roles of Drosophila endosymbionts and gut microbiota during infections.

Wolbachia-mediated reduction in the glutamate receptor mGluR promotes female promiscuity and bacterial spread

The molecular mechanisms by which parasites mediate host behavioral changes remain largely unexplored. Here, we examine Drosophila melanogaster infected with Wolbachia, a symbiont transmitted through the maternal germline, and find Wolbachia infection increases female receptivity to male courtship and hybrid mating. Wolbachia colonize regions of the brain that control sense perception and behavior. Quantitative global proteomics identify 177 differentially abundant proteins in infected female larval brains. Genetic alteration of the levels of three of these proteins in adults, the metabotropic glutamate receptor mGluR, the transcription factor TfAP-2, and the odorant binding protein Obp99b, each mimic the effect of Wolbachia on female receptivity. Furthermore, >700 Wolbachia proteins are detected in infected brains. Through abundance and molecular modeling analyses, we distinguish several Wolbachia-produced proteins as potential effectors. These results identify potential networks of host and Wolbachia proteins that modify behavior to promote mating success and aid the spread of Wolbachia.

Counting rare Wolbachia endosymbionts using digital droplet PCR

Wolbachia is the most widespread animal-associated intracellular microbe, living within the cells of over half of insect species. Since they can suppress pathogen replication and spread rapidly through insect populations, Wolbachia is at the vanguard of public health initiatives to control mosquito-borne diseases. Wolbachia's abilities to block pathogens and spread quickly are closely linked to their abundance in host tissues. The most common method for counting Wolbachia is quantitative polymerase chain reaction (qPCR), yet qPCR can be insufficient to count rare Wolbachia, necessitating tissue pooling and consequently compromising individual-level resolution of Wolbachia dynamics. Digital droplet PCR (ddPCR) offers superior sensitivity, enabling the detection of rare targets and eliminating the need for sample pooling. Here, we report three ddPCR assays to measure total Wolbachia abundance, Wolbachia abundance adjusted for DNA extraction efficiency, and Wolbachia density relative to host genome copies. Using Drosophila melanogaster with wMel Wolbachia as a model, we show these ddPCR assays can reliably detect as few as 7 to 12 Wolbachia gene copies in a 20 µL reaction. The designed oligos are homologous to sequences from at least 106 Wolbachia strains across supergroup A and 53 host species from the Drosophila, Scaptomyza, and Zaprionus genera, suggesting broad utility. These highly sensitive ddPCR assays are expected to significantly advance Wolbachia-host interactions research by enabling the collection of molecular data from individual insect tissues. Their ability to detect rare Wolbachia will be especially valuable in applied and natural field settings where pooling samples could obscure important variation.IMPORTANCEWolbachia bacteria live inside the cells of many animals, especially insects. In many insect species, almost every individual carries Wolbachia. How common Wolbachia becomes within a population often depends on how much of it is present in the insect's body. Therefore, accurately measuring Wolbachia levels is crucial for understanding how these bacteria interact with their hosts and spread. However, traditional molecular assays can lack the sensitivity needed for accurate, individual-level quantification of rare Wolbachia. Here, we present three highly sensitive digital droplet PCR assays for Wolbachia detection, offering superior sensitivity compared to existing methods. These assays will be useful for studies that measure Wolbachia abundance and related phenotypes in individual insects, providing enhanced resolution and improving efforts to characterize the mechanisms that govern phenotypic variation.

Wolbachia infection facilitates adaptive increase in male egg size in response to environmental changes

Under challenging conditions such as maladapted biotic and abiotic conditions, females can plastically adjust their egg size (gamete or zygote size) to counteract fitness declines early in life. Recent evidence suggests that endosymbionts may enhance this egg-size plasticity. Possible endosymbionts' modification of impact of multiple stressors is not well explored. Therefore, this study aims to test (1) whether Wolbachia infection influences the plasticity of parental investment in egg size under suboptimal environmental conditions and (2) whether the plasticity depends on the sex of eggs. We used three lines of the azuki bean beetle (Callosobruchus chinensis): a line coinfected with the wBruCon and wBruOri Wolbachia strains, a cured line infected solely with the wBruCon, and an uninfected (cured) line. These lines were subjected to either a control environment or a simulated climate change environment (elevated temperature and carbon dioxide levels, eT&CO2) to examine Wolbachia infection effects on parental investment in their offspring (egg size) and its subsequent impact on offspring fitness, including survival, development, and adult lifespan under starvation. After two days of eT&CO2 exposure, coinfected parents increased male egg size only. Larger eggs developed faster in both sexes and exhibited higher survival. However, offspring adult lifespan was not influenced by egg size but by environment, sex, Wolbachia infection, and development time: eT&CO2 reduced male lifespan but not female lifespan, the singly-infected line females lived longer than coinfected and uninfected line females, and shorter development time linked to longer lifespan. The negative correlation between development time and lifespan was higher under eT&CO2 but not sex-specific. This study is the first to demonstrate sex-specific egg size plasticity associated with Wolbachia infection in species with sex determination systems other than haplodiploid.

Wolbachia Screening in Aedes aegypti and Culex pipiens Mosquitoes from Madeira Island, Portugal

Mosquito-borne diseases such as dengue and West Nile virus pose serious public health risks. On Madeira Island, the presence of the mosquito species Aedes aegypti (Linnaeus, 1762) and Culex pipiens (Linnaeus, 1758) raises concerns about local transmission. In this study, we tested 100 Ae. aegypti and 40 Cx. pipiens mosquitoes collected exclusively in the municipality of Funchal, Madeira Island, to assess the presence and diversity of Wolbachia, a naturally occurring bacterium known to reduce mosquitos' ability to transmit viruses. Molecular identification confirmed that all Cx. pipiens specimens belonged to the molestus biotype, with three individuals identified as hybrids between molestus and pipiens biotypes. This is the first evidence of such hybrids in Madeira. Wolbachia was not detected in any of the Ae. aegypti samples. In contrast, all Cx. pipiens mosquitoes were positive, showing a 100% prevalence. Genetic characterization placed these infections within the wPip clade, supergroup B, sequence type 9. These findings provide key baseline data to inform future mosquito control strategies on the island. As Ae. aegypti showed no natural Wolbachia infection, introducing Wolbachia-infected mosquitoes may be necessary to implement such biocontrol approaches in Madeira.

Wolbachia Offers Protection Against Two Common Natural Viruses of Drosophila

Wolbachia pipientis is a maternally transmitted endosymbiont infecting more than half of terrestrial arthropod species. Wolbachia can express parasitic phenotypes such as manipulation of host reproduction and mutualist phenotypes such as protection against RNA virus infections. Because Wolbachia can invade populations by reproductive manipulation and block virus infection, it is used to modify natural insect populations. However, the ecological importance of virus protection is not yet clear, especially due to scarce information on Wolbachia protection against viruses that are common in nature. We used systemic infection to investigate whether Wolbachia protects its host by suppressing the titer of DMELDAV and DMelNora virus, two viruses that commonly infect Drosophila melanogaster flies in natural populations. Antiviral protection was tested in three systems to assess the impact of Wolbachia strains across species: (1) a panel of Wolbachia strains transfected into Drosophila simulans, (2) two Wolbachia strains introgressed into the natural host D. melanogaster, and (3) two native Wolbachia strains in their natural hosts Drosophila baimaii and Drosophila tropicalis. We showed that certain Wolbachia strains provide protection against DMelNora virus and DMELDAV, and this protection is correlated with Wolbachia density, which is consistent with what has been observed in protection against other RNA viruses. Additionally, we found that Wolbachia does not protect its original host, D. melanogaster, from DMELDAV infection. While native Wolbachia can reduce DMELDAV titers in D. baimaii, this effect was not detected in D. tropicalis. Although the Wolbachia protection-induced phenotype seems to depend on the virus, the specific Wolbachia strain, and the host species, our findings suggest that antiviral protection may be one of the mutualistic effects that helps explain why Wolbachia is so widespread in arthropod populations.

Influence of weather and seasonal factors on whitefly dynamics, associated endosymbiotic microbiomes, and Begomovirus transmission causing tomato leaf curl disease: insights from a metagenomic perspective

Introduction

Bemisia tabaci (Gennadius) is a globally significant agricultural pest, responsible for transmitting over 120 plant viruses, including those from the Begomovirus genus, which contribute to considerable crop losses. The species complex comprises cryptic species, associated with a diverse array of bacterial endosymbionts that play essential roles in host nutrition, virus transmission, and overall host adaptability. These endosymbionts are classified into primary and secondary categories, with primary endosymbionts forming obligatory, long-term associations, and secondary endosymbionts influencing factors such as biotype differentiation and vector competency. Notably, these microbial communities enhance B. tabaci's capacity to transmit viruses, including the tomato leaf curl virus (ToLCuV), which poses a significant threat to tomato production.

Methods

In this study, we examined the population dynamics of B. tabaci across three major tomato-growing regions in Karnataka, South India, focusing on their seasonal associations with endosymbionts and the incidence of tomato leaf curl disease (ToLCuD). Multiple regression analysis was employed to assess the influence of weather parameters on whitefly populations and disease prevalence. Additionally, we constructed a metagenomic profile to evaluate the effects of geographical location, seasonality, environmental factors, and agricultural practices on the bacterial communities associated with B. tabaci. Species-specific primers were used to validate the presence and diversity of these bacterial communities.

