Competition for amino acids between Wolbachia and the mosquito host, Aedes aegypti

Diet-Induced Nutritional Stress and Pathogen Interference in Wolbachia-Infected Aedes aegypti

The pathogen interference phenotype greatly restricts infection with dengue virus (DENV) and other pathogens in Wolbachia-infected Aedes aegypti, and is a vital component of Wolbachia-based mosquito control. Critically, the phenotype's causal mechanism is complex and poorly understood, with recent evidence suggesting that the cause may be species specific. To better understand this important phenotype, we investigated the role of diet-induced nutritional stress on interference against DENV and the avian malarial parasite Plasmodium gallinaceum in Wolbachia-infected Ae. aegypti, and on physiological processes linked to the phenotype. Wolbachia-infected mosquitoes were fed one of four different concentrations of sucrose, and then challenged with either P. gallinaceum or DENV. Interference against P. gallinaceum was significantly weakened by the change in diet however there was no effect on DENV interference. Immune gene expression and H2O2 levels have previously been linked to pathogen interference. These traits were assayed for mosquitoes on each diet using RT-qPCR and the Amplex Red Hydrogen Peroxide/Peroxidase Assay Kit, and it was observed that the change in diet did not significantly affect immune expression, but low carbohydrate levels led to a loss of ROS induction in Wolbachia-infected mosquitoes. Our data suggest that host nutrition may not influence DENV interference for Wolbachia-infected mosquitoes, but Plasmodium interference may be linked to both nutrition and oxidative stress. This pathogen-specific response to nutritional change highlights the complex nature of interactions between Wolbachia and pathogens in mosquitoes.

Gut microbiota in Drosophila melanogaster interacts with Wolbachia but does not contribute to Wolbachia-mediated antiviral protection

Animals experience near constant infection with microorganisms. A significant proportion of these microbiota reside in the alimentary tract. There is a growing appreciation for the roles gut microbiota play in host biology. The gut microbiota of insects, for example, have been shown to help the host overcome pathogen infection either through direct competition or indirectly by stimulating host immunity. These defenses may also be supplemented by coinfecting maternally inherited microbes such as Wolbachia. The presence of Wolbachia in a host can delay and/or reduce death caused by RNA viruses. Whether the gut microbiota of the host interacts with Wolbachia, or vice versa, the precise role of Wolbachia in antiviral protection is not known. In this study, we used 16S rDNA sequencing to characterise changes in gut microbiota composition in Drosophila melanogaster associated with Wolbachia infection and antibiotic treatment. We subsequently tested whether changes in gut composition via antibiotic treatment altered Wolbachia-mediated antiviral properties. We found that both antibiotics and Wolbachia significantly reduced the biodiversity of the gut microbiota without changing the total microbial load. We also showed that changing the gut microbiota composition with antibiotic treatment enhanced Wolbachia density but did not confer greater antiviral protection against Drosophila C virus to the host. We concluded there are significant interactions between Wolbachia and gut microbiota, but changing gut microbiota composition is not likely to be a means through which Wolbachia conveys antiviral protection to its host.

Wolbachia-Based Dengue Virus Inhibition Is Not Tissue-Specific in Aedes aegypti

Background

Dengue fever, caused by the dengue virus (DENV), is now the most common arbovirus transmitted disease globally. One novel approach to control DENV is to use the endosymbiotic bacterium, Wolbachia pipientis, to limit DENV replication inside the primary mosquito vector, Aedes aegypti. Wolbachia that is naturally present in a range of insects reduces the capacity for viruses, bacteria, parasites and fungi to replicate inside insects. Wolbachia’s mode of action is not well understood but may involve components of immune activation or competition with pathogens for limited host resources. The strength of Wolbachia-based anti DENV effects appear to correlate with bacterial density in the whole insect and in cell culture. Here we aimed to determine whether particular tissues, especially those with high Wolbachia densities or immune activity, play a greater role in mediating the anti DENV effect.

Methodology/findings

Ae. aegypti mosquito lines with and without Wolbachia (Wildtype) were orally fed DENV 3 and their viral loads subsequently measured over two time points post infection in the midgut, head, salivary glands, Malpighian tubules, fat body and carcass. We did not find correlations between Wolbachia densities and DENV loads in any tissue, nor with DENV loads in salivary glands, the endpoint of infection. This is in contrast with strong positive correlations between DENV loads in a range of tissues and salivary gland loads for Wildtype mosquitoes. Lastly, there was no evidence of a heightened role for tissues with known immune function including the fat body and the Malpighian tubules in Wolbachia’s limitation of DENV.

