Date of Award

Spring 2020


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Document Type


Degree Name

Master of Science (MS) in Biology




The outcomes of novel host-pathogen interactions are unpredictable but can result in epidemics or pandemics. Exploring the initial encounter between a pathogen and a novel host species can elucidate why some pathogens successfully infect and adapt on a novel host when others fail. Much of our understanding of host virulence after host shifts was developed using serial passage experiments (SPEs) in bacteria. Three accepted SPE generalities have emerged: virulence increases on the novel host, the evolved pathogen will become less able to infect its native host, and the pathogen experiences convergent evolution. This study tests the first two generalities using complex hosts (Drosophila sp) and the highly virulent Drosophila C virus (DCV). The fitness of DCV was utilized as a proxy for virulence (pathogen’s harm to host) and investigated over 10 serial passages of the pathogen. The number of eggs, pupae, and adults were measured along with the days to pupation and to adulthood. We observed significant decreases in fecundity for both hosts but no significant effects on developmental metrics. The novel host experienced a significantly larger decrease in survivability than the native host. This decrease in fecundity and increases in mortality indicate an increase in DCV virulence on the novel host. Viral load does not explain the increase in virulence. The virus evolved on the native host was equally virulent to the native host as the ancestral virus, in contrast to SPE predictions. This study supports the SPE generality of increased virulence in the novel host even with a physiologically complex host and a highly virulent pathogen.