Results

Meteorological data revealed a positive correlation between temperature and B. tabaci populations, which corresponded with an increased incidence of ToLCuD. Genetic characterization of the whitefly identified Asia II-5 and Asia II-7 cryptic species as the dominant forms in the surveyed regions, with Portiera emerging as the most prevalent endosymbiont. A more in-depth analysis of the microbial communities associated with B. tabaci, utilizing 16S rRNA metagenomic sequencing, revealed a dominance of the Proteobacteria phylum. The endosymbiotic bacterial consortium was primarily composed of Candidatus Portiera, Candidatus Hamiltonella, Candidatus Rickettsia, and Candidatus Arsenophonus.

Discussion

The metagenomic analysis revealed a highly diverse array of bacterial communities, with 92% of sequences classified under Proteobacteria, representing a spectrum of microbial types associated with B. tabaci ranging from parasitic and pathogenic to mutualistic. Within this phylum, Alphaproteobacteria were predominant, known for their role as facultative symbionts, while Gammaproteobacteria provided essential nutrients to arthropods, enhancing their survival and fitness. The interplay of continuous and intensive tomato cultivation, elevated temperatures, favorable host plants, and abundant viral inoculum creates an ideal environment for the proliferation of B. tabaci and the widespread transmission of ToLCuD. The presence of diverse cryptic species of B. tabaci, which are efficient viral vectors, further complicates the situation. These findings underscore the urgent need for integrated management strategies globally to control both whitefly populations and ToLCuD, ensuring the protection of tomato crops and the sustainability of farmer livelihoods.

Malagasy flea microbiota results from a combination of vertically transmitted and environmentally acquired microbes

Fleas (Insecta, Siphonaptera) are the known vectors of serious bacterial pathogens, such as Yersinia pestis and Rickettsia typhi. The microbiota of fleas has been poorly investigated although it has a known influence on vector competence. Here, we report and analyse the microbiota of 577 flea specimens from Madagascar, a hotspot of plague transmission. Importantly, endemic Malagasy fleas show low host specificity, allowing addressing the importance of vertebrate host species in microbiota composition, as well as that of abiotic variables. We describe through Illumina sequencing of 2 hypervariable regions of 16 S rDNA the bacterial composition of 577 flea specimens of Madagascar. We address the importance of biotic (mammalian host and flea species) and abiotic (season and sampling site) variables on bacterial community composition. Bacterial composition appears driven by flea species and season, but interestingly not by the flea's vertebrate hosts. These results suggest that the flea microbiota is at least in part acquired before they become adult, possibly in the immature off-host stages. Taken together, our results suggest that the microbiota of sampled fleas are composed of bacterial taxa with vertical transmission, such as Wolbachia which are prevalent in the present dataset, together with several bacterial taxa for which the occurrence is driven by environment factors, especially season and habitat. Given the importance of the microbiota in vector competence, we discuss the epidemiological consequences of environmentally-driven acquisition of microbiota in fleas on plague transmission in Madagascar.

Candidate DNA and RNA viruses of Drosophila suzukii from Canada and Germany, and their interactions with Wolbachia

Some species of insects harbour strains of the endosymbiotic bacteria Wolbachia that do not cause obvious reproductive manipulations, and so it is unclear why they persist in host populations. There is some evidence that some of these endosymbionts may provide their hosts with protection against viruses, which would help to explain their persistence, but few studies have explored associations between Wolbachia and naturally occurring, common viruses in natural populations. Here, we asked whether individuals of the invasive vinegar fly Drosophila suzukii infected with the wSuz strain of Wolbachia were less likely to be infected by naturally occurring viruses in its invaded range, in western North America and in Europe. First, using next-generation sequencing, we conducted a virome survey of adult and larval D. suzukii in British Columbia, Canada, finding eight candidate RNA viruses and two candidate DNA viruses; all but one have not been reported previously. Only the previously described Teise virus, an RNA virus, was abundant in our virome survey. We then screened individual flies from British Columbia and Germany for Teise virus and Wolbachia. Wolbachia-infected D. suzukii from the field were not less likely to be infected by Teise virus. Overall, our results do not provide conclusive evidence that wSuz provides strong protection for D. suzukii against viruses that are common in natural populations. However, the other viruses that we discovered in this study deserve further characterization in terms of their pathogenicity to D. suzukii and the frequency and dynamics of infection in wild populations.

Dissecting transmission to understand parasite evolution

Parasite transmission is a complex, multi-stage process that significantly impacts host-parasite dynamics. Transmission plays a key role in epidemiology and virulence evolution, where it is expected to trade off with virulence. However, the extent to which classical models on virulence-transmission relationships apply in the real world is unclear. This insight piece proposes a novel framework that breaks transmission into three distinct stages: within-host infectiousness, an intermediate between-host stage (biotic or abiotic), and new host infection. Each stage is influenced by intrinsic and extrinsic factors to the parasite, which together will determine its transmission success. Analyzing the transmission stages separately and how they affect each other might enhance our understanding of which host-, parasite- or environmental-driven factors might shape parasite evolution and inform us about new effectors to act on when designing disease control strategies.

Effectiveness of self-management programmes for athletes with patellofemoral pain syndrome: A systematic review

Background

Patellofemoral pain syndrome (PFPS) is a common condition that can severely hinder an athlete's ability to perform and compete at their full potential and overall well-being. The emergence of self-management programmes presents a potential treatment avenue for athletes with PFPS. Self-management programmes aim to empower patients through education, symptom monitoring, tailored exercises, and gradual activity progression while respecting pain thresholds. Despite their potential benefits, limited data evaluates the effectiveness of self-management programmes specifically tailored for athletes with PFPS.

Objectives

To assess the effectiveness of self-management programmes in improving pain and functional ability among athletes with PFPS.

Methods

A systematic search was conducted across PubMed/MEDLINE, Cochrane Library, CINAHL, and PsycINFO databases. Randomised and non-randomised controlled trials comparing self-management interventions to other treatments or control groups for improving pain and functional ability among athletes with patellofemoral pain syndrome were included. Studies including athletes with PFPS (defined as pain persisting for ≥12 weeks) were eligible. Four reviewers independently extracted data and assessed quality using the Physiotherapy Evidence Database (PEDro) scale.

Results

Three trials involving 139 participants met the eligibility criteria. Identified self-management programmes comprised exercises, mindfulness training, and educational programmes tailored to athletes. Two of the eligible studies identified found that exercise consisting of the combination of mindfulness and/or dry needling reported an enhanced recovery rate in the studied population with regards to PFPS. The other study reported no added benefit whether exercise or education was used as a modality for the self-management of PFPS.

Conclusion

While some studies showed promising results for self-management programmes involving exercises and mindfulness in managing patellofemoral pain syndrome among athletes, the overall evidence could be more extensive and consistent. High-quality controlled trials with consistent methodology and athletic populations are needed to identify the most effective self-management approaches for patellofemoral pain syndrome in athletes.

Wolbachia enhances the survival of Drosophila infected with fungal pathogens

BACKGROUND

Wolbachia bacteria of arthropods are at the forefront of basic and translational research on multipartite host-symbiont-pathogen interactions. These vertically transmitted microbes are the most widespread endosymbionts on the planet due to factors including host reproductive manipulation and fitness benefits. Importantly, some strains of Wolbachia can inhibit viral pathogenesis within and between arthropod hosts. Mosquitoes carrying the wMel Wolbachia strain of Drosophila melanogaster have a greatly reduced capacity to spread viruses like dengue and Zika to humans. While significant research efforts have focused on viruses, relatively little attention has been given to Wolbachia-fungal interactions despite the ubiquity of fungal entomopathogens in nature.

RESULTS

Here, we demonstrate that Wolbachia increase the longevity of their Drosophila melanogaster hosts when challenged with a spectrum of yeast and filamentous fungal pathogens. We find that this pattern can vary based on host genotype, sex, and fungal species. Further, Wolbachia correlates with higher fertility and reduced pathogen titers during initial fungal infection, indicating a significant fitness benefit. Finally, RNA sequencing results show altered expression of many immune and stress response genes in the context of Wolbachia and fungal infection, suggesting host immunity may be involved in the mechanism.

CONCLUSIONS

This study demonstrates Wolbachia's protective role in diverse fungal pathogen interactions and determines that the phenotype is broad, but with several variables that influence both the presence and strength of the phenotype. It also is a critical step forward to understanding how symbionts can protect their hosts from a variety of pathogens.

