Abstract The age of an adult female mosquito is a critical determinant of its ability to transmit a range of human pathogens such as the dengue viruses. Since most pathogens require a relatively long period of development in their mosquito vector before they can be transmitted to a new host, only older insects are of epidemiological importance. It has been proposed that life-shortening strains of the inherited bacterial symbiont Wolbachia may be potentially useful to shift mosquito population age structure towards younger individuals, thereby reducing pathogen transmission without eradicating the mosquito population. However, life-shortening Wolbachia strains do not occur in mosquitoes naturally. To facilitate the evaluation of this potential strategy, the major aim of this project was to transfer a life-shortening Wolbachia strain, wMelPop, from the fruit fly, Drosophila melanogaster, to the dengue vector, Aedes aegypti. After successful transinfection, I then characterized the wMelPop infection in this mosquito species, and the associated phenotypic effects of this Wolbachia strain on several aspects of Ae. aegypti life-history. The horizontal transfer of the obligate intracellular bacterium Wolbachia pipientis between invertebrate hosts hinges on the ability of Wolbachia to adapt to new intracellular environments. Previous attempts to directly transfer wMelPop from D. melanogaster to Ae. aegypti have been unsuccessful, presumably due to mal-adaptation of wMelPop to this novel mosquito host. In an attempt to adapt wMelPop to the mosquito intracellular environment to facilitate transfer, we serially passaged this infection for 3 years in Aedes cell culture. After long-term serial passage in mosquito cell lines, wMelPop was then initially reintroduced into its native host, D. melanogaster, by embryonic microinjection to examine the phenotypic outcome of this process. The cell line-adapted wMelPop strains were characterized by a loss of infectivity when reintroduced into Drosophila, grew to decreased densities, and had reduced abilities to cause life-shortening infection and cytoplasmic incompatibility (CI), when compared to the original wMelPop strain. These results were suggestive of adaptation of this Wolbachia strain to the mosquito intracellular environment. Subsequently, we successfully established wMelPop in Ae. aegypti using embryonic microinjection. Two independently transinfected Ae. aegypti lines, PGYP1 and PGYP2, were generated. Analogous to infection in Drosophila, the presence of wMelPop in both of these lines halved adult mosquito lifespan under a range of laboratory conditions. The new association is stable, and wMelPop is maternally inherited with high fidelity across Ae. aegypti life span. It is capable of inducing near perfect CI that does not diminish as male Ae. aegypti age, which should facilitate its invasion into natural field populations and persistence over time. Fecundity assays using young PGYP1 females revealed that wMelPop infection induced minimal costs for reproductive fitness for females during their first gonotrophic cycle. The phenotypic effects of wMelPop on several aspects of Ae. aegypti life-history were additionally characterized. Overall, minor costs of wMelPop infection for pre-imaginal survivorship, development and adult size were observed. However, wMelPop infection decreased the viability of quiescent Ae. aegypti eggs over time. Similarly, the reproductive fitness of wMelPop-infected Ae. aegypti females declined with age. These results revealed a general pattern associated with wMelPop pathogenesis in Ae. aegypti, where host fitness costs incurred by this infection increase during aging of both immature and adult life-history stages. We also discovered that wMelPop-infected Ae. aegypti have a reduced ability to utilise blood from non-human hosts for egg development. Blood feeding by wMelPop-infected females on mouse, guinea pig or chicken hosts resulted in a near complete abolishment of reproductive output associated with both a decline in the numbers of eggs oviposited as well as the hatching rate of successfully laid eggs. In contrast, the reproductive output of wMelPop-infected females fed human blood was normal, with intermediate effects observed with other vertebrate blood sources. Removal of Wolbachia from mosquitoes using antibiotic treatment restored egg development to standard levels on all blood sources. Further blood feeding assays over two reproductive cycles definitively illustrated a nutritional interaction between host blood source and egg development in wMelPop-infected Ae. aegypti. This dramatic phenotype may provide new insights into the nutritional basis of mosquito anthropophily. The results presented in this thesis highlight several important parameters required to theoretically model the infection dynamics of wMelPop, and its potential impacts on Ae. aegypti populations. Moreover, the successful establishment of wMelPop in Ae. aegypti forms the primary basis for further field-based evaluations, which will ultimately determine the viability of this Wolbachia-based strategy as an applied tool to reduce dengue transmission.
| Identifer | oai:union.ndltd.org:ADTP/254072 |
| Creators | Conor James McMeniman |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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