Establishment of novel wolbachia symbiosis and characterization of sex determination genes in Dengue mosquito vectors / Xiao Liang.

As one of the deadliest animals on earth, mosquitos transmit numerous diseases to humans, including dengue, Zika and malaria, which account for over one million human deaths every year. Due to insufficiency of traditional vector control strategies, significant efforts have recently been made to develop novel genetic approaches to either directly suppress mosquito populations or reduce mosquito's ability to transmit pathogens to humans. One of them is based on the maternally transmitted intracellular symbiotic bacterium Wolbachia. Estimated to infect more than 60% of arthropods in nature, Wolbachia can spread through host populations by means of a reproduction-interfering referred to as cytoplasmic incompatibility (CI). By altering the host's physiological environment, including immune priming or metabolic perturbation, Wolbachia can also confer antiviral resistance in mosquito vectors. Successful field trials have been conducted to release Wolbachia-infected mosquito males to induce incompatible matings for population suppression or spread Wolbachia into mosquito populations to reduce or block dengue transmission by population replacement. Both population suppression and replacement require for establishment of an artificial Wolbachia symbiosis in mosquito to make it incompatible with target populations. In order to develop a Wolbachia-based strategy for dengue/Zika control in Singapore and Mexico, I have established the transinfected line WB2. By comparing with another transinfected line WB1 which developed 15 years ago, I have demonstrated that wAlbB maintains a stable symbiosis with Ae. aegypti. Further assays show that Wolbachia induces strong resistance to dengue, Zika and Chikungunya viruses in WB2. WB2 line has now been released for field trials in both Mexico and Singapore. In order to improve Wolbachia-based mosquito control, transinfected mosquitoes must be optimized to display maximum pathogen blocking, the desired CI pattern, and the lowest possible fitness cost. Achieving such optimization, however, requires a better understanding of the interactions between the host and various Wolbabachia strains. Thus, we transferred the Wolbachia wMel strain into Ae. albopictus, resulting in a transinfected line, HM (wAlbAwAlbBwMel), no CI was induced when the triply infected males were crossed with the wild-type GUA females or with another triply infected HC females carrying wPip, wAlbA, and wAlbB, but removal of wAlbA from the HM line resulted in the expression of CI after crosses with lines infected by either one, two, or three strains of Wolbachia. These results show that introducing a novel strain of Wolbachia into a Wolbachia-infected host may result in complicated interactions between Wolbachia and the host and between the various Wolbachia strains, with competition likely to occur between strains in the same supergroup. In order to manage the potential risk of failure in population suppression in Singapore, I developed another Ae. aegypti carrying wMal. The transinfected line showed 100% maternal transmission. To facilitate developing a perfect sex separation approach for Wolbachia-based population suppression, I established the CRISPR/Cas9 approach to characterize the function of sex determination pathway genes in Ae. aegypti. By individually knocking out doublesex (dxl) and transformer-2 (tra-2), two essential genes in mosquito sex determination pathway, we show that dxl is not essential gene for female development while knockout of tra-2 results in male-biased sex ratio and absence of female mosquito with homozygous tra-2. These results indicate that the tra-2 is a potential sex determination target that can be explored to develop the female-specific lethality for mosquito sex separation.

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