Advances in antiviral strategies targeting mosquito-borne viruses: cellular, viral, and immune-related approaches

Mosquito-borne viruses (MBVs) are a major global health threat, causing significant morbidity and mortality. MBVs belong to several distinct viral families, each with unique characteristics. The primary families include Flaviviridae (e.g., Dengue, Zika, West Nile, Yellow Fever, Japanese Encephalitis), transmitted predominantly by Aedes and Culex mosquitoes; Togaviridae, which consists of the genus Alphavirus (e.g., Chikungunya, Eastern and Western Equine Encephalitis viruses), also transmitted by Aedes and Culex; Bunyaviridae (recently reorganized), containing viruses like Rift Valley Fever and Oropouche virus, transmitted by mosquitoes and sometimes sandflies; and Reoviridae, which includes the genus Orbivirus (e.g., West Nile and Bluetongue viruses), primarily affecting animals and transmitted by mosquitoes and sandflies. Despite extensive research, effective antiviral treatments for MBVs remain scarce, and current therapies mainly provide symptomatic relief and supportive care. This review examines the viral components and cellular and immune factors involved in the life cycle of MBVs. It also highlights recent advances in antiviral strategies targeting host factors such as lipid metabolism, ion channels, and proteasomes, as well as viral targets like NS2B-NS3 proteases and nonstructural proteins. Additionally, it explores immunomodulatory therapies to enhance antiviral responses and emphasizes the potential of drug repurposing, bioinformatics, artificial intelligence, and deep learning in identifying novel antiviral candidates. Continued research is crucial in mitigating MBVs' impact and preventing future outbreaks.

Symbiosis of intracellular bacteria Wolbachia with insects: a hundred years of study summarized

Wolbachia pipientis is an α-proteobacterium, which is a widespread intracellular symbiont in a number of Arthropoda and some Nematoda species. With insects, W. pipientis forms a symbiont-host system characterized by very close interactions between its components. The mutual effects of Wolbachia on the host and the host on Wolbachia are important biotic factors for both components of this symbiotic system. Wolbachia is able to affect both host reproduction and somatic organ function. Due to its prevalence among insects and a wide variety of both negative (cytoplasmic incompatibility and androcide are among the most well-known examples) and positive (increasing resistance to biotic and abiotic factors, providing vitamins and metabolites) effects on the host organism, Wolbachia is of great interest for both entomologists and microbiologists. The diversity of host phenotypes induced by Wolbachia provides a broad choice of evolutionary strategies (such as reproductive parasitism or mutually beneficial symbiont-host relationships) that it utilizes. The influence of Wolbachia is to be considered in the design of any experiment conducted on insects. The application of sequencing technologies has led to new approaches being created to study the existing relationships within the Wolbachia-insect system, but interpretation of the data obtained is challenging. Nevertheless, the prospects for the use of the whole-genome analysis data to study Wolbachia-host coevolution are beyond doubt. Ongoing projects to introduce Wolbachia strains, which provide antiviral host defense, into insect populations to control the spread of RNA-viruses are actively pursued, which could result in saving many human lives. The aim of this brief review is to summarize the data collected by scientists over the past hundred years of Wolbachia studies and the current understanding of its genetic diversity and mechanisms of interaction with the host, including those based on transcriptome analysis.

Wolbachia-based mosquito control: Environmental perspectives on population suppression and replacement strategies

Mosquito-borne diseases pose a significant threat to global health, and traditional mosquito control methods often fall short of effectiveness. A promising alternative is the biological control strategy of transinfecting mosquitoes with Wolbachia, a bacterium capable of outcompeting harmful pathogens and reducing the ability of mosquitoes to transmit diseases. However, Wolbachia infections are sensitive to abiotic environmental factors such as temperature and humidity, which can affect their densities in mosquitoes and, consequently, their ability to block pathogens. This review evaluates the effectiveness of different Wolbachia strains transinfected into mosquitoes in reducing mosquito-borne diseases. It explores how Wolbachia contributes to mosquito population control and pathogen interference, highlighting the importance of mathematical models in understanding Wolbachia transmission dynamics. Additionally, the review addresses the potential impact on arboviral transmission and the challenges posed by environmental fluctuations in mosquito control programs.

Wolbachia does not give an advantage to the ectoparasitoid Habrobracon hebetor (Say, 1836) when it develops on an infected host

The effect of Wolbachia on the viability and antimicrobial activity of the ectoparasitoid Habrobracon hebetor was evaluated in laboratory experiments. Two lines of the parasitoid, Wolbachia-infected (W+) and Wolbachia-free (W-), were used. Parasitoid larvae were fed with a host orally infected with a sublethal dose of Bacillus thuringiensis (Bt) and on the host uninfected with Bt. Parasitoid survival was assessed at developmental stages from second-instar larvae to adults. At all developmental stages, there were no statistically significant differences in survival between lines W+ and W-, regardless of host Bt infection. In both W+ and W- lines, the expression of lysozyme-like proteins, antimicrobial peptides (AMPs), and Hsp70 genes was analysed in fourth-instar larvae fed with an infected and uninfected host. In addition, lysozyme-like activity and antibacterial activity were evaluated. The expression of AMPs was significantly higher in W- larvae and did not get induced during the feeding on the Bt-infected host. mRNA expression of lysozyme-like proteins and lysozyme activity were significantly higher in W+ larvae than in W- larvae and did not get induced when the larvae were fed with the infected host. In whole-body homogenates of H. hebetor larvae fed with the uninfected host, antibacterial activity against gram-positive bacteria (Bacillus cereus and Bacillus subtilis) was significantly higher in the W+ line and did not get induced during the feeding with the Bt-infected host. Therefore, there is no obvious immunostimulatory effect of Wolbachia in H. hebetor larvae when they feed on a host infected with an entomopathogenic bacterium.

Endosymbionts as hidden players in tripartite pathosystem of interactions and potential candidates for sustainable viral disease management

The convoluted relationships between plants, viruses, and arthropod vectors housing bacterial endosymbionts are pivotal in the spread of harmful plant viral diseases. Endosymbionts play key roles in: manipulating host responses, influencing insect resistance to pesticides, shaping insect evolution, and bolstering virus acquisition, retention, and transmission. This interplay presents an innovative approach for developing sustainable strategies to manage plant diseases. Recent progress in targeting specific endosymbionts through genetic modifications, biotechnological advancements, and RNA interference shows potential for curbing viral spread and disease progression. Additionally, employing synthetic biology techniques like CRISPR/Cas9 to engineer endosymbionts and disrupt crucial interactions necessary for viral transmission in arthropod vectors holds promise for effective control measures. In this review, these obligate and facultative bacterial cruxes have been discussed to elaborate on their mechanistic involvement in the regulation and/or inhibition of tripartite pathways of interactions. Furthermore, we provide an in-depth understanding of endosymbionts' synergistic and antagonistic effects on: insect biology, plant immunity, and virus acquisition and transmission. Finally, we point out open questions for future research and provide research directions concerning the deployment of genetically engineered symbionts to affect plant-virus-vector interactions for sustainable disease management. By addressing existing knowledge gaps and charting future research paths, a deeper comprehension of the role of endosymbionts in plant-virus-vector interactions can pave the way for innovative and successful disease management strategies. The exploration of antiviral therapies, paratransgenesis, and pathogen-blocking tactics using engineered endosymbionts introduces pioneering solutions for lessening the impact of plant viral diseases and green pest management.

The infectivity of virus particles from Wolbachia-infected Drosophila

Viruses transmitted by arthropods pose a huge risk to human health. Wolbachia is an endosymbiotic bacterium that infects various arthropods and can block the viral replication cycle of several medically important viruses. As such, it has been successfully implemented in vector control strategies against mosquito-borne diseases, including Dengue virus. Whilst the mechanisms behind Wolbachia-mediated viral blocking are not fully characterised, it was recently shown that viruses grown in the presence of Wolbachia in some Dipteran cell cultures are less infectious than those grown in the absence of Wolbachia. Here, we investigate the breadth of this mechanism by determining if Wolbachia reduces infectivity in a different system at a different scale. To do this, we looked at Wolbachia's impact on insect viruses from two diverse virus families within the whole organism Drosophila melanogaster. Drosophila C virus (DCV; Family Dicistroviridae) and Flock House virus (FHV; Famliy Nodaviridae) were grown in adult D. melanogaster flies with and without Wolbachia strain wMelPop. Measures of the physical characteristics, infectivity, pathogenicity, and replicative properties of progeny virus particles did not identify any impact of Wolbachia on either DCV or FHV. Therefore, there was no evidence that changes in infectivity contribute to Wolbachia-mediated viral blocking in this system. Overall, this is consistent with growing evidence that the mechanisms behind Wolbachia viral blocking are dependent on the unique tripartite interactions occurring between the host, the Wolbachia strain, and the infecting virus.

