Replacing a native Wolbachia with a novel strain results in an increase in endosymbiont load and resistance to dengue virus in a mosquito vector

Diversity of Wolbachia infections in Sri Lankan mosquitoes with a new record of Wolbachia Supergroup B infecting Aedes aegypti vector populations

Wolbachia bacteria are common endosymbionts of insects and have recently been applied for controlling arboviral vectors, especially Aedes aegypti mosquito populations. However, several medically important mosquito species in Sri Lanka were present with limited information for the Wolbachia infection status. Therefore, the screening of Wolbachia in indigenous mosquitoes is required prior to a successful application of Wolbachia-based vector control strategy. In this study, screening of 78 mosquito species collected from various parts of the country revealed that 13 species were positive for Wolbachia infection, giving ~ 17% infection frequency of Wolbachia among the Sri Lankan mosquitoes. Twelve Wolbachia-positive mosquito species were selected for downstream Wolbachia strain genotyping using Multi Locus Sequencing Type (MLST), wsp gene, and 16S rRNA gene-based approaches. Results showed that these Wolbachia strains clustered together with the present Wolbachia phylogeny of world mosquito populations with some variations. Almost 90% of the mosquito populations were infected with supergroup B while the remaining were infected with supergroup A. A new record of Wolbachia supergroup B infection in Ae. aegypti, the main vectors of dengue, was highlighted. This finding was further confirmed by real-time qPCR, revealing Wolbachia density variations between Ae. aegypti and Ae. albopictus (p = 0.001), and between males and females (p < 0.05). The evidence of natural Wolbachia infections in Ae. aegypti populations in Sri Lanka is an extremely rare incident that has the potential to be used for arboviral vector control.

Wolbachia-carrying Aedes mosquitoes for preventing dengue infection

BACKGROUND

Dengue is a global health problem of high significance, with 3.9 billion people at risk of infection. The geographic expansion of dengue virus (DENV) infection has resulted in increased frequency and severity of the disease, and the number of deaths has increased in recent years. Wolbachia,an intracellular bacterial endosymbiont, has been under investigation for several years as a novel dengue-control strategy. Some dengue vectors (Aedes mosquitoes) can be transinfected with specific strains of Wolbachia, which decreases their fitness (ability to survive and mate) and their ability to reproduce, inhibiting the replication of dengue. Both laboratory and field studies have demonstrated the potential effect of Wolbachia deployments on reducing dengue transmission, and modelling studies have suggested that this may be a self-sustaining strategy for dengue prevention, although long-term effects are yet to be elucidated.

OBJECTIVES

To assess the efficacy of Wolbachia-carrying Aedes speciesdeployments (specifically wMel-, wMelPop-, and wAlbB- strains of Wolbachia) for preventing dengue virus infection.

SEARCH METHODS

We searched CENTRAL, MEDLINE, Embase, four other databases, and two trial registries up to 24 January 2024.

SELECTION CRITERIA

Randomized controlled trials (RCTs), including cluster-randomized controlled trials (cRCTs), conducted in dengue endemic or epidemic-prone settings were eligible. We sought studies that investigated the impact of Wolbachia-carrying Aedes deployments on epidemiological or entomological dengue-related outcomes, utilizing either the population replacement or population suppression strategy.

DATA COLLECTION AND ANALYSIS

Two review authors independently selected eligible studies, extracted data, and assessed the risk of bias using the Cochrane RoB 2 tool. We used odds ratios (OR) with the corresponding 95% confidence intervals (CI) as the effect measure for dichotomous outcomes. For count/rate outcomes, we planned to use the rate ratio with 95% CI as the effect measure. We used adjusted measures of effect for cRCTs. We assessed the certainty of evidence using GRADE.

MAIN RESULTS

One completed cRCT met our inclusion criteria, and we identified two further ongoing cRCTs. The included trial was conducted in an urban setting in Yogyakarta, Indonesia. It utilized a nested test-negative study design, whereby all participants aged three to 45 years who presented at healthcare centres with a fever were enrolled in the study provided they had resided in the study area for the previous 10 nights. The trial showed that wMel-Wolbachia infected Ae aegypti deployments probably reduce the odds of contracting virologically confirmed dengue by 77% (OR 0.23, 95% CI 0.15 to 0.35; 1 trial, 6306 participants; moderate-certainty evidence). The cluster-level prevalence of wMel Wolbachia-carrying mosquitoes remained high over two years in the intervention arm of the trial, reported as 95.8% (interquartile range 91.5 to 97.8) across 27 months in clusters receiving wMel-Wolbachia Ae aegypti deployments, but there were no reliable comparative data for this outcome. Other primary outcomes were the incidence of virologically confirmed dengue, the prevalence of dengue ribonucleic acid in the mosquito population, and mosquito density, but there were no data for these outcomes. Additionally, there were no data on adverse events.

AUTHORS' CONCLUSIONS

The included trial demonstrates the potential significant impact of wMel-Wolbachia-carrying Ae aegypti mosquitoes on preventing dengue infection in an endemic setting, and supports evidence reported in non-randomized and uncontrolled studies. Further trials across a greater diversity of settings are required to confirm whether these findings apply to other locations and country settings, and greater reporting of acceptability and cost are important.

Using Wolbachia to control rice planthopper populations: progress and challenges

Wolbachia have been developed as a tool for protecting humans from mosquito populations and mosquito-borne diseases. The success of using Wolbachia relies on the facts that Wolbachia are maternally transmitted and that Wolbachia-induced cytoplasmic incompatibility provides a selective advantage to infected over uninfected females, ensuring that Wolbachia rapidly spread through the target pest population. Most transinfected Wolbachia exhibit a strong antiviral response in novel hosts, thus making it an extremely efficient technique. Although Wolbachia has only been used to control mosquitoes so far, great progress has been made in developing Wolbachia-based approaches to protect plants from rice pests and their associated diseases. Here, we synthesize the current knowledge about the important phenotypic effects of Wolbachia used to control mosquito populations and the literature on the interactions between Wolbachia and rice pest planthoppers. Our aim is to link findings from Wolbachia-mediated mosquito control programs to possible applications in planthoppers.

Wolbachia-Virus interactions and arbovirus control through population replacement in mosquitoes

Following transfer into the primary arbovirus vector Aedes aegypti, several strains of the intracellular bacterium Wolbachia have been shown to inhibit the transmission of dengue, Zika, and chikungunya viruses, important human pathogens that cause significant morbidity and mortality worldwide. In addition to pathogen inhibition, many Wolbachia strains manipulate host reproduction, resulting in an invasive capacity of the bacterium in insect populations. This has led to the deployment of Wolbachia as a dengue control tool, and trials have reported significant reductions in transmission in release areas. Here, we discuss the possible mechanisms of Wolbachia-virus inhibition and the implications for long-term success of dengue control. We also consider the evidence presented in several reports that Wolbachia may cause an enhancement of replication of certain viruses under particular conditions, and conclude that these should not cause any concerns with respect to the application of Wolbachia to arbovirus control.

Immune-mediated competition benefits protective microbes over pathogens in a novel host species

Microbes that protect against infection inhabit hosts across the tree of life. It is unclear whether and how the host immune system may affect the formation of new protective symbioses. We investigated the transcriptomic response of Caenorhabditis elegans following novel interactions with a protective microbe (Enterococcus faecalis) able to defend against infection by pathogenic Staphylococcus aureus. We have previously shown that E. faecalis can directly limit pathogen growth within hosts. In this study, we show that colonisation by protective E. faecalis caused the differential expression of 1,557 genes in pathogen infected hosts, including the upregulation of immune genes such as lysozymes and C-type lectins. The most significantly upregulated host lysozyme gene, lys-7, impacted the competitive abilities of E. faecalis and S. aureus when knocked out. E. faecalis has an increased ability to resist lysozyme activity compared to S. aureus, suggesting that the protective microbe could gain a competitive advantage from this host response. Our finding that protective microbes can benefit from immune-mediated competition after introduction opens up new possibilities for biocontrol design and our understanding of symbiosis evolution. Crosstalk between the host immune response and microbe-mediated protection should favour the continued investment in host immunity and avoid the potentially risky evolution of host dependence.

