Genetics - Theses
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ItemEvolution of Wolbachia-Drosophila interactions and implications for Wolbachia-based biocontrol( 2010)Wolbachia is a maternally inherited α-proteobacteria that lives an obligate intracellular lifestyle in its eukaryotic hosts. Wolbachia pipientis was first identified in the 1920’s in the mosquito Culex pipiens; Wolbachia has since been detected in a broad range of hosts. Wolbachia induces a number of diverse reproductive manipulations in many of its hosts, implicating Wolbachia in control strategies for target species. However, changes in host bacterial interactions can evolve toward mutualism, and this requires investigation in target systems. This first part of this work examines changes in the Drosophila simulans-Wolbachia Riverside system in California. This system, studied for a period of more than two decades, provides a rare example in nature of a symbiotic relationship shifting from parasitism to mutualism. In my study, no changes in overall estimates of infection frequency, maternal transmission or cytoplasmic incompatibility were found, however there appears to be heterogeneity in maternal transmission rates and the ability of males to induce cytoplasmic incompatibility (CI). The source of CI variation in one isofemale line was subsequently investigated through reciprocal backcrossing with an old Wolbachia-infected strain, Riv88, and the variation was attributed to the Wolbachia genome. Further investigations are required to determine the extent of CI variation in the wild. These findings significant implications for the many applied projects that intend to exploit Wolbachia for its ability to manipulate host reproductive systems. The second part of this work investigates the extent to which evolution of a host-bacterial symbiosis can influence the effectiveness of control strategies using Wolbachia. A particular strain of Wolbachia, named popcorn (wMelPop), has been isolated for its ability to reduce host longevity. This bacterium has the capacity to alter the age structure of the target host populations. As many vector-borne diseases (eg: Dengue Fever, Malaria) require an incubation period within the vector prior to transmission to the final host, a reduction in the lifespan of vector populations has the potential to lower transmission levels. The long term stability of this bacterial strain has been investigated in its native host (Drosophila melanogaster), and a novel host (D. simulans). The longevity-shortening phenotype and several life-history traits that might influence the effectiveness of control strategies have been examined. Host genetic background and the indirect selection on the longevity phenotype (via early or late fecundity selection) were found to influence multiple life-history traits in these species. The source of this variation in D. melanogaster is attributed primarily to the host, but can also be influenced by Wolbachia. Strong host background effects were noted in D. simulans, which highlights the importance of testing multiple backgrounds for suitability of use in applied projects. This work has allowed for greater understanding of the potential for evolution within host-symbiont systems, and can be used to provide a framework for undertaking projects that will use Wolbachia as biological control agents. Future projects should consider the potential for evolution in detail under laboratory conditions before control strategies are implemented in the wild.