Interaction of the Wolbachia surface protein with a novel pro-viral protein from Aedes aegypti

Dengue virus (DENV) and other flaviviruses are prevented from replicating in mosquitoes by Wolbachia. To date, several reports have appeared that highlight multiple molecular and cellular pathways involved in the blocking mechanism, which underlines the complicated nature of the mechanism. Here, we developed a hypothesis on whether Wolbachia proteins interact with pro-viral host proteins by using a unique approach to study the antiviral mechanism based on Wolbachia-host protein-protein interaction. We selected Wolbachia surface protein (WSP) for co-immunoprecipitation because of its abundance and possible secretion. We first confirmed WSP's secretion in mosquito cells and found two host proteins, Ae. aegypti serine-threonine kinase (STK) and synaptic vesicle membrane (SVM) protein VAT-1, and one Wolbachia protein (wGroEL) interacting with WSP. We examined the role of STK and SVM genes in relation to DENV replication in Ae. aegypti mosquitoes and mosquito cell lines with and without Wolbachia. In DENV-infected Aag2 cells, the expression of SVM and STK was significantly increased. However, although these genes were induced in Wolbachia-infected Aag2 cells, they were downregulated after DENV infection. Silencing of STK, but not SVM, reduced DENV replication in Aag2 cells and mosquitoes. Conversely, RNA activation of STK, by utilizing promoter induction via short activating oligos, resulted in higher DENV replication in Wolbachia-infected and uninfected cell lines. Overall, our findings suggest that STK is a pro-viral gene, and Wolbachia WSP binds to STK, possibly making it less accessible for DENV replication.

IMPORTANCE

Wolbachia is an endosymbiotic bacterium that blocks the replication of arboviruses in transinfected Aedes aegypti mosquitoes. In this study, we focused on identifying the potential interaction of Wolbachia proteins with the host pro-viral proteins. For this, we embarked on identifying the interacting proteins with a major Wolbachia protein, WSP, which is both structural and also secreted into the host cells. An Ae. aegypti STK was identified, which is induced in DENV and Wolbachia-infected cells. Silencing or induction of the gene led to reduced and increased DENV replication in vitro. Consistently, knocking down the gene in mosquitoes resulted in decreased virus replication. We hypothesize that WSP may sequester STK, which is pro-viral, contributing to Wolbachia virus blocking.

The bitter truth: how insects cope with toxic plant alkaloids

Plants are unique organisms that have developed ingenious strategies to cope with environmental challenges, such as herbivorous insects. One of these strategies is the synthesis of a vast array of chemical compounds, known as specialized metabolites, that serve many ecological functions. Among the most fascinating and diverse groups of specialized metabolites are the alkaloids, which are characterized by the presence of a nitrogen atom within a heterocyclic ring. While some have medicinal and recreational applications, others are highly unpalatable and/or toxic. The effects of alkaloids on both humans and insects can be very diverse, affecting their physiology and behavior. Insects that feed on alkaloid-containing plants have evolved various mechanisms to cope with the consequences of these toxins. These include sequestration, where insects store alkaloids in specialized tissues or organs, enzymatic detoxification through enzymes such as cytochrome P450 monooxygenases and glutathione S-transferases, and behavioral adaptations such as selective feeding. In this review, we explore the relationships between plant alkaloids and the evolutionary adaptations that enable insects to exploit alkaloid-rich plants as food sources and ecological niches minimizing the harmful effects of these natural compounds. We aim to provide a comprehensive and updated overview of this fascinating and complex ecological interaction.

Counting rare Wolbachia endosymbionts using digital droplet PCR

Wolbachia is the most widespread animal-associated intracellular microbe, living within the cells of over half of insect species. Since they can suppress pathogen replication and spread rapidly through insect populations, Wolbachia is at the vanguard of public health initiatives to control mosquito-borne diseases. Wolbachia's abilities to block pathogens and spread quickly are closely linked to their abundance in host tissues. The most common method for counting Wolbachia is quantitative polymerase chain reaction (qPCR), yet qPCR can be insufficient to count rare Wolbachia, necessitating tissue pooling and consequently compromising individual-level resolution of Wolbachia dynamics. Digital droplet PCR (ddPCR) offers superior sensitivity, enabling the detection of rare targets and eliminating the need for sample pooling. Here, we report three ddPCR assays to measure total Wolbachia abundance, Wolbachia abundance adjusted for DNA extraction efficiency, and Wolbachia density relative to host genome copies. Using Drosophila melanogaster with wMel Wolbachia as a model, we show these ddPCR assays can reliably detect as few as 7 to 12 Wolbachia gene copies in a 20 μL reaction. The designed oligos are homologous to sequences from at least 106 Wolbachia strains across Supergroup A and 53 host species from the Drosophila, Scaptomyza, and Zaprionus genera, suggesting broad utility. These highly sensitive ddPCR assays are expected to significantly advance Wolbachia-host interactions research by enabling the collection of molecular data from individual insect tissues. Their ability to detect rare Wolbachia will be especially valuable in applied and natural field settings where pooling samples could obscure important variation.

Symbiotic microbial population composition of Apolygus lucorum under temperature and pesticide pressures

Insect population control using pesticides faces new challenges as global temperatures change. Symbiotic bacteria of insects play a key role in insect resistance to pesticides, and these symbiotic bacteria themselves are sensitive to the effects of temperature changes. Apolygus lucorum, a sucking pest, survives in a wide range of temperatures (15°C-35°C), and is presently controlled predominantly using the pesticide imidacloprid. Here, we investigated the effects of temperature and imidacloprid on A. lucorum microbial population composition using 16S rRNA sequencing. We found that the application of imidacloprid in high-temperature environments led to an increase in the species diversity of bacteria in the body of A. lucorum. High temperatures may disrupt the symbiotic relationship between certain bacteria and A. lucorum, such as Cedecea neteri. High temperatures led to a decrease in the abundance of Cedecea neteri. Agathobaculum butyriciproducens, Advenella migardefenensis, and Akkermansia muciniphila were very sensitive to temperature and were strongly affected by temperature changes. Microorganisms that were greatly affected by the concentration of imidacloprid in the community include Aeromonas caviae and Akkermansia muciniphila. The aim of this study is to reveal the dynamics and diversity of symbiotic bacteria of A. lucorum treated with imidacloprid at a range of temperatures. These results provide insight into new strategies for pest control in a changing climate.

Wolbachia-based emerging strategies for control of vector-transmitted disease

Dengue fever is a mosquito-transmitted disease of great public health importance. Dengue lacks adequate vaccine protection and insecticide-based methods of mosquito control are proving increasingly ineffective. Here we review the emerging use of mosquitoes transinfected with the obligate intracellular bacterium Wolbachia pipientis for vector control. Wolbachia often induces cytoplasmic incompatibility in its mosquito hosts, resulting in infertile progeny between an infected male and an uninfected female. Wolbachia infection also suppresses the replication of pathogens in the mosquito, a process known as "pathogen blocking". Two strategies have emerged. The first one releases Wolbachia carriers (both male and female) to replace the wild mosquito population, a process driven by cytoplasmic incompatibility and that becomes irreversible once a threshold is reached. This suppresses disease transmission mainly by pathogen blocking and frequently requires a single intervention. The second strategy floods the field population with an exclusively male population of Wolbachia-carrying mosquitoes to generate infertile hybrid progeny. In this case, transmission suppression depends largely on decreasing the population density of mosquitoes driven by infertility and requires continued mosquito release. The efficacy of both Wolbachia-based approaches has been conclusively demonstrated by randomized and non-randomized studies of deployments across the world. However, results conducted in one setting cannot be directly or easily extrapolated to other settings because dengue incidence is highly affected by the conditions into which the mosquitoes are released. Compared to traditional vector control methods, Wolbachia-based approaches are much more environmentally friendly and can be effective in the medium/long term. On the flip side, they are much more complex and cost-intensive operations, requiring a substantial investment, infrastructure, trained personnel, coordination between agencies, and community engagement. Finally, we discuss recent evidence suggesting that the release of Wolbachia-transinfected mosquitoes has a moderate potential risk of spreading potentially dangerous genes in the environment.

Wolbachia in Antarctic terrestrial invertebrates: Absent or undiscovered?

Interactions between a host organism and its associated microbiota, including symbiotic bacteria, play a crucial role in host adaptation to changing environmental conditions. Antarctica provides a unique environment for the establishment and maintenance of symbiotic relationships. One of the most extensively studied symbiotic bacteria in invertebrates is Wolbachia pipientis, which is associated with a wide variety of invertebrates. Wolbachia is known for manipulating host reproduction and having obligate or facultative mutualistic relationships with various hosts. However, there is a lack of clear understanding of the prevalence of Wolbachia in terrestrial invertebrates in Antarctica. We present the outcomes of a literature search for information on the occurrence of Wolbachia in each of the major taxonomic groups of terrestrial invertebrates (Acari, Collembola, Diptera, Rotifera, Nematoda, Tardigrada). We also performed profiling of prokaryotes based on three marker genes and Kraken2 in available whole genome sequence data obtained from Antarctic invertebrate samples. We found no reports or molecular evidence of Wolbachia in these invertebrate groups in Antarctica. We discuss possible reasons underlying this apparent absence and suggest opportunities for more targeted future research to confirm bacteria's presence or absence.