Conclusion/significance

We conclude that the efficacy of DENV blocking in Wolbachia infected mosquitoes is not reliant on any particular tissue. This work therefore suggests that the mechanism of Wolbachia-based antiviral effects is either systemic or acts locally via processes that are fundamental to diverse cell types. We further conclude that the relationship between DENV blocking and Wolbachia density is not linear in mosquito tissues

Dengue fever caused by the dengue virus (DENV) is transmitted by the mosquito, Aedes aegypti. To control the disease, an intracellular bacterium called Wolbachia has been introduced into Ae. aegypti where it blocks/limits success of infection of DENV. The mechanistic basis of blocking is not well understood but may involve Wolbachia activating the host immune system or competing with DENV for host resources. The strength of blocking appears to correlate with Wolbachia density. Here, we aimed to determine if any particular tissues inside the mosquito play a greater role in blocking. Tissues were chosen based on their Wolbachia density and their roles in infection and immunity. Wolbachia infected and uninfected mosquitoes were orally infected with DENV and Wolbachia density and DENV load were assessed in midgut, salivary gland, head, Malpighian tubules, fat body and carcass. Wolbachia density did not correlate with DENV loads in the same tissues nor with DENV loads in the salivary glands. We also showed that no one tissue appeared to play a greater role in blocking. In summary, these finding suggest that in the mosquito a threshold Wolbachia density may be required for DENV blocking. Our findings also suggest that blocking may involve mechanisms that are fundamental to all cells.

Interaction of Wolbachia and Bloodmeal Type in Artificially Infected Aedes albopictus (Diptera: Culicidae)

Maternally inherited Wolbachia bacteria are being introduced into vector mosquito populations, with the goal of reducing the transmission of diseases such as dengue fever. The infection dynamics of Wolbachia depends upon the ability of Wolbachia to manipulate host reproduction as well as any fitness costs imposed upon the host. Some vector mosquito species are opportunistic blood feeders, utilizing both human and nonhuman vertebrate hosts, and the effects of bloodmeal source on Wolbachia phenotype is not well understood. Here we transfer wMelPop Wolbachia from Drosophila melanogaster (Meigen) into wild-type Aedes albopictus (Skuse) and characterize the resulting triple infection by examining for an effect of human and mouse blood on the Wolbachia infection persistence and phenotypes. When provided with human blood, the triple Wolbachia infection was persistent, with high maternal inheritance and relatively little fecundity cost, and a pattern of imperfect unidirectional cytoplasmic incompatibility was observed in mating experiments between wild-type and triply infected individuals. With mouse blood, reduced female fecundity and low maternal inheritance were observed in wMelPop-infected females, which affected the typical pattern of unidirectional CI. Our findings indicate the interactive effects of Wolbachia infection and blood source drive distinct shifts in the Wolbachia-host symbiotic association.

The rich somatic life of Wolbachia

Wolbachia is an intracellular endosymbiont infecting most arthropod and some filarial nematode species that is vertically transmitted through the maternal lineage. Due to this primary mechanism of transmission, most studies have focused on Wolbachia interactions with the host germline. However, over the last decade many studies have emerged highlighting the prominence of Wolbachia in somatic tissues, implicating somatic tissue tropism as an important aspect of the life history of this endosymbiont. Here, we review our current understanding of Wolbachia-host interactions at both the cellular and organismal level, with a focus on Wolbachia in somatic tissues.

