CHARACTERIZATION OF \u3cem\u3eWOLBACHIA\u3c/em\u3e AND ITS INTERACTION IN HOST MOSQUITOES

Wolbachia are maternally inherited, obligate, intracellular bacteria inducing a form of sterility known as cytoplasmic incompatibility. Wolbachia based strategies have been proposed for the control of disease vectors. One example is to use a population replacement strategy to drive into natural population a novel Wolbachia that modifies the age structure of a vector population, reducing disease transmission. In this research, the effects of a life-shortening stain of Wolbachia (popcorn Wolbachia) are transferred into the mosquitoes Aedes albopictus (Chapter Two and Three) and A. aegypti (Chapter Four and Five). In Chapter Two, the Wolbachia symbiosis significantly reduced fecundity and egg hatches in A. albopictus, with Wolbachia being highly pathogenic in this mosquito species. In Chapter Three, the relationship between popcorn Wolbachia and its host (in a triple-infected mosquito strain) varied with the mosquito diet. Feeding on mouse blood was associated with the loss of infection, whereas the infection was maintained in human blood-fed mosquito lines. Egg viability of triple infected mosquito was reduced only with mouse blood. In Chapter Four, the reduced competitiveness (e.g., low survival and increased developmental time) of infected A. aegypti immatures was associated with popcorn Wolbachia, relative to uninfected individuals in low food condition. In Chapter Five, the decreased survival of immature A. aegypti was associated with popcorn Wolbachia in the presence of potential predators (i.e., older A. aegypti or A. albopictus larvae). Using a novel behavioral assay, a delayed larval reaction to light avoidance was observed to be associated with the infection, suggesting Wolbachia effects on immature host behaviors. In Chapter Six, popcorn Wolbachia and wAlbB infected A. aegypti showed similar reproduction potential. No reduced level of CI or mating competitiveness was observed in wAlbB infected males. The results suggest the wAlbB infection in A. aegypti can be an additional agent for Wolbachia-based control strategies. In Chapter Seven, a filtering system using commercially available sieves was able to separate immature mosquitoes from water, preventing escape of mosquitoes. In Chapter Eight, an inexpensive artificial blood feeding was designed for feeding multiple mosquito cages. The results support the use of these tools to facilitate mass rearing of mosquitoes

Suboptimal Larval Habitats Modulate Oviposition of the Malaria Vector Mosquito Anopheles coluzzii.

Selection of oviposition sites by gravid females is a critical behavioral step in the reproductive cycle of Anopheles coluzzii, which is one of the principal Afrotropical malaria vector mosquitoes. Several studies suggest this decision is mediated by semiochemicals associated with potential oviposition sites. To better understand the chemosensory basis of this behavior and identify compounds that can modulate oviposition, we examined the generally held hypothesis that suboptimal larval habitats give rise to semiochemicals that negatively influence the oviposition preference of gravid females. Dual-choice bioassays indicated that oviposition sites conditioned in this manner do indeed foster significant and concentration dependent aversive effects on the oviposition site selection of gravid females. Headspace analyses derived from aversive habitats consistently noted the presence of dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS) and 6-methyl-5-hepten-2-one (sulcatone) each of which unitarily affected An. coluzzii oviposition preference. Electrophysiological assays across the antennae, maxillary palp, and labellum of gravid An. coluzzii revealed differential responses to these semiochemicals. Taken together, these findings validate the hypothesis in question and suggest that suboptimal environments for An. coluzzii larval development results in the release of DMDS, DMTS and sulcatone that impact the response valence of gravid females

Life-Shortening \u3cem\u3eWolbachia\u3c/em\u3e Infection Reduces Population Growth of \u3cem\u3eAedes aegypti\u3c/em\u3e

Wolbachia bacteria are being introduced into natural populations of vector mosquitoes, with the goal of reducing the transmission of human diseases such as Zika and dengue fever. The successful establishment of Wolbachia infection is largely dependent on the effects of Wolbachia infection to host fitness, but the effects of Wolbachia infection on the individual life-history traits of immature mosquitoes can vary. Here, the effects of life-shortening Wolbachia (wMelPop) on population growth of infected individuals were evaluated by measuring larval survival, developmental time and adult size of Aedes aegypti in intra- (infected or uninfected only) and inter-group (mixed with infected and uninfected) larval competition assays. At low larval density conditions, the population growth of wMelPop infected and uninfected individuals was similar. At high larval densities, wMelPop infected individuals had a significantly reduced population growth rate relative to uninfected individuals, regardless of competition type. We discuss the results in relation to the invasion of the wMelPop Wolbachia infection into naturally uninfected populations

\u3cem\u3eWolbachia\u3c/em\u3e infections that reduce immature insect survival: predicted impacts on population replacement

BACKGROUND: The evolutionary success of Wolbachia bacteria, infections of which are widespread in invertebrates, is largely attributed to an ability to manipulate host reproduction without imposing substantial fitness costs. Here, we describe a stage-structured model with deterministic immature lifestages and a stochastic adult female lifestage. Simulations were conducted to better understand Wolbachia invasions into uninfected host populations. The model includes conventional Wolbachia parameters (the level of cytoplasmic incompatibility, maternal inheritance, the relative fecundity of infected females, and the initial Wolbachia infection frequency) and a new parameter termed relative larval viability (RLV), which is the survival of infected larvae relative to uninfected larvae. RESULTS: The results predict the RLV parameter to be the most important determinant for Wolbachia invasion and establishment. Specifically, the fitness of infected immature hosts must be close to equal to that of uninfected hosts before population replacement can occur. Furthermore, minute decreases in RLV inhibit the invasion of Wolbachia despite high levels of cytoplasmic incompatibility, maternal inheritance, and low adult fitness costs. CONCLUSIONS: The model described here takes a novel approach to understanding the spread of Wolbachia through a population with explicit dynamics. By combining a stochastic female adult lifestage and deterministic immature/adult male lifestages, the model predicts that even those Wolbachia infections that cause minor decreases in immature survival are unlikely to invade and spread within the host population. The results are discussed in relation to recent theoretical and empirical studies of natural population replacement events and proposed applied research, which would use Wolbachia as a tool to manipulate insect populations