Wolbachia-Based Approaches to Controlling Mosquito-Borne Viral Threats: Innovations, AI Integration, and Future Directions in the Context of Climate Change

Wolbachia-based mosquito control strategies have gained significant attention as a sustainable approach to reduce the transmission of vector-borne diseases such as dengue, Zika, and chikungunya. These endosymbiotic bacteria can limit the ability of mosquitoes to transmit pathogens, offering a promising alternative to traditional chemical-based interventions. With the growing impact of climate change on mosquito population dynamics and disease transmission, Wolbachia interventions represent an adaptable and resilient strategy for mitigating the public health burden of vector-borne diseases. Changes in temperature, humidity, and rainfall patterns can alter mosquito breeding habitats and extend the geographical range of disease vectors, increasing the urgency for effective control measures. This review highlights innovations in Wolbachia-based mosquito control and explores future directions in the context of climate change. It emphasizes the integration of Wolbachia with other biological approaches and the need for multidisciplinary efforts to address climate-amplified disease risks. As ecosystems shift, Wolbachia interventions could be crucial in reducing mosquito-borne diseases, especially in vulnerable regions. AI integration in Wolbachia research presents opportunities to enhance mosquito control strategies by modeling ecological data, predicting mosquito dynamics, and optimizing intervention outcomes. Key areas include refining release strategies, real-time monitoring, and scaling interventions. Future opportunities lie in advancing AI-driven approaches for integrating Wolbachia with other vector control measures, promoting adaptive, data-driven responses to climate-amplified disease transmission.

Insect Lipid Metabolism in the Presence of Symbiotic and Pathogenic Viruses and Bacteria

Insects, like most animals, have intimate interactions with microorganisms that can influence the insect host's lipid metabolism. In this chapter, we describe what is known so far about the role prokaryotic microorganisms play in insect lipid metabolism. We start exploring microbe-insect lipid interactions focusing on endosymbionts, and more specifically the gut microbiota that has been predominantly studied in Drosophila melanogaster. We then move on to an overview of the work done on the common and well-studied endosymbiont Wolbachia pipientis, also in interaction with other microbes. Taking a slightly different angle, we then look at the effect of human pathogens, including dengue and other viruses, on the lipids of mosquito vectors. We extend the work on human pathogens and include interactions with the endosymbiont Wolbachia that was identified as a natural tool to reduce the spread of mosquito-borne diseases. Research on lipid metabolism of plant disease vectors is up and coming and we end this chapter by highlighting current knowledge in that field.

Another tool in the toolbox: Aphid-specific Wolbachia protect against fungal pathogens

Aphids harbor nine common facultative symbionts, most mediating one or more ecological interactions. Wolbachia pipientis, well-studied in other arthropods, remains poorly characterized in aphids. In Pentalonia nigronervosa and P. caladii, global pests of banana, Wolbachia was initially hypothesized to function as a co-obligate nutritional symbiont alongside the traditional obligate Buchnera. However, genomic analyses failed to support this role. Our sampling across numerous populations revealed that more than 80% of Pentalonia aphids carried an M-supergroup strain of Wolbachia (wPni). The lack of fixation further supports a facultative status for Wolbachia, while high infection frequencies in these entirely asexual aphids strongly suggest Wolbachia confers net fitness benefits. Finding no correlation between Wolbachia presence and food plant use, we challenged Wolbachia-infected aphids with common natural enemies. Bioassays revealed that Wolbachia conferred significant protection against a specialized fungal pathogen (Pandora neoaphidis) but not against generalist pathogens or parasitoids. Wolbachia also improved aphid fitness in the absence of enemy challenge. Thus, we identified the first clear benefits for aphid-associated Wolbachia and M-supergroup strains specifically. Aphid-Wolbachia systems provide unique opportunities to merge key models of symbiosis to better understand infection dynamics and mechanisms underpinning symbiont-mediated phenotypes.

Variable effects of transient Wolbachia infections on alphaviruses in Aedes aegypti

Wolbachia pipientis (= Wolbachia) has promise as a tool to suppress virus transmission by Aedes aegypti mosquitoes. However, Wolbachia can have variable effects on mosquito-borne viruses. This variation remains poorly characterized, yet the multimodal effects of Wolbachia on diverse pathogens could have important implications for public health. Here, we examine the effects of transient somatic infection with two strains of Wolbachia (wAlbB and wMel) on the alphaviruses Sindbis virus (SINV), O'nyong-nyong virus (ONNV), and Mayaro virus (MAYV) in Ae. aegypti. We found variable effects of Wolbachia including enhancement and suppression of viral infections, with some effects depending on Wolbachia strain. Both wAlbB- and wMel-infected mosquitoes showed enhancement of SINV infection rates one week post-infection, with wAlbB-infected mosquitoes also having higher viral titers than controls. Infection rates with ONNV were low across all treatments and no significant effects of Wolbachia were observed. The effects of Wolbachia on MAYV infections were strikingly strain-specific; wMel strongly blocked MAYV infections and suppressed viral titers, while wAlbB had more modest effects. The variable effects of Wolbachia on vector competence underscore the importance of further research into how this bacterium impacts the virome of wild mosquitoes including the emergent human pathogens they transmit.

No Evidence for Wolbachia Effects on the Thermal Preference of the Invasive Pest Liriomyza huidobrensis

Heritable endosymbiont Wolbachia is prevalent among arthropods, serving multiple functions for their hosts. However, the role of Wolbachia in mediating thermal preference selection remains largely unexplored. In this study, we utilized a custom-built thermal gradient to evaluate the thermal preference (Tp) of 1367 individuals of the invasive leaf-miner Liriomyza huidobrensis with or without Wolbachia wLhui from Yunnan and Xinjiang populations. Under meticulously controlled conditions and with a vast sample size, we found no significant difference in the mean Tp between wLhui-infected and uninfected leaf miners from either population when host age and sex were not considered. Furthermore, generalized linear model (GLM) analysis revealed no significant correlation between average Tp and age, sex, or Wolbachia infection, nor interactions among these factors, except in the Xinjiang population, where Tp was strongly associated with host age. Finally, we discuss the ecological implications of these findings and propose future research directions on Wolbachia-mediated host Tp in the leaf miner. Overall, our findings do not provide evidence that Wolbachia significantly affects the thermal preference of L. huidobrensis. Further studies across different systems are needed to investigate the complex interactions between Wolbachia and insect thermal behavior.

Bivalve microbiomes are shaped by host species, size, parasite infection, and environment

Many factors affect an organism's microbiome including its environment, proximity to other organisms, and physiological condition. As filter feeders, bivalves have highly plastic microbiomes that are especially influenced by the surrounding seawater, yet they also maintain a unique core set of microbes. Using 16S ribosomal RNA sequencing, we characterized the bacterial microbiomes of four species of bivalves native to the Mid-Atlantic East Coast of North America: Crassostrea virginica, Macoma balthica, Ameritella mitchelli, and Ischadium recurvum and assessed the impact of their external environment, internal parasites, and size on their microbial communities. We found significant differences in bacterial amplicon sequence variants (ASVs) across species, with each species harboring a core ASV present across all individuals. We further found that some C. virginica co-cultured with I. recurvum had high abundances of the I. recurvum core ASV. We identified ASVs associated with infection by the parasites Perkinsus marinus and Zaops ostreum as well others associated with bivalve size. Several of these ASV are candidates for further investigation as potential probiotics, as they were found positively correlated with bivalve size and health. This research represents the first description of the microbiomes of A. mitchelli, I. recurvum, and M. balthica. We document that all four species have highly plastic microbiomes, while maintaining certain core bacteria, with important implications for growth, health, and adaptation to new environments.

Wolbachia affect female mate preference and offspring fitness in a parasitoid wasp

BACKGROUND

Wolbachia are widespread intracellular bacteria in insects that often have high rates of spread due to their impact on insect reproduction. These bacteria may also affect the mating behavior of their host with impacts on the fitness of host progeny. In this study, we investigated the impact of Wolbachia on a preference for mating with young or old males in the parasitoid wasp Habrobracon hebetor.

RESULTS

Our results showed that uninfected females from a tetracycline-treated line preferred to mate with young males, whereas Wolbachia-infected females had no preference. Time to mating was relatively shorter in the infected lines. Regardless of Wolbachia infection status, progeny resulting from matings with young males showed higher fitness than those from crosses with old males, and infected females crossed with infected young males showed the highest performance.

CONCLUSION

These results suggest an impact of Wolbachia on female mate preference and offspring fitness although it is unclear how this phenomenon increases Wolbachia transmission of infected wasps. © 2024 Society of Chemical Industry.

Comparative analysis of the microbiota of sand fly vectors of Leishmania major and L. tropica in a mixed focus of cutaneous leishmaniasis in southeast Tunisia; ecotype shapes the bacterial community structure

Phlebotomine sand flies are vectors of the protozoan parasite Leishmania spp. Although the intestinal microbiota is involved in a wide range of biological and physiological processes and has the potential to alter vector competence, little is known about the impact of host species and environment on the gut microbiome. To address this issue, a comparative analysis of the microbiota of sand fly vector populations of Leishmania major and L. tropica in a mixed focus of cutaneous leishmaniasis in Tunisia was performed. Bacterial 16S rRNA gene amplification and Illumina MiSeq sequencing were used to characterize and compare the overall bacterial and fungal composition of field-collected sand flies: Phlebotomus papatasi, Ph. perniciosus, Ph. riouxi, and Ph. sergenti. Thirty-eight bacterial genera belonging to five phyla were identified in 117 female specimens. The similarities and differences between the microbiome data from different samples collected from three collections were determined using principal coordinate analysis (PCoA). Substantial variations in the bacterial composition were found between geographically distinct populations of the same sand fly species, but not between different species at the same location, suggesting that the microbiota content was structured according to environmental factors rather than host species. These findings suggest that host phylogeny may play a minor role in determining the insect gut microbiota, and its potential to affect the transmission of the Leishmania parasite appear to be very low. These results highlight the need for further studies to decode sand fly Leishmania-microbiota interactions, as even the same bacterial species, such as Enterococcus faecalis, can exert completely opposite effects when confronted with different pathogens within various host insects and vice versa.