Wolbachia wAlbB inhibit dengue and Zika infection in the mosquito Aedes aegypti with an Australian background

Biological control of mosquito vectors using the endosymbiotic bacteria Wolbachia is an emerging strategy for the management of human arboviral diseases. We recently described the development of a strain of Aedes aegypti infected with the Wolbachia strain wAlbB (referred to as the wAlbB2-F4 strain) through simple backcrossing of wild type Australian mosquitoes with a wAlbB infected Ae. aegypti strain from the USA. Field releases of male wAlbB2-F4 mosquitoes resulted in the successful suppression of wild populations of mosquitoes in the trial sites by exploiting the strain's Wolbachia-induced cytoplasmic incompatibility. We now demonstrate that the strain is resistant to infection by dengue and Zika viruses and is genetically similar to endemic Queensland populations. There was a fourfold reduction in the proportion of wAlbB2-F4 mosquitoes that became infected following a blood meal containing dengue 2 virus (16.7%) compared to wild type mosquitoes (69.2%) and a 6-7 fold reduction in the proportion of wAlbB2-F4 mosquitoes producing virus in saliva following a blood meal containing an epidemic strain of Zika virus (8.7% in comparison to 58.3% in wild type mosquitoes). Restriction-site Associated DNA (RAD) sequencing revealed that wAlbB2-F4 mosquitoes have > 98% Australian ancestry, confirming the successful introduction of the wAlbB2 infection into the Australian genomic background through backcrossing. Genotypic and phenotypic analyses showed the wAlbB2-F4 strain retains the insecticide susceptible phenotype and genotype of native Australian mosquitoes. We demonstrate that the Wolbachia wAlbB2-F4, in addition to being suitable for population suppression programs, can also be effective in population replacement programs given its inhibition of virus infection in mosquitoes. The ease at which a target mosquito population can be transfected with wAlbB2, while retaining the genotypes and phenotypes of the target population, shows the utility of this strain for controlling the Ae. aegypti mosquitoes and the pathogens they transmit.

Attempts to use breeding approaches in Aedes aegypti to create lines with distinct and stable relative Wolbachia densities

Wolbachia is an insect endosymbiont being used for biological control in the mosquito Aedes aegypti because it causes cytoplasmic incompatibility (CI) and limits viral replication of dengue, chikungunya, and Zika viruses. While the genetic mechanism of pathogen blocking (PB) is not fully understood, the strength of both CI and PB are positively correlated with Wolbachia densities in the host. Wolbachia densities are determined by a combination of Wolbachia strain and insect genotype, as well as interactions with the environment. We employed both artificial selection and inbreeding with the goal of creating lines of Ae. aegypti with heritable and distinct Wolbachia densities so that we might better dissect the mechanism underlying PB. We were unable to shift the mean relative Wolbachia density in Ae. aegypti lines by either strategy, with relative densities instead tending to cycle over a narrow range. In lieu of this, we used Wolbachia densities in mosquito legs as predictors of relative densities in the remaining individual's carcass. Because we worked with outbred mosquitoes, our findings indicate either a lack of genetic variation in the mosquito for controlling relative density, natural selection against extreme densities, or a predominance of environmental factors affecting densities. Our study reveals that there are moderating forces acting on relative Wolbachia densities that may help to stabilize density phenotypes post field release. We also show a means to accurately bin vector carcasses into high and low categories for non-DNA omics-based studies of Wolbachia-mediated traits.

Control of arboviruses vectors using biological control by Wolbachia pipientis: a short review

The number of arbovirus cases has increased in recent years, demonstrating a need for investing in effective control actions. Among these actions, are strategies using biological control vectors, a field where Wolbachia pipientis has shown itself as useful. Wolbachia pipientis, an obligatory intracellular Gram-negative bacteria, which parasites arthropods naturally or through laboratory-induced infections, is capable of manipulating the reproduction of its host. A systematic literature review gathering studies on this bacteria over last 10 years (2007-2021) was performed given its important role in the reduction of insect disease vectors. 111 articles were found, from which 78 were used in this study. Information on the Wolbachia biology, mechanism of action and potential for the biological control of insect disease vectors was gathered. The present study may contribute to the knowledge surrounding the bacterium, as well as stimulate the production of other studies with the same theme.

Establishment of Wolbachia infection in Aedes aegypti from Pakistan via embryonic microinjection and semi-field evaluation of general fitness of resultant mosquito population

Background

Dengue is a mosquito-borne viral disease that is mainly spread by Aedes aegypti. It is prevalent on five continents, predominantly in tropical and sub-tropical zones across the world. Wolbachia bacteria have been extensively used in vector control strategies worldwide. The focus of the current study was to obtain a natural population of Ae. aegypti harbouring Wolbachia and to determine the impact of this bacteria on the new host in a semi-field environment.

Methods

Wolbachia-infected Aedes albopictus was collected from the city of Lahore, Punjab, Pakistan, and Wolbachia were successfully introduced into laboratory-reared Ae. aegypti via embryonic microinjection. The stable vertical transmission of wAlbB in the host population was observed for eight generations, and the impact of Wolbachia on the general fitness of the host was evaluated in semi-field conditions.

Results

In the laboratory and semi-field experiments, wAlbB Wolbachia presented a strong cytoplasmic incompatibility (CI) effect, evidenced as zero egg hatching, in crosses between Wolbachia-infected males and wild (uninfected) females of Ae. aegypti. Wolbachia infection had no noticeable impact on the general fitness (P > 0.05), fecundity, body size (females and males) and mating competitiveness of the new host, Ae. aegypti. However, there was a significant decrease in female fertility (egg hatch) (P < 0.001). In addition, under starvation conditions, there was a remarkable decrease (P < 0.0001) in the life span of Wolbachia-infected females compared to uninfected females (4 vs. > 5 days, respectively).

Conclusions

Wolbachia strain wAlbB has a great potential to control the dengue vector in Ae. aegypti populations by producing 100% CI with a limited burden on its host in natural field conditions. This strain can be used as a biological tool against vector-borne diseases.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05317-4.

Metagenome Sequencing Reveals the Microbiome of Aedes albopictus and Its Possible Relationship With Dengue Virus Susceptibility

Dengue fever virus (DENV) is a mosquito-borne flavivirus that poses a serious risk to human health. Aedes albopictus is a widely distributed vector of dengue fever in China. Based on the impact of physiological activity, the microbiome in A. albopictus will provide a novel environment-friendly approach to control DENV transmission. We performed metagenomic sequencing on A. albopictus before and after exposure to DENV blood meal to detect microbiome variation of A. albopictus with different susceptibilities to DENV. The dominant phyla in A. albopictus microbiome were Proteobacteria and Ascomycota, and the dominant genera were Aspergillus and Metarhizium. Gammaproteobacteria bacterium, Lactobacillus harbinensis, and Neurospora crassa differed significantly after DENV infection. There were 15 different microorganisms found to be involved in mosquito immunity and metabolism, such as Alphaproteobacteria bacterium, Methyloglobulus morosus, and Shigella sonnei, which might have an impact on the DENV susceptibility of A. albopictus. It was hypothesized that the lack of specific bacteria may lead to increased susceptibility of A. albopictus to DENV. Interventions in the microbiome composition or specific bacteria of A. albopictus may affect the susceptibility to DENV and control the mosquito-borne diseases efficiently.

Cytoplasmic incompatibility in hybrid zones: infection dynamics and resistance evolution

Cytoplasmic incompatibility is an endosymbiont-induced mating incompatibility common in arthropods. Unidirectional cytoplasmic incompatibility impairs crosses between infected males and uninfected females, whereas bidirectional cytoplasmic incompatibility occurs when two host lineages are infected with reciprocally in compatible endosymbionts. Bidirectional cytoplasmic incompatibility is unstable in unstructured populations, but may be stable in hybrid zones. Stable coexistence of incompatible host lineages should generate frequent incompatible crosses. Therefore, hosts are expected to be under selection to resist their endosymbionts. Here, we for mulate a mathematical model of hybrid zones where two bidirectionally incompatible host lineages meet. We expand this model to consider the invasion of a hypothetical resistance allele. To corroborate our mathematical predictions, we test each prediction with stochastic, individual-based simulations. Our models suggest that hybrid zones may sustain stable coinfections of bidirectionally incompatible endosymbiont strains. Over a range of conditions, host are under selection for resistance against cytoplasmic incompatibility. Under asymetric migration, a resistance allele can facilitate infection turnover and subsequently either persist or become lost. The predictions we present may inform our understanding of the cophylogenetic relationship between the endosym biont Wolbachia and its hosts.