Estimating the effects of temperature on transmission of the human malaria parasite, Plasmodium falciparum

Despite concern that climate change could increase the human risk to malaria in certain areas, the temperature dependency of malaria transmission is poorly characterized. Here, we use a mechanistic model fitted to experimental data to describe how Plasmodium falciparum infection of the African malaria vector, Anopheles gambiae, is modulated by temperature, including its influences on parasite establishment, conversion efficiency through parasite developmental stages, parasite development rate, and overall vector competence. We use these data, together with estimates of the survival of infected blood-fed mosquitoes, to explore the theoretical influence of temperature on transmission in four locations in Kenya, considering recent conditions and future climate change. Results provide insights into factors limiting transmission in cooler environments and indicate that increases in malaria transmission due to climate warming in areas like the Kenyan Highlands, might be less than previously predicted

A non-destructive sugar-feeding assay for parasite detection and estimating the extrinsic incubation period of Plasmodium falciparum in individual mosquito vectors

Despite its epidemiological importance, the time Plasmodium parasites take to achieve development in the vector mosquito (the extrinsic incubation period, EIP) remains poorly characterized. A novel non-destructive assay designed to estimate EIP in single mosquitoes, and more broadly to study Plasmodium–Anopheles vectors interactions, is presented. The assay uses small pieces of cotton wool soaked in sugar solution to collect malaria sporozoites from individual mosquitoes during sugar feeding to monitor infection status over time. This technique has been tested across four natural malaria mosquito species of Africa and Asia, infected with Plasmodium falciparum (six field isolates from gametocyte-infected patients in Burkina Faso and the NF54 strain) and across a range of temperatures relevant to malaria transmission in field conditions. Monitoring individual infectious mosquitoes was feasible. The estimated median EIP of P. falciparum at 27 °C was 11 to 14 days depending on mosquito species and parasite isolate. Long-term individual tracking revealed that sporozoites transfer onto cotton wool can occur at least until day 40 post-infection. Short individual EIP were associated with short mosquito lifespan. Correlations between mosquito/parasite traits often reveal trade-offs and constraints and have important implications for understanding the evolution of parasite transmission strategies

Wolbachia infections that reduce immature insect survival: Predicted impacts on population replacement

<p>Abstract</p> <p>Background</p> <p>The evolutionary success of <it>Wolbachia </it>bacteria, infections of which are widespread in invertebrates, is largely attributed to an ability to manipulate host reproduction without imposing substantial fitness costs. Here, we describe a stage-structured model with deterministic immature lifestages and a stochastic adult female lifestage. Simulations were conducted to better understand <it>Wolbachia </it>invasions into uninfected host populations. The model includes conventional <it>Wolbachia </it>parameters (the level of cytoplasmic incompatibility, maternal inheritance, the relative fecundity of infected females, and the initial <it>Wolbachia </it>infection frequency) and a new parameter termed relative larval viability (<it>RLV</it>), which is the survival of infected larvae relative to uninfected larvae.</p> <p>Results</p> <p>The results predict the <it>RLV </it>parameter to be the most important determinant for <it>Wolbachia </it>invasion and establishment. Specifically, the fitness of infected immature hosts must be close to equal to that of uninfected hosts before population replacement can occur. Furthermore, minute decreases in <it>RLV </it>inhibit the invasion of <it>Wolbachia </it>despite high levels of cytoplasmic incompatibility, maternal inheritance, and low adult fitness costs.</p> <p>Conclusions</p> <p>The model described here takes a novel approach to understanding the spread of <it>Wolbachia </it>through a population with explicit dynamics. By combining a stochastic female adult lifestage and deterministic immature/adult male lifestages, the model predicts that even those <it>Wolbachia </it>infections that cause minor decreases in immature survival are unlikely to invade and spread within the host population. The results are discussed in relation to recent theoretical and empirical studies of natural population replacement events and proposed applied research, which would use <it>Wolbachia </it>as a tool to manipulate insect populations.</p

Rethinking the extrinsic incubation period of malaria parasites

The time it takes for malaria parasites to develop within a mosquito, and become transmissible, is known as the extrinsic incubation period, or EIP. EIP is a key parameter influencing transmission intensity as it combines with mosquito mortality rate and competence to determine the number of mosquitoes that ultimately become infectious. In spite of its epidemiological significance, data on EIP are scant. Current approaches to estimate EIP are largely based on temperature-dependent models developed from data collected on parasite development within a single mosquito species in the 1930s. These models assume that the only factor affecting EIP is mean environmental temperature. Here, we review evidence to suggest that in addition to mean temperature, EIP is likely influenced by genetic diversity of the vector, diversity of the parasite, and variation in a range of biotic and abiotic factors that affect mosquito condition. We further demonstrate that the classic approach of measuring EIP as the time at which mosquitoes first become infectious likely misrepresents EIP for a mosquito population. We argue for a better understanding of EIP to improve models of transmission, refine predictions of the possible impacts of climate change, and determine the potential evolutionary responses of malaria parasites to current and future mosquito control tools