Host range of a parasitoid wasp is linked to host susceptibility to its mutualistic viral symbiont

Parasitoid wasps are one of the most species-rich groups of animals on Earth, due to their ability to successfully develop as parasites of nearly all types of insects. Unlike most known parasitoid wasps that specialize towards one or a few host species, Diachasmimorpha longicaudata is a generalist that can survive within multiple genera of tephritid fruit fly hosts, including many globally important pest species. Diachasmimorpha longicaudata has therefore been widely released to suppress pest populations as part of biological control efforts in tropical and subtropical agricultural ecosystems. In this study, we investigated the role of a mutualistic poxvirus in shaping the host range of D. longicaudata across three genera of agricultural pest species: two of which are permissive hosts for D. longicaudata parasitism and one that is a nonpermissive host. We found that permissive hosts Ceratitis capitata and Bactrocera dorsalis were highly susceptible to manual virus injection, displaying rapid virus replication and abundant fly mortality. However, the nonpermissive host Zeugodacus cucurbitae largely overcame virus infection, exhibiting substantially lower mortality and no virus replication. Investigation of transcriptional dynamics during virus infection demonstrated hindered viral gene expression and limited changes in fly gene expression within the nonpermissive host compared with the permissive species, indicating that the host range of the viral symbiont may influence the host range of D. longicaudata wasps. These findings also reveal that viral symbiont activity may be a major contributor to the success of D. longicaudata as a generalist parasitoid species and a globally successful biological control agent.

Comparative genomics of the carmine cochineal symbiont Candidatus Dactylopiibacterium carminicum reveals possible protection to the host against viruses via CRISPR/Cas

We present new genomes from the bacterial symbiont Candidatus Dactylopiibacterium carminicum obtained from non-domesticated carmine cochineals belonging to the scale insect Dactylopius (Hemiptera: Coccoidea: Dactylopiidae). As Dactylopiibacterium has not yet been cultured in the laboratory, metagenomes and metatranscriptomics have been key in revealing putative symbiont functions. Dactylopiibacterium is a nitrogen-fixing beta-proteobacterium that may be vertically transmitted and shows differential gene expression inside the cochineal depending on the tissue colonized. Here we found that all cochineal species tested had Dactylopiibacterium carminicum which has a highly conserved genome. All Dactylopiibacterium genomes analyzed had genes involved in nitrogen fixation and plant polymer degradation. Dactylopiibacterium genomes resemble those from free-living plant bacteria, some found as endophytes. Notably, we found here a new putative novel function where the bacteria may protect the insect from viruses, since all Dactylopiibacterium genomes contain CRISPRs with a spacer matching nucleopolyhedrovirus that affects insects.

Endocytosed dsRNAs induce lysosomal membrane permeabilization that allows cytosolic dsRNA translocation for Drosophila RNAi responses

RNA interference (RNAi) is a gene-silencing mechanism triggered by the cytosolic entry of double-stranded RNAs (dsRNAs). Many animal cells internalize extracellular dsRNAs via endocytosis for RNAi induction. However, it is not clear how the endocytosed dsRNAs are translocated into the cytosol across the endo/lysosomal membrane. Herein, we show that in Drosophila S2 cells, endocytosed dsRNAs induce lysosomal membrane permeabilization (LMP) that allows cytosolic dsRNA translocation. LMP mediated by dsRNAs requires the lysosomal Cl-/H+ antiporter ClC-b/DmOstm1. In clc-b or dmostm1 knockout S2 cells, extracellular dsRNAs are endocytosed and reach the lysosomes normally but fail to enter the cytosol. Pharmacological induction of LMP restores extracellular dsRNA-directed RNAi in clc-b or dmostm1-knockout cells. Furthermore, clc-b or dmostm1 mutant flies are defective in extracellular dsRNA-directed RNAi and its associated antiviral immunity. Therefore, endocytosed dsRNAs have an intrinsic ability to induce ClC-b/DmOstm1-dependent LMP that allows cytosolic dsRNA translocation for RNAi responses in Drosophila cells.

The endosymbiont Spiroplasma poulsonii increases Drosophila melanogaster resistance to pathogens by enhancing iron sequestration and melanization

Facultative endosymbiotic bacteria, such as Wolbachia and Spiroplasma species, are commonly found in association with insects and can dramatically alter their host physiology. Many endosymbionts are defensive and protect their hosts against parasites or pathogens. Despite the widespread nature of defensive insect symbioses and their importance for the ecology and evolution of insects, the mechanisms of symbiont-mediated host protection remain poorly characterized. Here, we utilized the fruit fly Drosophila melanogaster and its facultative endosymbiont Spiroplasma poulsonii to characterize the mechanisms underlying symbiont-mediated host protection against bacterial and fungal pathogens. Our results indicate a variable effect of S. poulsonii on infection outcome, with endosymbiont-harboring flies being more resistant to Rhyzopus oryzae, Staphylococcus aureus, and Providencia alcalifaciens but more sensitive or as sensitive as endosymbiont-free flies to the infections with Pseudomonas species. Further focusing on the protective effect, we identified Transferrin-mediated iron sequestration induced by Spiroplasma as being crucial for the defense against R. oryzae and P. alcalifaciens. In the case of S. aureus, enhanced melanization in Spiroplasma-harboring flies plays a major role in protection. Both iron sequestration and melanization induced by Spiroplasma require the host immune sensor protease Persephone, suggesting a role of proteases secreted by the symbiont in the activation of host defense reactions. Hence, our work reveals a broader defensive range of Spiroplasma than previously appreciated and adds nutritional immunity and melanization to the defensive arsenal of symbionts.

IMPORTANCE

Defensive endosymbiotic bacteria conferring protection to their hosts against parasites and pathogens are widespread in insect populations. However, the mechanisms by which most symbionts confer protection are not fully understood. Here, we studied the mechanisms of protection against bacterial and fungal pathogens mediated by the Drosophila melanogaster endosymbiont Spiroplasma poulsonii. We demonstrate that besides the previously described protection against wasps and nematodes, Spiroplasma also confers increased resistance to pathogenic bacteria and fungi. We identified Spiroplasma-induced iron sequestration and melanization as key defense mechanisms. Our work broadens the known defense spectrum of Spiroplasma and reveals a previously unappreciated role of melanization and iron sequestration in endosymbiont-mediated host protection. We propose that the mechanisms we have identified here may be of broader significance and could apply to other endosymbionts, particularly to Wolbachia, and potentially explain their protective properties.

A comprehensive review of Wolbachia-mediated mechanisms to control dengue virus transmission in Aedes aegypti through innate immune pathways

The Dengue virus (DENV), primarily spread by Aedes aegypti and also by Aedes albopictus in some regions, poses significant global health risks. Alternative techniques are urgently needed because the current control mechanisms are insufficient to reduce the transmission of DENV. Introducing Wolbachia pipientis into Ae. aegypti inhibits DENV transmission, however, the underlying mechanisms are still poorly understood. Innate immune effector upregulation, the regulation of autophagy, and intracellular competition between Wolbachia and DENV for lipids are among the theories for the mechanism of inhibition. Furthermore, mainly three immune pathways Toll, IMD, and JAK/STAT are involved in the host for the suppression of the virus. These pathways are activated by Wolbachia and DENV in the host and are responsible for the upregulation and downregulation of many genes in mosquitoes, which ultimately reduces the titer of the DENV in the host. The functioning of these immune pathways depends upon the Wolbachia, host, and virus interaction. Here, we summarize the current understanding of DENV recognition by the Ae. aegypti's immune system, aiming to create a comprehensive picture of our knowledge. Additionally, we investigated how Wolbachia regulates the activation of multiple genes associated with immune priming for the reduction of DENV.

In vitro extracellular replication of Wolbachia endobacteria

Obligate intracellular endobacteria of the genus Wolbachia are widespread in arthropods and several filarial nematodes. Control programs for vector-borne diseases (dengue, Zika, malaria) and anti-filarial therapy with antibiotics are based on this important endosymbiont. Investigating Wolbachia, however, is impeded by the need for host cells. In this study, the requirements for Wolbachia wAlbB growth in a host cell-free in vitro culture system were characterized via qPCRs. A cell lysate fraction from Aedes albopictus C6/36 insect cells containing cell membranes and medium with fetal bovine serum were identified as requisite for cell-free replication of Wolbachia. Supplementation with the membrane fraction of insect cell lysate increased extracellular Wolbachia replication by 4.2-fold. Replication rates in the insect cell-free culture were lower compared to Wolbachia grown inside insect cells. However, the endobacteria were able to replicate for up to 12 days and to infect uninfected C6/36 cells. Cell-free Wolbachia treated with the lipid II biosynthesis inhibitor fosfomycin had an enlarged phenotype, seen previously for intracellular Wolbachia in C6/36 cells, indicating that the bacteria were unable to divide. In conclusion, we have developed a cell-free culture system in which Wolbachia replicate for up to 12 days, providing an in vitro tool to elucidate the biology of these endobacteria, e.g., cell division by using compounds that may not enter the C6/36 cells. A better understanding of Wolbachia biology, and in particular host-symbiont interactions, is key to the use of Wolbachia in vector control programs and to future drug development against filarial diseases.