Effects of Larval Nutrition on Wolbachia-Based Dengue Virus Interference in Aedes aegypti (Diptera: Culicidae)

In order to assess the broad-scale applicability of field releases of Wolbachia for the biological control of insect-transmitted diseases, we determined the relationship between the larval diet of Aedes aegypti L. mosquitoes infected with Wolbachia strains and their susceptibility to dengue virus (DENV) infection via intrathoracic injection and oral inoculation. Larvae were reared on diets that varied in the quantity of food which had the effect of modifying development time and adult body size. Wolbachia wMel infection was associated with highly significant reductions in dengue serotype 2 (DENV-2) infection rates of between 80 and 97.5% following intrathoracic injection of adults emerging from three diet levels. Reductions were 100% in two diet level treatments following oral inoculation. Similarly, wMelPop infection was associated with highly significant reductions in DENV-2 infection rates of between 95 and 100% for intrathoracic injection and 97.5 and 100% for oral inoculation across diet level treatments. Larval diet level had no significant effect on DENV-2 infection rates in the presence of Wolbachia infection in mosquitoes that were intrathoracically injected with the virus. This indicates that the effectiveness of Wolbachia on vector competence disruption within Ae. aegypti is unlikely to be compromised by variable larval nutrition in field settings.

Rapid and Non-destructive Detection and Identification of Two Strains of Wolbachia in Aedes aegypti by Near-Infrared Spectroscopy

The release of Wolbachia infected mosquitoes is likely to form a key component of disease control strategies in the near future. We investigated the potential of using near-infrared spectroscopy (NIRS) to simultaneously detect and identify two strains of Wolbachia pipientis (wMelPop and wMel) in male and female laboratory-reared Aedes aegypti mosquitoes. Our aim is to find faster, cheaper alternatives for monitoring those releases than the molecular diagnostic techniques that are currently in use. Our findings indicate that NIRS can differentiate females and males infected with wMelPop from uninfected wild type samples with an accuracy of 96% (N = 299) and 87.5% (N = 377), respectively. Similarly, females and males infected with wMel were differentiated from uninfected wild type samples with accuracies of 92% (N = 352) and 89% (N = 444). NIRS could differentiate wMelPop and wMel transinfected females with an accuracy of 96.6% (N = 442) and males with an accuracy of 84.5% (N = 443). This non-destructive technique is faster than the standard polymerase chain reaction diagnostic techniques. After the purchase of a NIRS spectrometer, the technique requires little sample processing and does not consume any reagents.

Near infrared spectroscopy (NIRS) is a technique that measures specific frequencies of light absorbed by C-H, O-H, S-H and N-H functional groups. Mosquito samples are grouped based upon absorption differences between their chemical properties. In this study, we used NIRS to differentiate 1) Aedes aegypti infected with either of the two strains of intracellular bacterium Wolbachia (wMel and wMelPop) from wild type Ae. aegypti and 2) Aedes aegypti infected with wMel from those infected with wMelPoP. NIRS facilitated the differentiation of wMel and wMelPop from wild type samples and samples infected with either of the Wolbachia infected strains with high prediction accuracies over their lifespan. Predictive models were derived from initial calibration data sets and validated against independent cohorts. Prediction accuracies were high (82–98%) regardless of the cohort mosquitoes were sampled from. The results show that NIRS may have real potential as an alternative method for monitoring Wolbachia incidence in mosquitoes. A rapid, simple and cost-effective surveillance tool suitable for resource-poor areas and large urban release programs would be of great utility for evaluating Wolbachia-based interventions. The models developed during this study require further validation using field collections.

The influence of larval competition on Brazilian Wolbachia-infected Aedes aegypti mosquitoes

BACKGROUND

With field releases starting in Brazil, particular interest must be given to understanding how the endosymbiotic bacterium Wolbachia pipientis affects Aedes aegypti mosquitoes with a Brazilian genetic background. Currently, there is limited information on how the bacterium affects phenotypic traits such as larval development rate, metabolic reserves and morphometric parameters in Ae. aegypti. Here, we analyze for the first time, the effect of Wolbachia on these key phenotypes and consider how this might impact the potential of the bacterium as a disease control agent in Brazil.

METHODS

We examined the influence of the wMel strain of Wolbachia in laboratory Ae. aegypti with a Brazilian genetic background, reared under different larval densities. Pupae formation was counted daily to assess differences in development rates. Levels of metabolic reserves and morphometric parameters were assessed in adults resulting from each larval condition.

RESULTS

wMel infection led to more rapid larval development at higher densities for both males and females, with no effect under less crowded conditions in females. Infection also led to reduced body size at both high and low density, but not at intermediate density, although the scale of this difference was maintained regardless of larval density, in comparison to uninfected individuals. Wing shape also varied significantly between infected and uninfected mosquitoes due to larval density. Glycogen levels in uninfected mosquitoes decreased under higher larval density, but were consistently high with Wolbachia infection, regardless of larval density.

