Factors influencing the intriguing persistencce of a Wolbachia symbiont in spotted wing Drosophila




McPherson, Audrey E.

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Wolbachia is a maternally inherited, endosymbiotic bacterium that infects at least 40% of terrestrial arthropods. As a facultative symbiont in the majority of its hosts, Wolbachia commonly act as a reproductive parasite; however, there are a number of Wolbachia strains that do not cause reproductive manipulations in their hosts and have no apparent fitness enhancement, yet are stably maintained in populations at low to intermediate frequencies. How these strains of Wolbachia persist in nature has been a long-standing question and is still unresolved. One explanation for the persistence of such strains is that they provide a context-dependent fitness advantage to their hosts. In this thesis, I investigate one such strain of Wolbachia, wSuz, which infects the agricultural pest, Drosophila suzukii, also known as spotted wing Drosophila. To explore the possibility that wSuz may be involved in pathogen protection, I screened wild flies for Wolbachia and two naturally occurring RNA viruses, Teise Virus and a recently discovered virus related to Motts Mill Virus. I did not find an association between Wolbachia and virus infection. Additionally, I designed an experiment to test whether Wolbachia increases host fitness at high larval densities. Intriguingly, although there was no effect of density, the frequency of Wolbachia infection changed dramatically in just one generation, but in opposite directions in replicate experiments that were performed a month apart. These results support the hypothesis that Wolbachia frequencies can change quickly across generations and provide some type of condition-dependent benefit. The maintenance of Wolbachia remains a mystery, but my study provides some exciting clues about what conditions may be playing a role.



endosymbiont, Wolbachia, viral protection, context-dependent fitness advantage, larval density, Drosophila suzukii, rapid frequency change