Designing effective Wolbachia release programs for mosquito and arbovirus control

Mosquitoes carrying endosymbiotic bacteria called Wolbachia are being released in mosquito and arbovirus control programs around the world through two main approaches: population suppression and population replacement. Open field releases of Wolbachia-infected male mosquitoes have achieved over 95% population suppression by reducing the fertility of wild mosquito populations. The replacement of populations with Wolbachia-infected females is self-sustaining and can greatly reduce local dengue transmission by reducing the vector competence of mosquito populations. Despite many successful interventions, significant questions and challenges lie ahead. Wolbachia, viruses and their mosquito hosts can evolve, leading to uncertainty around the long-term effectiveness of a given Wolbachia strain, while few ecological impacts of Wolbachia releases have been explored. Wolbachia strains are diverse and the choice of strain to release should be made carefully, taking environmental conditions and the release objective into account. Mosquito quality control, thoughtful community awareness programs and long-term monitoring of populations are essential for all types of Wolbachia intervention. Releases of Wolbachia-infected mosquitoes show great promise, but existing control measures remain an important way to reduce the burden of mosquito-borne disease.

Development of an RNAi-based microalgal larvicide for the control of Aedes aegypti

BACKGROUND

Mosquito-borne diseases affect over half of the human population globally. Multiple studies have shown that chemical insecticides are ineffective because of resistance. Therefore, environmentally safe mosquito population control tools need to be developed. Ribonucleic acid interference (RNAi) is a reverse genetic mechanism recently introduced as a new pest control tool. This technique represents a new class of biorational technology that could combat the increased global incidence of insecticide resistance. The technique has the potential of becoming a critical component of integrated vector control programs.

METHODS

A 3-hydroxykynurenine transaminase (3-HKT) RNAi expression plasmid was constructed, generated and transformed into Chlamydomonas and Chlorella algae. The transgenic algae were then used to feed Ae. aegypti mosquito larvae. The feeding experiments were conducted on a small and large scale with 10 and about 300 larvae, respectively. The mortality rate of the larvae was calculated over 30 days. In addition, histological examination of the insect tissues was performed to examine the extent of tissue damage.

RESULTS

The integumentary system and midguts of larvae fed with transgenic Chlamydomonas were severely damaged. The mortality rate of the larvae fed with transgenic Chlamydomonas ranged from 60 to 100% in small-scale tests. The survival rate of adult mosquitoes was 0.0% in a large-scale feeding experiment when the larvae were fed with transgenic Chlamydomonas. Moreover, when the larvae were fed with transgenic Chlorella, the mortality rate ranged from 6.7% to 43% compared to that fed wild-type Chlorella.

CONCLUSIONS

3HKT RNAi transgenic algae are in some scales lethal to Ae. aegypti. The findings of this study indicate that technology based on microalgae RNAi may provide a new way to control mosquito populations.

Transcriptional Response of Wolbachia to Dengue Virus Infection in Cells of the Mosquito Aedes aegypti

Aedes aegypti transmits one of the most significant mosquito-borne viruses, dengue virus (DENV). The absence of effective vaccines and clinical treatments and the emergence of insecticide resistance in A. aegypti necessitate novel vector control strategies. A new approach uses the endosymbiotic bacterium Wolbachia pipientis to reduce the spread of arboviruses. However, the Wolbachia-mediated antiviral mechanism is not well understood. To shed light on this mechanism, we investigated an unexplored aspect of Wolbachia-virus-mosquito interaction. We used RNA sequencing to examine the transcriptional response of Wolbachia to DENV infection in A. aegypti Aag2 cells transinfected with the wAlbB strain of Wolbachia. Our results suggest that genes encoding an endoribonuclease (RNase HI), a regulator of sigma 70-dependent gene transcription (6S RNA), essential cellular, transmembrane, and stress response functions and primary type I and IV secretion systems were upregulated, while a number of transport and binding proteins of Wolbachia, ribosome structure, and elongation factor-associated genes were downregulated due to DENV infection. Furthermore, bacterial retrotransposon, transposable, and phage-related elements were found among the up- and downregulated genes. We show that Wolbachia elicits a transcriptional response to virus infection and identify differentially expressed Wolbachia genes mostly at the early stages of virus infection. These findings highlight Wolbachia's ability to alter its gene expression in response to DENV infection of the host cell. IMPORTANCE Aedes aegypti is a vector of several pathogenic viruses, including dengue, Zika, chikungunya, and yellow fever viruses, which are of importance to human health. Wolbachia is an endosymbiotic bacterium currently used in transinfected mosquitoes to suppress replication and transmission of dengue viruses. However, the mechanism of Wolbachia-mediated virus inhibition is not fully understood. While several studies have shown mosquitoes' transcriptional responses to dengue virus infection, none have investigated these responses in Wolbachia, which may provide clues to the inhibition mechanism. Our results suggest changes in the expression of a number of functionally important Wolbachia genes upon dengue virus infection, including those involved in stress responses, providing insights into the endosymbiont's reaction to virus infection.

Vector Competence for DENV-2 Among Aedes albopictus (Diptera: Culicidae) Populations in China

Aedes albopictus is a vector of over 20 arboviruses that has spread throughout the world, mainly in the second half of the twentieth century. Approximately 50–100 million people are infected with dengue virus (DENV) transmitted by Aedes mosquitoes each year, leading to heavy economic burdens for both governments and individuals, among countless other negative consequences. Understanding the vector competence of vector species is critical for effectively preventing and controlling vector-borne diseases. Accordingly, in this study, vector competence was evaluated by quantitative analysis of DENV-2 loads in mosquito tissues (midguts, heads, and salivary glands) and whole mosquitoes through real-time quantitative polymerase chain reaction (RT-qPCR) analysis. Wolbachia and the expression of immune-associated genes (Rel1, Rel2, Dicer2, and STAT) in mosquitoes were also detected by RT-qPCR to explore their impact on vector competence. The amount of DENV-2 in the mosquito midguts, heads, and salivary glands from southern-western China were found to be lower than those from eastern-central-northern China. The DENV-2 loads in whole mosquitoes showed a negative correlation with Rel1 gene (r = -0.285, P = 0.011) and STAT gene expression levels (r = -0.289, P = 0.009). In terms of Wolbachia strains, the density of the wAlbB strain was found to be significantly higher than that of the wAlbA strain in the eight Ae. albopictus populations, and the relative density of the wAlbB strain in mosquitoes from southern-western China was higher than those from eastern-central-northern China. The relative density of the wAlbB strain showed a negative correlation with the mean loads of DENV-2 in the heads (r = -0.729, P = 0.040), salivary glands (r = -0.785, P = 0.021), and whole mosquitoes (r = -0.909, P = 0.002). Thus, there are lower DENV-2 loads in the mosquitoes from southern-western China, which may be related to the innate immunity of mosquitoes as affected by Rel1 in the Toll pathway, STAT in the JAK-STAT pathway, and the relative density of the wAlbB strain.