CONCLUSIONS

We demonstrate that the wMel Wolbachia strain can positively influence some important host fitness traits, and that this interaction is directly linked to the conditions in which the host is reared. Combined with previously published data, these results suggest that this Wolbachia strain could be successfully used as part of the Eliminate Dengue Program in Brazil.

Macronutrients mediate the functional relationship between Drosophila and Wolbachia

Wolbachia are maternally inherited bacterial endosymbionts that naturally infect a diverse array of arthropods. They are primarily known for their manipulation of host reproductive biology, and recently, infections with Wolbachia have been proposed as a new strategy for controlling insect vectors and subsequent human-transmissible diseases. Yet, Wolbachia abundance has been shown to vary greatly between individuals and the magnitude of the effects of infection on host life-history traits and protection against infection is correlated to within-host Wolbachia abundance. It is therefore essential to better understand the factors that modulate Wolbachia abundance and effects on host fitness. Nutrition is known to be one of the most important mediators of host-symbiont interactions. Here, we used nutritional geometry to quantify the role of macronutrients on insect-Wolbachia relationships in Drosophila melanogaster. Our results show fundamental interactions between diet composition, host diet selection, Wolbachia abundance and effects on host lifespan and fecundity. The results and methods described here open a new avenue in the study of insect-Wolbachia relationships and are of general interest to numerous research disciplines, ranging from nutrition and life-history theory to public health.

Costs of Three Wolbachia Infections on the Survival of Aedes aegypti Larvae under Starvation Conditions

The mosquito Aedes aegypti, the principal vector of dengue virus, has recently been infected experimentally with Wolbachia: intracellular bacteria that possess potential as dengue biological control agents. Wolbachia depend on their hosts for nutrients they are unable to synthesize themselves. Consequently, competition between Wolbachia and their host for resources could reduce host fitness under the competitive conditions commonly experienced by larvae of Ae. aegypti in the field, hampering the invasion of Wolbachia into natural mosquito populations. We assess the survival and development of Ae. aegypti larvae under starvation conditions when infected with each of three experimentally-generated Wolbachia strains: wMel, wMelPop and wAlbB, and compare their fitness to wild-type uninfected larvae. We find that all three Wolbachia infections reduce the survival of larvae relative to those that are uninfected, and the severity of the effect is concordant with previously characterized fitness costs to other life stages. We also investigate the ability of larvae to recover from extended food deprivation and find no effect of Wolbachia on this trait. Aedes aegypti larvae of all infection types were able to resume their development after one month of no food, pupate rapidly, emerge at a large size, and exhibit complete cytoplasmic incompatibility and maternal transmission. A lowered ability of Wolbachia-infected larvae to survive under starvation conditions will increase the threshold infection frequency required for Wolbachia to establish in highly competitive natural Ae. aegypti populations and will also reduce the speed of invasion. This study also provides insights into survival strategies of larvae when developing in stressful environments.

Exposure to West Nile Virus Increases Bacterial Diversity and Immune Gene Expression in Culex pipiens

Complex interactions between microbial residents of mosquitoes and arboviruses are likely to influence many aspects of vectorial capacity and could potentially have profound effects on patterns of arbovirus transmission. Such interactions have not been well studied for West Nile virus (WNV; Flaviviridae, Flavivirus) and Culex spp. mosquitoes. We utilized next-generation sequencing of 16S ribosomal RNA bacterial genes derived from Culex pipiens Linnaeus following WNV exposure and/or infection and compared bacterial populations and broad immune responses to unexposed mosquitoes. Our results demonstrate that WNV infection increases the diversity of bacterial populations and is associated with up-regulation of classical invertebrate immune pathways including RNA interference (RNAi), Toll, and Jak-STAT (Janus kinase-Signal Transducer and Activator of Transcription). In addition, WNV exposure alone, without the establishment of infection, results in similar alterations to microbial and immune signatures, although to a lesser extent. Multiple bacterial genera were found in greater abundance in WNV-exposed and/or infected mosquitoes, yet the most consistent and notable was the genus Serratia.

