The genetic basis of Wolbachia-induced male killing in Drosophila

Abstract

Microbe-animal symbioses are ubiquitous in nature. Microbes of particular host tissues often have unique functions or effects on the host. Within arthropods, microorganisms of the reproductive tissues may have profound effects on host physiology, health, fitness, and evolution. Among the most extraordinary microbes in this context are the reproductive manipulators that hijack host reproductive processes to facilitate their own spread. Here, I present my dissertation research on the genetics and mechanism of Wolbachia-induced reproductive parasitism in Drosophila. Wolbachia is the most widespread endosymbiont in nature and many strains specialize in manipulating host reproduction in various ways. Despite decades of research on Wolbachia, the genetic basis of parasitism remained a mystery. However, due to its profound effects on the evolution of arthropods, its ability to block transmission of many viruses that infect humans, and its driving mechanisms, it is critical to identify the genes and mechanisms underlying its interactions with its hosts. Here, I describe the research leading to the discoveries of three genes underlying reproductive parasitism, with a focus on male killing. I begin with the research that was first conducted to identify candidate genes for reproductive parasitism in the wMel strain of Wolbachia in Drosophila melanogaster. This work identified two genes, cifA and cifB (cytoplasmic incompatibility or CI factors A and B) that together induce the CI phenotype. This work also describes the establishment of the techniques and strains used to transgenically test parasitism phenotypes. Building on this foundation, I then describe the discovery and assessment of wmk (WO-mediated killing) as another wMel gene responsible for the male killing parasitism phenotype. I show that when transgenically expressed, wmk recapitulates many aspects of natural male-killing infection. In addition, I show experiments demonstrating that out of all potential candidates for male killing based on multiple lines of evidence, only wmk is able to recapitulate male killing, and all other gene candidates fail to do so. I then demonstrate that transgenic expression of wmk homologs in strains non-native to D. melanogaster results in complex phenotypes. Through this work, I conclude that cifA and cifB induce the CI phenotype and that wmk is the current best candidate for male killing by Wolbachia, thus solving a century-long genetic murder mystery.