Controlling the development of the dengue vector Aedes aegypti using HR3 RNAi transgenic Chlamydomonas

The Aedes aegypti mosquito plays an important role in the spread of diseases, including epidemic ones, such as dengue fever, Zika virus disease, yellow fever, and chikungunya disease. To control the population of Ae.aegypti, we transferred an HR3 RNAi fragment into the microalgae Chlamydomonas, which serves as food for Ae.aegypti larvae. Results showed that the HR3 RNAi transgenic algal strains were lethal to Ae.aegypti. The integumentary system of larvae fed with HR3 RNAi transgenic algal strains was severely damaged. Muscles of the larvae were unevenly distributed and disordered, and their midgut showed disintegration of the intestinal cavity. RNA-Seq results demonstrated that on the 4th day of inoculation with the transgenic algae, the abundance of early expressed genes in the 20E signal transduction pathway of larvae fed with the HR3 RNAi transgenic algal strain significantly reduced. These genes include E74, E75, E93, and 20E receptor complex EcR/USP and FTZ-F1 gene regulated by HR3. In later experiments, a scale test of 300 Ae.aegypti eggs per group was carried out for 30 days, and the survival rate of Ae.aegypti fed with the HR3 RNAi transgenic strain was only 1.3%. These results indicate that the HR3 RNAi transgenic strain exerts obvious insecticidal effect.

Stable Introduction of Plant-Virus-Inhibiting Wolbachia into Planthoppers for Rice Protection

Progress has been made in developing the maternally inherited endosymbiotic bacterium Wolbachia as a tool for protecting humans from mosquito-borne diseases. In contrast, Wolbachia-based approaches have not yet been developed for the protection of plants from insect pests and their associated diseases, with a major challenge being the establishment of artificial Wolbachia infections expressing desired characteristics in the hemipterans that transmit the majority of plant viruses. Here, we report stable introduction of Wolbachia into the brown planthopper, Nilaparvata lugens, the most destructive rice pest that annually destroys millions of hectares of staple crops. The Wolbachia strain wStri from the small brown planthopper, Laodelphax striatellus, was transferred to this new host, where it showed high levels of cytoplasmic incompatibility, enabling rapid invasion of laboratory populations. Furthermore, wStri inhibited infection and transmission of Rice ragged stunt virus and mitigated virus-induced symptoms in rice plants, opening up the development of Wolbachia-based strategies against major agricultural pests and their transmitted pathogens. VIDEO ABSTRACT.

Symbiont-mediated fly survival is independent of defensive symbiont genotype in the Drosophila melanogaster-Spiroplasma-wasp interaction

When a parasite attacks an insect, the outcome is commonly modulated by the presence of defensive heritable symbionts residing within the insect host. Previous studies noted markedly different strengths of Spiroplasma-mediated fly survival following attack by the same strain of wasp. One difference between the two studies was the strain of Spiroplasma used. We therefore performed a laboratory experiment to assess whether Spiroplasma-mediated protection depends upon the strain of Spiroplasma. We perform this analysis using the two strains of male-killing Spiroplasma used previously, and examined response to challenge by two strains of Leptopilina boulardi and two strains of Leptopilina heterotoma wasp. We found no evidence Spiroplasma strain affected fly survival following wasp attack. In contrast, analysis of the overall level of protection, including the fecundity of survivors of wasp attack, did indicate the two Spiroplasma strains tested varied in protective efficiency against three of the four wasp strains tested. These data highlight the sensitivity of symbiont-mediated protection phenotypes to laboratory conditions, and the importance of common garden comparison. Our results also indicate that Spiroplasma strains can vary in protective capacity in Drosophila, but these differences may exist in the relative performance of survivors of wasp attack, rather than in survival of attack per se.

Wolbachia modulates prevalence and viral load of Culex pipiens densoviruses in natural populations

The inadequacy of standard mosquito control strategies calls for ecologically safe novel approaches, for example the use of biological agents such as the endosymbiotic α-proteobacteria Wolbachia or insect-specific viruses (ISVs). Understanding the ecological interactions between these "biocontrol endosymbionts" is thus a fundamental step. Wolbachia are transmitted vertically from mother to offspring and modify their hosts' phenotypes, including reproduction (e.g., cytoplasmic incompatibility) and survival (e.g., viral interference). In nature, Culex pipiens (sensu lato) mosquitoes are always found infected with genetically diverse Wolbachia called wPip that belong to five phylogenetic groups. In recent years, ISVs have also been discovered in these mosquito species, although their interactions with Wolbachia in nature are unknown. Here, we studied the interactions between a widely prevalent ISV, the Culex pipiens densovirus (CpDV, Densovirinae), and Wolbachia in northern Tunisian C. pipiens populations. We showed an influence of different Wolbachia groups on CpDV prevalence and a general positive correlation between Wolbachia and CpDV loads. By investigating the putative relationship between CpDV diversification and wPip groups in the different sites, we detected a signal linked to wPip groups in CpDV phylogeny in sites where all larvae were infected by the same wPip group. However, no such signal was detected where the wPip groups coexisted, suggesting CpDV horizontal transfer between hosts. Overall, our results provide good evidence for an ecological influence of Wolbachia on an ISV, CpDV, in natural populations and highlight the importance of integrating Wolbachia in our understanding of ISV ecology in nature.

Age, tissue, genotype and virus infection regulate Wolbachia levels in Drosophila

The bacterial symbiont Wolbachia can protect insects against viral pathogens, and the varying levels of antiviral protection are correlated with the endosymbiont load within the insects. To understand why Wolbachia strains differ in their antiviral effects, we investigated the factors controlling Wolbachia density in five closely related strains in their natural Drosophila hosts. We found that Wolbachia density varied greatly across different tissues and between flies of different ages, and these effects depended on the host-symbiont association. Some endosymbionts maintained largely stable densities as flies aged while others increased, and these effects in turn depended on the tissue being examined. Measuring Wolbachia rRNA levels in response to viral infection, we found that viral infection itself also altered Wolbachia levels, with Flock House virus causing substantial reductions in symbiont loads late in the infection. This effect, however, was virus-specific as Drosophila C virus had little impact on Wolbachia in all of the five host systems. Because viruses have strong tissue tropisms and antiviral protection is thought to be cell-autonomous, these effects are likely to affect the virus-blocking phenomenon. However, we were unable to find any evidence of a correlation between Wolbachia and viral titres within the same tissues. We conclude that Wolbachia levels within flies are regulated in a complex host-symbiont-virus-dependent manner and this trinity is likely to influence the antiviral effects of Wolbachia.

A Role for the Insulin Receptor in the Suppression of Dengue Virus and Zika Virus in Wolbachia-Infected Mosquito Cells

Wolbachia-infected mosquitoes are refractory to super-infection with arthropod-borne pathogens, but the role of host cell signaling proteins in pathogen-blocking mechanisms remains to be elucidated. Here, we use an antibody microarray approach to provide a comprehensive picture of the signaling response of Aedes aegypti-derived cells to Wolbachia. This approach identifies the host cell insulin receptor as being downregulated by the bacterium. Furthermore, siRNA-mediated knockdown and treatment with a small-molecule inhibitor of the insulin receptor kinase concur to assign a crucial role for this enzyme in the replication of dengue and Zika viruses in cultured mosquito cells. Finally, we show that the production of Zika virus in Wolbachia-free live mosquitoes is impaired by treatment with the selective inhibitor mimicking Wolbachia infection. This study identifies Wolbachia-mediated downregulation of insulin receptor kinase activity as a mechanism contributing to the blocking of super-infection by arboviruses.

Evidence for the natural occurrence of Wolbachia in Aedes aegypti mosquitoes

Wolbachia, a Gram-negative bacterium, is widely known to be present in arthropods and nematodes. Of late, great impetus is given to employ this intracellular bacterium, as an alternative to conventional biocontrol agents for the control of mosquitoes because of its inherent ability to induce sperm-egg incompatibility, feminisation etc. By employing molecular tools, we have shown the presence of Wolbachia from Aedes aegypti mosquito population collected from Coimbatore, India by PCR amplifying the Ae. aegypti mosquito genome with Wolbachia specific 16S rRNA, wsp and ftsZ gene primers. The phylogenetic analysis of these gene sequences incorporating MLST and GenBank reference sequences has confirmed the occurrence of Wolbachia supergroup B in Ae. aegypti. In addition, qRT-PCR results have shown the dynamics of Wolbachia across the developmental stages of mosquito. The absence of Wolbachia in tetracycline-treated Ae. aegypti mosquitoes evidenced by transmission electron microscopy reinforced our finding conclusively. After confirming their persistence through generations, we have designated Wolbachia from Ae. aegypti as wAegB. In our considered view, wAegB could play a dynamic role in impeding mosquito multiplication and consequently impinging transmission of the dreadful dengue.