The microbiome modulates arbovirus transmission in mosquitoes

Mosquito-transmitted arthropod-borne viruses (arboviruses) such as dengue virus, chikungunya virus, and West Nile virus constitute a major public health burden and are increasing in severity and frequency worldwide. The microbiota associated with mosquitoes (comprised of viruses, bacteria, fungi and protozoa) can profoundly influence many host phenotypes including vector competence, which can either be enhanced or suppressed. Thus, the tripartite interactions between the mosquito vector, its microbiota and the pathogens they transmit offer novel possibilities to control arthropod-borne diseases.

Native Wolbachia from Aedes albopictus Blocks Chikungunya Virus Infection In Cellulo

Wolbachia, a widespread endosymbiont of terrestrial arthropods, can protect its host against viral and parasitic infections, a phenotype called "pathogen blocking". However, in some cases Wolbachia may have no effect or even enhance pathogen infection, depending on the host-Wolbachia-pathogen combination. The tiger mosquito Aedes albopictus is naturally infected by two strains of Wolbachia, wAlbA and wAlbB, and is a competent vector for different arboviruses such as dengue virus (DENV) and Chikungunya virus (CHIKV). Interestingly, it was shown in some cases that Ae. albopictus native Wolbachia strains are able to inhibit DENV transmission by limiting viral replication in salivary glands, but no such impact was measured on CHIKV replication in vivo. To better understand the Wolbachia/CHIKV/Ae. albopictus interaction, we generated a cellular model using Ae. albopictus derived C6/36 cells that we infected with the wAlbB strain. Our results indicate that CHIKV infection is negatively impacted at both RNA replication and virus assembly/secretion steps in presence of wAlbB. Using FISH, we observed CHIKV and wAlbB in the same mosquito cells, indicating that the virus is still able to enter the cell in the presence of the bacterium. Further work is needed to decipher molecular pathways involved in Wolbachia-CHIKV interaction at the cellular level, but this cellular model can be a useful tool to study the mechanism behind virus blocking phenotype induced by Wolbachia. More broadly, this underlines that despite Wolbachia antiviral potential other complex interactions occur in vivo to determine mosquito vector competence in Ae. albopictus.

Effect of repeat human blood feeding on Wolbachia density and dengue virus infection in Aedes aegypti

Background

The introduction of the endosymbiotic bacterium, Wolbachia into Aedes aegypti populations is a novel approach to reduce disease transmission. The presence of Wolbachia limits the ability of the mosquito to transmit dengue virus (DENV) and the strength of this effect appears to correlate with Wolbachia densities in the mosquito. There is also some evidence that Wolbachia densities may increase following the consumption of a bloodmeal. Here we have examined whether multiple blood feeds lead to increases in density or associated changes in Wolbachia-mediated blocking of DENV.

Methods

The Wolbachia infected Aedes aegypti mosquito line was used for the study. There were three treatment groups; a non-blood fed control, a second group fed once and a third group fed twice on human blood. All groups were orally infected with DENV-2 and then their midguts and salivary glands were dissected 10–11 days post infection. RNA/DNA was simultaneously extracted from each tissue and subsequently used for DENV RNA copies and Wolbachia density quantification, respectively.

Results

We found variation between replicate vector competence experiments and no clear evidence that Wolbachia numbers increased in either the salivary glands or remainder of the body with feeding and hence saw no corresponding improvements in DENV blocking.

Conclusions

Aedes aegypti are “sip” feeders returning often to obtain bloodmeals and hence it is important to assess whether repeat blood feeding improved the efficacy of Wolbachia-based DENV blocking. Our work suggests in the laboratory context when Wolbachia densities are high that repeat feeding does not improve blocking and hence this ability should likely be stable with respect to feeding cycle in the field.

Exome and transcriptome sequencing of Aedes aegypti identifies a locus that confers resistance to Brugia malayi and alters the immune response