Inter-kingdom communication and the sympoietic way of life

Organisms are now seen as holobionts, consortia of several species that interact metabolically such that they sustain and scaffold each other's existence and propagation. Sympoiesis, the development of the symbiotic relationships that form holobionts, is critical for our understanding the origins and maintenance of biodiversity. Rather than being the read-out of a single genome, development has been found to be sympoietic, based on multigenomic interactions between zygote-derived cells and symbiotic microbes. These symbiotic and sympoietic interactions are predicated on the ability of cells from different kingdoms of life (e.g., bacteria and animals) to communicate with one another and to have their chemical signals interpreted in a manner that facilitates development. Sympoiesis, the creation of an entity by the interactions of other entities, is commonly seen in embryogenesis (e.g., the creation of lenses and retinas through the interaction of brain and epidermal compartments). In holobiont sympoiesis, interactions between partners of different domains of life interact to form organs and biofilms, wherein each of these domains acts as the environment for the other. If evolution is forged by changes in development, and if symbionts are routinely involved in our development, then changes in sympoiesis can constitute an important factor in evolution.

Bacteria are a major determinant of Orsay virus transmission and infection in Caenorhabditis elegans

The microbiota is a key determinant of the physiology and immunity of animal hosts. The factors governing the transmissibility of viruses between susceptible hosts are incompletely understood. Bacteria serve as food for Caenorhabditis elegans and represent an integral part of the natural environment of C. elegans. We determined the effects of bacteria isolated with C. elegans from its natural environment on the transmission of Orsay virus in C. elegans using quantitative virus transmission and host susceptibility assays. We observed that Ochrobactrum species promoted Orsay virus transmission, whereas Pseudomonas lurida MYb11 attenuated virus transmission relative to the standard laboratory bacterial food Escherichia coli OP50. We found that pathogenic Pseudomonas aeruginosa strains PA01 and PA14 further attenuated virus transmission. We determined that the amount of Orsay virus required to infect 50% of a C. elegans population on P. lurida MYb11 compared with Ochrobactrum vermis MYb71 was dramatically increased, over three orders of magnitude. Host susceptibility was attenuated even further in the presence of P. aeruginosa PA14. Genetic analysis of the determinants of P. aeruginosa required for attenuation of C. elegans susceptibility to Orsay virus infection revealed a role for regulators of quorum sensing. Our data suggest that distinct constituents of the C. elegans microbiota and potential pathogens can have widely divergent effects on Orsay virus transmission, such that associated bacteria can effectively determine host susceptibility versus resistance to viral infection. Our study provides quantitative evidence for a critical role for tripartite host-virus-bacteria interactions in determining the transmissibility of viruses among susceptible hosts.

Wolbachia impacts microbiome diversity and fitness-associated traits for Drosophila melanogaster in a seasonally fluctuating environment

The microbiome contributes to many different host traits, but its role in host adaptation remains enigmatic. The fitness benefits of the microbiome often depend on ecological conditions, but theory suggests that fluctuations in both the microbiome and environment modulate these fitness benefits. Moreover, vertically transmitted bacteria might constrain the ability of both the microbiome and host to respond to changing environments. Drosophila melanogaster provides an excellent system to investigate the impacts of interactions between the microbiome and the environment. To address this question, we created field mesocosms of D. melanogaster undergoing seasonal environmental change with and without the vertically transmitted bacteria, Wolbachia pipientis. Sampling temporal patterns in the microbiome revealed that Wolbachia constrained microbial diversity. Furthermore, Wolbachia and a dominant member of the microbiome, Commensalibacter, were associated with differences in two higher-order fitness traits, starvation resistance and lifespan. Our work here suggests that the interplay between the abiotic context and microbe-microbe interactions may shape key host phenotypes that underlie adaptation to changing environments. We conclude by exploring the consequences of complex interactions between Wolbachia and the microbiome for our understanding of eco-evolutionary processes that shape host-microbiome interactions.

Insect-microbe interactions and their influence on organisms and ecosystems

Microorganisms are important associates of insect and arthropod species. Insect-associated microbes, including bacteria, fungi, and viruses, can drastically impact host physiology, ecology, and fitness, while many microbes still have no known role. Over the past decade, we have increased our knowledge of the taxonomic composition and functional roles of insect-associated microbiomes and viromes. There has been a more recent shift toward examining the complexity of microbial communities, including how they vary in response to different factors (e.g., host genome, microbial strain, environment, and time), and the consequences of this variation for the host and the wider ecological community. We provide an overview of insect-microbe interactions, the variety of associated microbial functions, and the evolutionary ecology of these relationships. We explore the influence of the environment and the interactive effects of insects and their microbiomes across trophic levels. Additionally, we discuss the potential for subsequent synergistic and reciprocal impacts on the associated microbiomes, ecological interactions, and communities. Lastly, we discuss some potential avenues for the future of insect-microbe interactions that include the modification of existing microbial symbionts as well as the construction of synthetic microbial communities.

Biological function molecular pathways and druggability of DNMT2/TRDMT1

5-methylcytosine (m5C) is among the most common epigenetic modification in DNA and RNA molecules, and plays an important role in the animal development and disease pathogenesis. Interestingly, unlike other m5C DNA methyltransferases (DNMTs), DNMT2/TRDMT1 has the double-substrate specificity and adopts a DNMT-similar catalytic mechanism to methylate RNA. Moreover, it is widely involved in a variety of physiological regulatory processes, such as the gene expression, precise protein synthesis, immune response, and disease occurrence. Thus, comprehending the epigenetic mechanism and function of DNMT2/TRDMT1 will probably provide new strategies to treat some refractory diseases. Here, we discuss recent studies on the spatiotemporal expression pattern and post-translational modifications of DNMT2/TRDMT1, and summarize the research advances in substrate characteristics, catalytic recognition mechanism, DNMT2/TRDMT1-related genes or proteins, pharmacological application, and inhibitor development. This review will shed light on the pharmacological design by targeting DNMT2/TRDMT1 to treat parasitic, viral and oncologic diseases.

Lack of abundant core virome in Culex mosquitoes from a temperate climate region despite a mosquito species-specific virome

In arthropod-associated microbial communities, insect-specific viruses (ISVs) are prevalent yet understudied due to limited infectivity outside their natural hosts. However, ISVs might play a crucial role in regulating mosquito populations and influencing arthropod-borne virus transmission. Some studies have indicated a core virome in mosquitoes consisting of mostly ISVs. Employing single mosquito metagenomics, we comprehensively profiled the virome of native and invasive mosquito species in Belgium. This approach allowed for accurate host species determination, prevalence assessment of viruses and Wolbachia, and the identification of novel viruses. Contrary to our expectations, no abundant core virome was observed in Culex mosquitoes from Belgium. In that regard, we caution against rigidly defining mosquito core viromes and encourage nuanced interpretations of other studies. Nonetheless, our study identified 45 viruses of which 28 were novel, enriching our understanding of the mosquito virome and ISVs. We showed that the mosquito virome in this study is species-specific and less dependent on the location where mosquitoes from the same species reside. In addition, because Wolbachia has previously been observed to influence arbovirus transmission, we report the prevalence of Wolbachia in Belgian mosquitoes and the detection of several Wolbachia mobile genetic elements. The observed prevalence ranged from 83% to 92% in members from the Culex pipiens complex.IMPORTANCECulex pipiens mosquitoes are important vectors for arboviruses like West Nile virus and Usutu virus. Virome studies on individual Culex pipiens, and on individual mosquitoes in general, have been lacking. To mitigate this, we sequenced the virome of 190 individual Culex and 8 individual Aedes japonicus mosquitoes. We report the lack of a core virome in these mosquitoes from Belgium and caution the interpretation of other studies in this light. The discovery of new viruses in this study will aid our comprehension of insect-specific viruses and the mosquito virome in general in relation to mosquito physiology and mosquito population dynamics.

Intracellular defensive symbiont is culturable and capable of transovarial, vertical transmission

Insects frequently form heritable associations with beneficial bacteria that are vertically transmitted from parent to offspring. Long-term vertical transmission has repeatedly resulted in genome reduction and gene loss, rendering many such bacteria incapable of establishment in axenic culture. Among aphids, heritable endosymbionts often provide context-specific benefits to their hosts. Although these associations have large impacts on host phenotypes, experimental approaches are often limited by an inability to cultivate these microbes. Here, we report the axenic culture of Candidatus Fukatsuia symbiotica strain WIR, a heritable bacterial endosymbiont of the pea aphid, Acyrthosiphon pisum. Whole-genome sequencing revealed similar genomic features and high sequence similarity to previously described strains, suggesting that the cultivation techniques used here may be applicable to Ca. F. symbiotica strains from distantly related aphids. Microinjection of cultured Ca. F. symbiotica into uninfected aphids revealed that it can reinfect developing embryos and that infections are maintained in subsequent generations via transovarial maternal transmission. Artificially infected aphids exhibit phenotypic and life history traits similar to those observed for native infections. Our results show that Ca. F. symbiotica may be a useful tool for experimentally probing the molecular mechanisms underlying host-symbiont interactions in a heritable symbiosis.