Identification and molecular characterization of Wolbachia strains in natural populations of Aedes albopictus in China

BACKGROUND

Aedes albopictus is naturally infected with Wolbachia spp., maternally transmitted bacteria that influence the reproduction of hosts. However, little is known regarding the prevalence of infection, multiple infection status, and the relationship between Wolbachia density and dengue outbreaks in different regions. Here, we assessed Wolbachia infection in natural populations of Ae. albopictus in China and compared Wolbachia density between regions with similar climates, without dengue and with either imported or local dengue.

RESULTS

To explore the prevalence of Wolbachia infection, Wolbachia DNA was detected in mosquito samples via PCR amplification of the 16S rRNA gene and the surface protein gene wsp. We found that 93.36% of Ae. albopictus in China were positive for Wolbachia. After sequencing gatB, coxA, hcpA, ftsZ, fbpA and wsp genes of Wolbachia strains, we identified a new sequence type (ST) of wAlbB (464/465). Phylogenetic analysis indicated that wAlbA and wAlbB strains formed a cluster with strains from other mosquitoes in a wsp-based maximum likelihood (ML) tree. However, in a ML tree based on multilocus sequence typing (MLST), wAlbB STs (464/465) did not form a cluster with Wolbachia strains from other mosquitoes. To better understand the association between Wolbachia spp. and dengue infection, the prevalence of Wolbachia in Ae. albopictus from different regions (containing local dengue cases, imported dengue cases and no dengue cases) was determined. We found that the prevalence of Wolbachia was lower in regions with only imported dengue cases.

CONCLUSIONS

The natural prevalence of Wolbachia infections in China was much lower than in other countries or regions. The phylogenetic relationships among Wolbachia spp. isolated from field-collected Ae. albopictus reflected the presence of dominant and stable strains. However, wAlbB (464/465) and Wolbachia strains did not form a clade with Wolbachia strains from other mosquitoes. Moreover, lower densities of Wolbachia in regions with only imported dengue cases suggest a relationship between fluctuations in Wolbachia density in field-collected Ae. albopictus and the potential for dengue invasion into these regions.

Transfection of Culicoides sonorensis biting midge cell lines with Wolbachia pipientis

Background

Biting midges of the genus Culicoides vector multiple veterinary pathogens and are difficult to control. Endosymbionts particularly Wolbachia pipientis may offer an alternative to control populations of Culicoides and/or impact disease transmission in the form of population suppression or replacement strategies.

Methods

Culicoides sonorensis cell lines were transfected with a Wolbachia infection using a modified shell vial technique. Infections were confirmed using PCR and cell localization using fluorescent in situ hybridization (FISH). The stability of Wolbachia infections and density was determined by qPCR. qPCR was also used to examine immune genes in the IMD, Toll and JACK/STAT pathways to determine if Wolbachia were associated with an immune response in infected cells.

Results

Here we have transfected two Culicoides sonorensis cell lines (W3 and W8) with a Wolbachia infection (walbB) from donor Aedes albopictus Aa23 cells. PCR and FISH showed the presence of Wolbachia infections in both C. sonorensis cell lines. Infection densities were higher in the W8 cell lines when compared to W3. In stably infected cells, genes in the immune Toll, IMD and JAK/STAT pathways were upregulated, along with Attacin and an Attacin-like anti-microbial peptides.

Conclusions

The successful introduction of Wolbachia infections in C. sonorensis cell lines and the upregulation of immune genes, suggest the utility of using Wolbachia for a population replacement and/or population suppression approach to limit the transmission of C. sonorensis vectored diseases. Results support the further investigation of Wolbachia induced pathogen inhibitory effects in Wolbachia-infected C. sonorensis cell lines and the introduction of Wolbachia into C. sonorensis adults via embryonic microinjection to examine for reproductive phenotypes and host fitness effects of a novel Wolbachia infection.

Wolbachia-mediated antiviral protection is cell-autonomous

Vector-borne viral diseases pose significant risks to human health. To control the transmission of these viruses, a number of approaches are required. The ability of the intracellular bacteria Wolbachia to limit viral accumulation and transmission in some arthropod hosts, highlights its potential as a biocontrol agent. Whilst Wolbachia can reduce the transmission of several epidemiologically important viruses, protection is not consistent amongst all insects, viruses and strains of Wolbachia, which confounds elucidation of the mechanisms that underly this protection. Evidence of different mechanisms has emerged, but is not always consistent, suggesting the tripartite interaction may be complex. Here we provide evidence that Wolbachia-mediated antiviral protection is dependent on the presence of Wolbachia in individual cells, and cannot be conferred to surrounding cells. Our results suggest that protection is cell-autonomous, and this has several mechanistic implications, which can direct future research.

Evolutionary Ecology of Wolbachia Releases for Disease Control

Wolbachia is an endosymbiotic Alphaproteobacteria that can suppress insect-borne diseases through decreasing host virus transmission (population replacement) or through decreasing host population density (population suppression). We contrast natural Wolbachia infections in insect populations with Wolbachia transinfections in mosquitoes to gain insights into factors potentially affecting the long-term success of Wolbachia releases. Natural Wolbachia infections can spread rapidly, whereas the slow spread of transinfections is governed by deleterious effects on host fitness and demographic factors. Cytoplasmic incompatibility (CI) generated by Wolbachia is central to both population replacement and suppression programs, but CI in nature can be variable and evolve, as can Wolbachia fitness effects and virus blocking. Wolbachia spread is also influenced by environmental factors that decrease Wolbachia titer and reduce maternal Wolbachia transmission frequency. More information is needed on the interactions between Wolbachia and host nuclear/mitochondrial genomes, the interaction between invasion success and local ecological factors, and the long-term stability of Wolbachia-mediated virus blocking.

Incompatible and sterile insect techniques combined eliminate mosquitoes

The radiation-based sterile insect technique (SIT) has successfully suppressed field populations of several insect pest species, but its effect on mosquito vector control has been limited. The related incompatible insect technique (IIT)-which uses sterilization caused by the maternally inherited endosymbiotic bacteria Wolbachia-is a promising alternative, but can be undermined by accidental release of females infected with the same Wolbachia strain as the released males. Here we show that combining incompatible and sterile insect techniques (IIT-SIT) enables near elimination of field populations of the world's most invasive mosquito species, Aedes albopictus. Millions of factory-reared adult males with an artificial triple-Wolbachia infection were released, with prior pupal irradiation of the released mosquitoes to prevent unintentionally released triply infected females from successfully reproducing in the field. This successful field trial demonstrates the feasibility of area-wide application of combined IIT-SIT for mosquito vector control.

Importance of endosymbionts Wolbachia and Rickettsia in insect resistance development

Endosymbionts play important roles in protecting hosts from environmental stress, such as natural enemies, heat, and toxins. Many insects are infected with the facultative nonessential endosymbionts Wolbachia and Rickettsia, which are the crux in this review, although other relevant symbiont genera will also be treated. Insecticide resistance of hosts can be related to infections with Wolbachia and Rickettsia. These endosymbionts commonly increase host susceptibility to chemical insecticides, but cases of increased resistance also exist. The symbiont-mediated insecticide resistance/susceptibility varies with species of insect, species of symbiont, and chemical compound. Changes in insecticide resistance levels of insects can be associated with fluctuations in population density of endosymbionts. Effects of endosymbionts on host fitness, metabolism, immune system, and gene expression may determine how endosymbionts influence insecticide resistance. A clearer understanding of these interactions can improve our knowledge about drivers of decreasing insecticide resistance.