Many mosquito species are naturally polymorphic for their abilities to transmit parasites, a feature which is of great interest for controlling vector-borne disease. Aedes aegypti, the primary vector of dengue and yellow fever and a laboratory model for studying lymphatic filariasis, is genetically variable for its capacity to harbor the filarial nematode Brugia malayi. The genome of Ae. aegypti is large and repetitive, making genome resequencing difficult and expensive. We designed exome captures to target protein-coding regions of the genome, and used association mapping in a wild Kenyan population to identify a single, dominant, sex-linked locus underlying resistance. This falls in a region of the genome where a resistance locus was previously mapped in a line established in 1936, suggesting that this polymorphism has been maintained in the wild for the at least 80 years. We then crossed resistant and susceptible mosquitoes to place both alleles of the gene into a common genetic background, and used RNA-seq to measure the effect of this locus on gene expression. We found evidence for Toll, IMD, and JAK-STAT pathway activity in response to early stages of B. malayi infection when the parasites are beginning to die in the resistant genotype. We also found that resistant mosquitoes express anti-microbial peptides at the time of parasite-killing, and that this expression is suppressed in susceptible mosquitoes. Together, we have found that a single resistance locus leads to a higher immune response in resistant mosquitoes, and we identify genes in this region that may be responsible for this trait.

The impact of host diet on Wolbachia titer in Drosophila

While a number of studies have identified host factors that influence endosymbiont titer, little is known concerning environmental influences on titer. Here we examined nutrient impact on maternally transmitted Wolbachia endosymbionts in Drosophila. We demonstrate that Drosophila reared on sucrose- and yeast-enriched diets exhibit increased and reduced Wolbachia titers in oogenesis, respectively. The yeast-induced Wolbachia depletion is mediated in large part by the somatic TOR and insulin signaling pathways. Disrupting TORC1 with the small molecule rapamycin dramatically increases oocyte Wolbachia titer, whereas hyper-activating somatic TORC1 suppresses oocyte titer. Furthermore, genetic ablation of insulin-producing cells located in the Drosophila brain abolished the yeast impact on oocyte titer. Exposure to yeast-enriched diets altered Wolbachia nucleoid morphology in oogenesis. Furthermore, dietary yeast increased somatic Wolbachia titer overall, though not in the central nervous system. These findings highlight the interactions between Wolbachia and germline cells as strongly nutrient-sensitive, and implicate conserved host signaling pathways by which nutrients influence Wolbachia titer.

Vector competence of Aedes aegypti mosquitoes for filarial nematodes is affected by age and nutrient limitation

Mosquitoes are one of the most important vectors of human disease. The ability of mosquitoes to transmit disease is dependent on the age structure of the population, as mosquitoes must survive long enough for the parasites to complete their development and infect another human. Age could have additional effects due to mortality rates and vector competence changing as mosquitoes senesce, but these are comparatively poorly understood. We have investigated these factors using the mosquito Aedes aegypti and the filarial nematode Brugia malayi. Rather than observing any effects of immune senescence, we found that older mosquitoes were more resistant, but this only occurred if they had previously been maintained on a nutrient-poor diet of fructose. Constant blood feeding reversed this decline in vector competence, meaning that the number of parasites remained relatively unchanged as mosquitoes aged. Old females that had been maintained on fructose also experienced a sharp spike in mortality after an infected blood meal ("refeeding syndrome") and few survived long enough for the parasite to develop. Again, this effect was prevented by frequent blood meals. Our results indicate that old mosquitoes may be inefficient vectors due to low vector competence and high mortality, but that frequent blood meals can prevent these effects of age.

Wolbachia strain wAlbB confers both fitness costs and benefit on Anopheles stephensi

Background

Wolbachia is a maternally transmitted intracellular bacterium that is estimated to infect up to 65% of insect species, but it is not naturally present in Anopheles malaria vectors. Wolbachia-based strategies for malaria vector control can be developed either through population replacement to reduce vectorial capacity or through population suppression to reduce the mosquito population. We have previously generated An. stephensi mosquitoes carrying a stable wAlbB Wolbachia infection and have demonstrated their ability to invade wild-type laboratory populations and confer resistance to Plasmodium on these populations.

Methods

We assessed wAlbB-associated fitness by comparing the female fecundity, immature development and survivorship, body size, male mating competiveness, and adult longevity of the infected An. stephensi to that of wild-type mosquitoes.

Results

We found that wAlbB reduced female fecundity and caused a minor decrease in male mating competiveness. We also observed that wAlbB increased the life span of both male and female mosquitoes when they were maintained solely on sugar meals; however, there was no impact on the life span of blood-fed females. In addition, wAlbB did not influence either immature development and survivorship or adult body sizes.

Conclusions

These results provide significant support for developing Wolbachia-based strategies for malaria vector control.