IMPORTANCE

Diverse eukaryotic organisms form stable, symbiotic relationships with bacteria that provide benefits to their hosts. While these associations are often biologically important, they can be difficult to probe experimentally because intimately host-associated bacteria are difficult to access within host tissues, and most cannot be cultured. This is especially true for the intracellular, maternally inherited bacteria associated with many insects, including aphids. Here, we demonstrate that a pea aphid-associated strain of the heritable endosymbiont, Candidatus Fukatsuia symbiotica, can be grown outside of its host using standard microbiology techniques and can readily re-establish infection that is maintained across host generations. These artificial infections recapitulate the effects of native infections, making this host-symbiont pair a useful experimental system.

Reduction of Wolbachia in Diaphorina citri (Hemiptera: Liviidae) increases phytopathogen acquisition and decreases fitness

Wolbachia pipientis is a maternally inherited intracellular bacterium that infects a wide range of arthropods. Wolbachia can have a significant impact on host biology and development, often due to its effects on reproduction. We investigated Wolbachia-mediated effects in the Asian citrus psyllid, Diaphorina citri Kuwayama, which transmits Candidatus Liberibacter asiaticus (CLas), the causal agent of citrus greening disease. Diaphorina citri are naturally infected with Wolbachia; therefore, investigating Wolbachia-mediated effects on D. citri fitness and CLas transmission required artificial reduction of this endosymbiont with the application of doxycycline. Doxycycline treatment of psyllids reduced Wolbachia infection by approximately 60% in both male and female D. citri. Psyllids treated with doxycycline exhibited higher CLas acquisition in both adults and nymphs as compared with negative controls. In addition, doxycycline-treated psyllids exhibited decreased fitness as measured by reduced egg and nymph production as well as adult emergence as compared with control lines without the doxycycline treatment. Our results indicate that Wolbachia benefits D. citri by improving fitness and potentially competes with CLas by interfering with phytopathogen acquisition. Targeted manipulation of endosymbionts in this phytopathogen vector may yield disease management tools.

Wolbachia endosymbionts in Drosophila regulate the resistance to Zika virus infection in a sex dependent manner

Drosophila melanogaster has been used extensively for dissecting the genetic and functional bases of host innate antiviral immunity and virus-induced pathology. Previous studies have shown that the presence of Wolbachia endosymbionts in D. melanogaster confers resistance to infection by certain viral pathogens. Zika virus is an important vector-borne pathogen that has recently expanded its range due to the wide geographical distribution of the mosquito vector. Here, we describe the effect of Wolbachia on the immune response of D. melanogaster adult flies following Zika virus infection. First, we show that the presence of Wolbachia endosymbionts promotes the longevity of uninfected D. melanogaster wild type adults and increases the survival response of flies following Zika virus injection. We find that the latter effect is more pronounced in females rather than in males. Then, we show that the presence of Wolbachia regulates Zika virus replication during Zika virus infection of female flies. In addition, we demonstrate that the antimicrobial peptide-encoding gene Drosocin and the sole Jun N-terminal kinase-specific MAPK phosphatase Puckered are upregulated in female adult flies, whereas the immune and stress response gene TotM is upregulated in male individuals. Finally, we find that the activity of RNA interference and Toll signaling remain unaffected in Zika virus-infected female and male adults containing Wolbachia compared to flies lacking the endosymbionts. Our results reveal that Wolbachia endosymbionts in D. melanogaster affect innate immune signaling activity in a sex-specific manner, which in turn influences host resistance to Zika virus infection. This information contributes to a better understanding of the complex interrelationship between insects, their endosymbiotic bacteria, and viral infection. Interpreting these processes will help us design more effective approaches for controlling insect vectors of infectious disease.

A specific innate immune response silences the virulence of Pseudomonas aeruginosa in a latent infection model in the Drosophila melanogaster host

Microbial pathogenicity often depends on the route of infection. For instance, P. aeruginosa or S. marcescens cause acute systemic infections when low numbers of bacteria are injected into D. melanogaster flies whereas flies succumb much slower to the continuous ingestion of these pathogens, even though both manage to escape from the gut compartment and reach the hemocoel. Here, we have developed a latent P. aeruginosa infection model by feeding flies on the bacteria for a short period. The bacteria stably colonize internal tissues yet hardly cause any damage since latently-infected flies live almost as long as noninfected control flies. The apparently dormant bacteria display particular characteristics in terms of bacterial colony morphology, composition of the outer cell wall, and motility. The virulence of these bacteria can however be reactivated upon wounding the host. We show that melanization but not the cellular or the systemic humoral response is the predominant host defense that establishes latency and may coerce the bacteria to a dormant state. In addition, the lasting activation of the melanization responses in latently-infected flies provides a degree of protection to the host against a secondary fungal infection. Latent infection by an ingested pathogen protects against a variety of homologous or heterologous systemic secondary infectious challenges, a situation previously described for the endosymbiotic Wolbachia bacteria, a guard against viral infections.

IMD-mediated innate immune priming increases Drosophila survival and reduces pathogen transmission

Invertebrates lack the immune machinery underlying vertebrate-like acquired immunity. However, in many insects past infection by the same pathogen can 'prime' the immune response, resulting in improved survival upon reinfection. Here, we investigated the mechanistic basis and epidemiological consequences of innate immune priming in the fruit fly Drosophila melanogaster when infected with the gram-negative bacterial pathogen Providencia rettgeri. We find that priming in response to P. rettgeri infection is a long-lasting and sexually dimorphic response. We further explore the epidemiological consequences of immune priming and find it has the potential to curtail pathogen transmission by reducing pathogen shedding and spread. The enhanced survival of individuals previously exposed to a non-lethal bacterial inoculum coincided with a transient decrease in bacterial loads, and we provide strong evidence that the effect of priming requires the IMD-responsive antimicrobial-peptide Diptericin-B in the fat body. Further, we show that while Diptericin B is the main effector of bacterial clearance, it is not sufficient for immune priming, which requires regulation of IMD by peptidoglycan recognition proteins. This work underscores the plasticity and complexity of invertebrate responses to infection, providing novel experimental evidence for the effects of innate immune priming on population-level epidemiological outcomes.

Regulation of ubiquitination and antiviral activity of Cactin by deubiquitinase Usp14 in Drosophila

Cactin, a highly conserved protein, plays a crucial role in various physiological processes in eukaryotes, including innate immunity. Recently, the function of Cactin in the innate immunity of Drosophila has been explored, revealing that Cactin regulates a non-canonical signaling pathway associated with the Toll and Imd pathways via the Cactin-Deaf1 axis. In addition, Cactin exhibits specific antiviral activity against the Drosophila C virus (DCV) in Drosophila, with an unknown mechanism. During DCV infection, it has been confirmed that the protein level and antiviral activity of Cactin are regulated by ubiquitination. However, the precise ubiquitination and deubiquitination mechanisms of Cactin in Drosophila remain unexplored. In this study, we identified ubiquitin-specific protease 14 (Usp14) as a major deubiquitinase for Cactin through comprehensive deubiquitinase screening. Our results demonstrate that Usp14 interacts with the C_Cactus domain of Cactin via its USP domain. Usp14 efficiently removes K48- and K63-linked polyubiquitin chains from Cactin, thereby preventing its degradation through the ubiquitin-proteasome pathway. Usp14 significantly inhibits DCV replication in Drosophila cells by stabilizing Cactin. Moreover, Usp14-deficient fruit flies exhibit increased susceptibility to DCV infection compared to wild-type flies. Collectively, our findings reveal the regulation of ubiquitination and antiviral activity of Cactin by the deubiquitinase Usp14, providing valuable insights into the modulation of Cactin-mediated antiviral activity in Drosophila.IMPORTANCEViral infections pose a severe threat to human health, marked by high pathogenicity and mortality rates. Innate antiviral pathways, such as Toll, Imd, and JAK-STAT, are generally conserved across insects and mammals. Recently, the multi-functionality of Cactin in innate immunity has been identified in Drosophila. In addition to regulating a non-canonical signaling pathway through the Cactin-Deaf1 axis, Cactin exhibits specialized antiviral activity against the Drosophila C virus (DCV) with an unknown mechanism. A previous study emphasized the significance of the Cactin level, regulated by the ubiquitin-proteasome pathway, in modulating antiviral signaling. However, the regulatory mechanisms governing Cactin remain unexplored. In this study, we demonstrate that Usp14 stabilizes Cactin by preventing its ubiquitination and subsequent degradation. Furthermore, Usp14 plays a crucial role in regulating the antiviral function mediated by Cactin. Therefore, our findings elucidate the regulatory mechanism of Cactin in Drosophila, offering a potential target for the prevention and treatment of viral infections.