Sharing cells with Wolbachia: the transovarian vertical transmission of Culex pipiens densovirus

Culex pipiens densovirus (CpDV), a single stranded DNA virus, has been isolated from Culex pipiens mosquitoes but differs from other mosquito densoviruses in terms of genome structure and sequence identity. Its transmission from host to host, the nature of its interactions with both its host and host's endosymbiotic bacteria Wolbachia are not known. Here, we report the presence of CpDV in the ovaries and eggs of Cx. pipiens mosquitoes in close encounters with Wolbachia. In the ovaries, CpDV amount significantly differed between mosquito lines harbouring different strains of Wolbachia and these differences were not linked to variations in Wolbachia densities. CpDV was vertically transmitted in all laboratory lines to 17%-20% of the offspring. For some females, however, the vertical transmission reached 90%. Antibiotic treatment that cured the host from Wolbachia significantly decreased both CpDV quantity and vertical transmission suggesting an impact of host microbiota, including Wolbachia, on CpDV transmission. Overall our results show that CpDV is transmitted vertically via transovarian path along with Wolbachia with which it shares the same cells. Our results are primordial to understand the dynamics of densovirus infection, their persistence and spread in populations considering their potential use in the regulation of mosquito vector populations.

Antivirus effectiveness of ivermectin on dengue virus type 2 in Aedes albopictus

Background

Dengue fever is the most rapidly spreading mosquito-borne viral disease over the past 50 years, with a 30-fold increase in global incidence. Dengue vector control is a key component for the dengue control strategy, since no absolutely effective vaccine or drug is available yet. However, the rapid rise and spread of mosquito insecticide resistance have become major threats to the efficiency of insecticide-based vector control activities. Thus, innovative vector control tools are badly needed. This study aims to confirm the antivirus effectiveness of ivermectin on dengue virus type 2 (DENV-2) in Aedes albopictus (Skuse, 1894), then to explore its potential use in the combating to the dengue epidemics.

Methods

Aedes albopictus were first infected with DENV-2 in human whole blood, and at the fourth day after infectious blood feeding, they were divided into eight groups. Seven of them were held for six days with access to 0, 2, 4, 8, 16, 32 and 64 ng/ml ivermectin, respectively, and the last one was set as a historical control group, which was stored at -80°C until being detected at the same time with the other groups. Each mosquito was detected using real-time fluorescent RT-PCR kit. DENV-2 RNA concentration (copies/ml) and infection rate in each group were compared.

Results

Both of quantitatively and qualitatively inhibiting effects of ivermectin have been detected in this study. Generally, DENV-2 replicated well in Aedes albopictus without ivermectin intervention, whose virus loads exhibited significantly higher when the mosquitoes were holding from 4 days to 10 days after infectious blood feeding. In contrast, with the treatment of ivermectin, the infection rate was reduced by as much as 49.63%. The regression equation between infection rates (Y₂) and ivermectin concentration log₂ values (X₂) was obtained as Y₂ = 91.41–7.21*X₂ with R² = 0.89.

Conclusion

Ivermectin can directly or indirectly inhibit DENV-2 multiplication in Aedes albopictus. Moreover, the actual concentration for application in zooprophylaxis needs to be confirmed in the further field trials.

Dengue fever is one of neglected vector-borne tropical diseases with a 30-fold increase in global incidence recently. In 2012, World Health Organization set a goal to reduce dengue mortality by at least 50% by 2020. Being faced with more challenges in the dengue control programs, such as the increase of dengue outbreaks, lacking absolutely effective vaccine, rise of vector insecticide resistance and so on; innovative vector control tools are urgently needed for current control programs on dengue fever. To find a new avenue in vector control, we for the first time assessed the inhibiting effectiveness of ivermectin on dengue virus type 2 (DENV-2) inside Aedes mosquitoes. We found that about 80% Aedes albopictus mosquitoes were effectively infected with DENV-2 without treatment of ivermectin. But in the groups of ivermectin treatment, the infection rate of DENV-2 and the median of virus loads were significantly reduced by up to 49.63% and 99.99%, respectively. Both quantitatively and qualitatively inhibiting effects of ivermectin were detected. We found out that ivermectin was able to effectively inhibit the DENV-2 multiplication in Aedes albopictus, which may gave us a hint that using ivermectin in some control programs as a zooprophylaxis to block dengue epidemic through inhibiting DENV-2 in field Aedes mosquitoes.

Reduced diversity of gut microbiota in two Aedes mosquitoes species in areas of recent invasion

Aedes mosquitoes are considered highly successful global invasive species and vectors of several pathogens of relevance for public health. Their midgut's microbiota can play an important role in affecting not only their vectorial competence but also their fitness, physiology, food digestion, metabolism, immunity and adaptation to new environmental conditions. Using high-throughput sequencing we compared the microbiota of Aedes albopictus collected in Italy with those reported in populations from France and Vietnam. We also analysed Aedes koreicus gut microbiota for the first time. We found remarkable individual difference along with common bacterial taxa in both species. Ae. albopictus collected in Italy had a lower richness and a different composition of microbiota in respect to specimens collected in France and Vietnam. It also showed a core microbiota formed mainly of bacteria of the genus Pseudomonas. Overall, the two Aedes species (Ae. albopictus and Ae. koreicus) collected in Italy, showed a large core microbiota with 75.98% of the identified Operational Taxonomic Units. Furthermore, Ae. albopictus had 2.5% prevalence of Wolbachia and 0.07% of Asaia spp, while Ae. koreicus had 14.42% of Asaia spp. and no Wolbachia. This study provides new informations on the spatial variation of the midgut bacterial communities in mosquitoes of medical relevance within areas of recent invasion and provide the basis for further studies aimed at assessing the effects of such variation on vectorial capacity for a range of pathogens.

Changes in Larval Mosquito Microbiota Reveal Non-target Effects of Insecticide Treatments in Hurricane-Created Habitats

Ephemeral aquatic habitats and their associated microbial communities (microbiomes) play important roles in the growth and development of numerous aquatic insects, including mosquitoes (Diptera). Biological control agents, such as Bacillus thuringiensis israelensis (Bti) or insect growth regulators (e.g., methoprene), are commonly used to control mosquitoes in these habitats. However, it is unknown how commonly used control compounds affect the mosquito internal microbiome and potentially alter their life history traits. The objectives of this study were threefold: characterize the internal microbiota of Aedes larvae (Culicidae) in ephemeral forested mosquito habitat using high-throughput amplicon based sequencing, assess how mosquito control treatments affect the internal microbial communities of larval mosquitoes, and determine if changes to the microbiome resulted from direct or indirect treatment effects. The larval microbiome varied in community composition and diversity with development stage and treatment, suggesting potential effects of control compounds on insect microbial ecology. While microbial community differences due to Bti treatment were a result of indirect effects on larval development, methoprene had significant impacts on bacterial and algal taxa that could not be explained by indirect treatment effects. These results provide new information on the interactions between pesticide treatments and insect microbial communities.

The Maternal Effect Gene Wds Controls Wolbachia Titer in Nasonia

Maternal transmission of intracellular microbes is pivotal in establishing long-term, intimate symbioses. For germline microbes that exert negative reproductive effects on their hosts, selection can theoretically favor the spread of host genes that counteract the microbe's harmful effects. Here, we leverage a major difference in bacterial (Wolbachia pipientis) titers between closely related wasp species with forward genetic, transcriptomic, and cytological approaches to map two quantitative trait loci that suppress bacterial titers via a maternal effect. Fine mapping and knockdown experiments identify the gene Wolbachia density suppressor (Wds), which dominantly suppresses bacterial transmission from mother to embryo. Wds evolved by lineage-specific non-synonymous changes driven by positive selection. Collectively, our findings demonstrate that a genetically simple change arose by positive Darwinian selection in less than a million years to regulate maternally transmitted bacteria via a dominant, maternal effect gene.

Antiviral systems in vector mosquitoes

Mosquito-borne viral diseases represent a major challenge to human public health. As natural vectors of arboviruses, mosquitoes can be infected by a virus, but they have evolved multiple mechanisms to tolerate constant infection and restrict viral replication via their antiviral immune system. In a state of continuous infection, a mosquito can transmit an arbovirus while obtaining a blood meal from a mammalian host. During infection, the virus is mainly inhibited through a small RNA-mediated interference mechanism. Within mosquitoes, the invaded viruses are recognized based on pathogen-associated molecular patterns, leading to the production of cytokines. These cytokines in turn bind pattern recognition receptors and activate Toll, IMD and other immune signalling pathways to expand the immune response and induce antiviral activity via immune effectors. Interestingly, the gut microbiota and Wolbachia also play a role in mosquito antiviral immunity, which is very similar to acquired immunity. This review describes the advances made in understanding various aspects of mosquito antiviral immune molecular mechanisms in detail and explores some of the unresolved issues related to the mosquito immune system.

Conflict in the Intracellular Lives of Endosymbionts and Viruses: A Mechanistic Look at Wolbachia-Mediated Pathogen-blocking

At the forefront of vector control efforts are strategies that leverage host-microbe associations to reduce vectorial capacity. The most promising of these efforts employs Wolbachia, a maternally transmitted endosymbiotic bacterium naturally found in 40% of insects. Wolbachia can spread through a population of insects while simultaneously inhibiting the replication of viruses within its host. Despite successes in using Wolbachia-transfected mosquitoes to limit dengue, Zika, and chikungunya transmission, the mechanisms behind pathogen-blocking have not been fully characterized. Firstly, we discuss how Wolbachia and viruses both require specific host-derived structures, compounds, and processes to initiate and maintain infection. There is significant overlap in these requirements, and infection with either microbe often manifests as cellular stress, which may be a key component of Wolbachia’s anti-viral effect. Secondly, we discuss the current understanding of pathogen-blocking through this lens of cellular stress and develop a comprehensive view of how the lives of Wolbachia and viruses are fundamentally in conflict with each other. A thorough understanding of the genetic and cellular determinants of pathogen-blocking will significantly enhance the ability of vector control programs to deploy and maintain effective Wolbachia-mediated control measures.

Group B Wolbachia Strain-Dependent Inhibition of Arboviruses

Mosquito-borne viruses, including Zika virus (ZIKV) and dengue virus (DENV), are global threats that continue to infect millions annually. Historically, efforts to combat the spread of these diseases have sought to eradicate the mosquito population. This has had limited success. Recent efforts to combat the spread of these diseases have targeted the mosquito population and the mosquito's ability to transmit viruses by altering the mosquito's microbiome. The introduction of particular strains of Wolbachia bacteria into mosquitos suppresses viral growth and blocks disease transmission. This novel strategy is being tested worldwide to reduce DENV and has early indications of success. The Wolbachia genus comprised divergent strains that are divided in major phylogenetic clades termed supergroups. All Wolbachia field trials currently utilize supergroup A Wolbachia in Aedes aegypti mosquitos to limit virus transmission. Here we discuss our studies of Wolbachia strains not yet used in virus control strategies but that show strong potential to reduce ZIKV replication. These strains are important opportunities in the search for novel tools to reduce the levels of mosquito-borne viruses and provide additional models for mechanistic studies.

Family level variation in Wolbachia-mediated dengue virus blocking in Aedes aegypti

BACKGROUND

The mosquito vector Aedes aegypti is responsible for transmitting a range of arboviruses including dengue (DENV) and Zika (ZIKV). The global reach of these viruses is increasing due to an expansion of the mosquito's geographic range and increasing urbanization and human travel. Vector control remains the primary means for limiting these diseases. Wolbachia pipientis is an endosymbiotic bacterium of insects that has the ability to block the replication of pathogens, including flaviviruses such as DENV or ZIKV, inside the body of the vector. A strain of Wolbachia called wMel is currently being released into wild mosquito populations to test its potential to limit virus transmission to humans. The mechanism that underpins the virus blocking effect, however, remains elusive.

METHODS

We used a modified full-sib breeding design in conjunction with vector competence assays in wildtype and wMel-infected Aedes aegypti collected from the field. All individuals were injected with DENV-2 intrathoracically at 5-6 days of age. Tissues were dissected 7 days post-infection to allow quantification of DENV and Wolbachia loads.

RESULTS

We show the first evidence of family level variation in Wolbachia-mediated blocking in mosquitoes. This variation may stem from either genetic contributions from the mosquito and Wolbachia genomes or environmental influences on Wolbachia. In these families, we also tested for correlations between strength of blocking and expression level for several insect immunity genes with possible roles in blocking, identifying two genes of interest (AGO2 and SCP-2).

CONCLUSIONS

In this study we show variation in Wolbachia-mediated DENV blocking in Aedes aegypti that may arise from genetic contributions and environmental influences on the mosquito-Wolbachia association. This suggests that Wolbachia-mediated blocking may have the ability to evolve through time or be expressed differentially across environments. The long-term efficacy of Wolbachia in the field will be dependent on the stability of blocking. Understanding the mechanism of blocking will be necessary for successful development of strategies that counter the emergence of evolved resistance or variation in its expression under diverse field conditions.

malERA: An updated research agenda for basic science and enabling technologies in malaria elimination and eradication

Basic science holds enormous power for revealing the biological mechanisms of disease and, in turn, paving the way toward new, effective interventions. Recognizing this power, the 2011 Research Agenda for Malaria Eradication included key priorities in fundamental research that, if attained, could help accelerate progress toward disease elimination and eradication. The Malaria Eradication Research Agenda (malERA) Consultative Panel on Basic Science and Enabling Technologies reviewed the progress, continuing challenges, and major opportunities for future research. The recommendations come from a literature of published and unpublished materials and the deliberations of the malERA Refresh Consultative Panel. These areas span multiple aspects of the Plasmodium life cycle in both the human host and the Anopheles vector and include critical, unanswered questions about parasite transmission, human infection in the liver, asexual-stage biology, and malaria persistence. We believe an integrated approach encompassing human immunology, parasitology, and entomology, and harnessing new and emerging biomedical technologies offers the best path toward addressing these questions and, ultimately, lowering the worldwide burden of malaria.

Wolbachia-mediated virus blocking in the mosquito vector Aedes aegypti

Viruses transmitted by mosquitoes such as dengue, Zika and West Nile cause a threat to global health due to increased geographical range and frequency of outbreaks. The bacterium Wolbachia pipientis may be the solution reducing disease transmission. Though commonly missing in vector species, the bacterium was artificially and stably introduced into Aedes aegypti to assess its potential for biocontrol. When infected with Wolbachia, mosquitoes become refractory to infection by a range of pathogens, including the aforementioned viruses. How the bacterium is conferring this phenotype remains unknown. Here we discuss current hypotheses in the field for the mechanistic basis of pathogen blocking and evaluate the evidence from mosquitoes and related insects.

Variable Inhibition of Zika Virus Replication by Different Wolbachia Strains in Mosquito Cell Cultures

Mosquito-borne arboviruses are a major source of human disease. One strategy to reduce arbovirus disease is to reduce the mosquito's ability to transmit virus. Mosquito infection with the bacterial endosymbiont Wolbachia pipientis wMel is a novel strategy to reduce Aedes mosquito competency for flavivirus infection. However, experiments investigating cyclic environmental temperatures have shown a reduction in maternal transmission of wMel, potentially weakening the integration of this strain into a mosquito population relative to that of other Wolbachia strains. Consequently, it is important to investigate additional Wolbachia strains. All Zika virus (ZIKV) suppression studies are limited to the wMel Wolbachia strain. Here we show ZIKV inhibition by two different Wolbachia strains: wAlbB (isolated from Aedes albopictus mosquitoes) and wStri (isolated from the planthopper Laodelphax striatellus) in mosquito cells. Wolbachia strain wStri inhibited ZIKV most effectively. Single-cycle infection experiments showed that ZIKV RNA replication and nonstructural protein 5 translation were reduced below the limits of detection in wStri-containing cells, demonstrating early inhibition of virus replication. ZIKV replication was rescued when Wolbachia was inhibited with a bacteriostatic antibiotic. We observed a partial rescue of ZIKV growth when Wolbachia-infected cells were supplemented with cholesterol-lipid concentrate, suggesting competition for nutrients as one of the possible mechanisms of Wolbachia inhibition of ZIKV. Our data show that wAlbB and wStri infection causes inhibition of ZIKV, making them attractive candidates for further in vitro mechanistic and in vivo studies and future vector-centered approaches to limit ZIKV infection and spread.IMPORTANCE Zika virus (ZIKV) has swiftly spread throughout most of the Western Hemisphere. This is due in large part to its replication in and spread by a mosquito vector host. There is an urgent need for approaches that limit ZIKV replication in mosquitoes. One exciting approach for this is to use a bacterial endosymbiont called Wolbachia that can populate mosquito cells and inhibit ZIKV replication. Here we show that two different strains of Wolbachia, wAlbB and wStri, are effective at repressing ZIKV in mosquito cell lines. Repression of virus growth is through the inhibition of an early stage of infection and requires actively replicating Wolbachia Our findings further the understanding of Wolbachia viral inhibition and provide novel tools that can be used in an effort to limit ZIKV replication in the mosquito vector, thereby interrupting the transmission and spread of the virus.

Symbiont strain is the main determinant of variation in Wolbachia-mediated protection against viruses across Drosophila species

Wolbachia is a common heritable bacterial symbiont in insects. Its evolutionary success lies in the diverse phenotypic effects it has on its hosts coupled to its propensity to move between host species over evolutionary timescales. In a survey of natural host-symbiont associations in a range of Drosophila species, we found that 10 of 16 Wolbachia strains protected their hosts against viral infection. By moving Wolbachia strains between host species, we found that the symbiont genome had a much greater influence on the level of antiviral protection than the host genome. The reason for this was that the level of protection depended on the density of the symbiont in host tissues, and Wolbachia rather than the host-controlled density. The finding that virus resistance and symbiont density are largely under the control of symbiont genes in this system has important implications both for the evolution of these traits and for public health programmes using Wolbachia to prevent mosquitoes from transmitting disease.

Mosquito Microbiome Dynamics, a Background for Prevalence and Seasonality of West Nile Virus

Symbiotic microbial communities augment host phenotype, including defense against pathogen carriage and infection. We sampled the microbial communities in 11 adult mosquito host species from six regions in southern Ontario, Canada over 3 years. Of the factors examined, we found that mosquito species was the largest driver of the microbiota, with remarkable phylosymbiosis between host and microbiota. Seasonal shifts of the microbiome were consistently repeated over the 3-year period, while region had little impact. Both host species and seasonal shifts in microbiota were associated with patterns of West Nile virus (WNV) in these mosquitoes. The highest prevalence of WNV, with a seasonal spike each year in August, was in the Culex pipiens/restuans complex, and high WNV prevalence followed a decrease in relative abundance of Wolbachia in this species. Indeed, mean temperature, but not precipitation, was significantly correlated with Wolbachia abundance. This suggests that at higher temperatures Wolbachia abundance is reduced leading to greater susceptibility to WNV in the subsequent generation of C. pipiens/restuans hosts. Different mosquito genera harbored significantly different bacterial communities, and presence or abundance of Wolbachia was primarily associated with these differences. We identified several operational taxonomic units (OTUs) of Wolbachia that drive overall microbial community differentiation among mosquito taxa, locations and timepoints. Distinct Wolbachia OTUs were consistently found to dominate microbiomes of Cx. pipiens/restuans, and of Coquilletidia perturbans. Seasonal fluctuations of several other microbial taxa included Bacillus cereus, Enterococcus, Methylobacterium, Asaia, Pantoea, Acinetobacter johnsonii, Pseudomonas, and Mycoplasma. This suggests that microbiota may explain some of the variation in vector competence previously attributed to local environmental processes, especially because Wolbachia is known to affect carriage of viral pathogens.

Biological Control Strategies for Mosquito Vectors of Arboviruses

Historically, biological control utilizes predatory species and pathogenic microorganisms to reduce the population of mosquitoes as disease vectors. This is particularly important for the control of mosquito-borne arboviruses, which normally do not have specific antiviral therapies available. Although development of resistance is likely, the advantages of biological control are that the resources used are typically biodegradable and ecologically friendly. Over the past decade, the advancement of molecular biology has enabled optimization by the manipulation of genetic materials associated with biological control agents. Two significant advancements are the discovery of cytoplasmic incompatibility induced by Wolbachia bacteria, which has enhanced replacement programs, and the introduction of dominant lethal genes into local mosquito populations through the release of genetically modified mosquitoes. As various arboviruses continue to be significant public health threats, biological control strategies have evolved to be more diverse and become critical tools to reduce the disease burden of arboviruses.

Downregulation of Aedes aegypti chromodomain helicase DNA binding protein 7/Kismet by Wolbachia and its effect on dengue virus replication

Dengue virus (DENV) is a mosquito-transmitted virus imposing a significant burden on human health around the world. Since current control strategies are not sufficient, there is an urgent need to find alternative methods to control DENV transmission. It has been demonstrated that introduction of Wolbachia pipientis in Aedes aegypti mosquitoes can impede DENV transmission with the mechanism(s) not fully understood. Recently, a number of studies have found the involvement of chromodomain DNA binding helicases in case of Human Immunodeficiency virus (HIV) and Influenza A virus infection. In this study, we have identified three chromodomain helicase DNA binding protein (CHD) genes in Ae. aegypti and looked at their response in the case of Wolbachia and DENV infections. Foremost amongst them we have found that AeCHD7/Kismet is significantly downregulated in the presence of Wolbachia infection only in female mosquitoes. Furthermore, AeCHD7 levels showed significant increase during DENV infection, and AeCHD7 depletion led to severe reduction in the replication of DENV. Our data have identified AeCHD7 as a novel Ae. aegypti host factor that is important for DENV replication, and Wolbachia downregulates it, which may contribute towards the mechanism(s) of limiting DENV replication.

Wolbachia increases the susceptibility of a parasitoid wasp to hyperparasitism

The success of maternally transmitted endosymbiotic bacteria, such as Wolbachia, is directly linked to their host reproduction but in direct conflict with other parasites that kill the host before it reaches reproductive maturity. Therefore, symbionts that have evolved strategies to increase their host’s ability to evade lethal parasites may have high penetrance, while detrimental symbionts would be selected against, leading to lower penetrance or extinction from the host population. In a natural population of the parasitoid wasp Hyposoter horticola in the Åland Islands (Finland), the Wolbachia strain wHho persists at an intermediate prevalence (∼50%). Additionally, there is a negative correlation between the prevalence of Wolbachia and a hyperparasitoid wasp, Mesochorus cf. stigmaticus, in the landscape. Using a manipulative field experiment, we addressed the persistence of Wolbachia at this intermediate level, and tested whether the observed negative correlation could be due to Wolbachia inducing either susceptibility or resistance to parasitism. We show that infection with Wolbachia does not influence the ability of the wasp to parasitize its butterfly host, Melitaea cinxia, but that hyperparasitism of the wasp increases in the presence of wHho. Consequently, the symbiont is detrimental, and in order to persist in the host population, must also have a positive effect on fitness that outweighs the costly burden of susceptibility to widespread parasitism.

A Native Wolbachia Endosymbiont Does Not Limit Dengue Virus Infection in the Mosquito Aedes notoscriptus (Diptera: Culicidae)

The endosymbiotic bacterium Wolbachia pipientis infects many species of insects and has been transinfected into the mosquito Aedes aegypti (L.), the primary vector of dengue virus (DENV). Recently, it has been shown that Wolbachia blocks the replication and transmission of RNA viruses, such as DENV, in a number of mosquito species including Ae. aegypti and Aedes albopictus (Skuse), which is naturally infected with Wolbachia and considered a secondary vector for DENV. The mosquito species Aedes notoscriptus (Skuse) is highly prevalent in Australia, including in areas where DENV outbreaks have been recorded. The mosquito has been implicated in the transmission of Ross River and Barmah Forest viruses, but not DENV. We investigated whether Wolbachia naturally infects this mosquito species and whether it has an impact on the ability of Ae. notoscriptus to transmit DENV. We show, for the first time, that Ae. notoscriptus is naturally infected with a strain of Wolbachia that belongs to supergroup B and is localized only in the ovaries. However, Wolbachia infection in Ae. notoscriptus did not induce resistance to DENV and had no effect on overall DENV infection rate or titer. The presence of a native Wolbachia in Ae. notoscriptus cannot explain why this mosquito is an ineffective vector of DENV.