Viruses and antiviral responses of an invasive fruit pest, Drosophila suzukii

Drosophila suzukii (Matsamura) is an invasive dipteran pest of soft fruit crops. Native to Japan and SE Asia it was first detected in the Mediterranean growing regions of Europe and the western states of the USA in 2008. Since then it has been expanding its range across both continents causing huge economic damage to the horticultural industries there. Current control measures are heavily dependent on broad spectrum insecticides and labour intensive cultural control. Therefore, there is a large incentive to investigate alternative, more environmentally benign, control methods such as biological control or biopesticides. The viruses of D. suzukii offer a potential source of pathogens suitable for the development of such a biopesticide. Chapter 2 explores the diversity of viruses found naturally associating with D. suzukii in both its native and naturalised ranges. In it, I describe 18 new RNA viruses belonging to a variety of virus clades. Although none of these viruses belong to those clades traditionally used as biological control agents, we suggest further work for the development of a viral control agent based on our data. Not only are the viruses of D. suzukii of direct applied interest to the horticultural industry, they also offer a powerful model system for the study of virus host dynamics in the wild. The ecosystems recently invaded by this pest contain many other species of Drosophila which harbour their own raft of viral pathogens. In chapter 3 I explore the extent to which these viruses are shared between species and how virus prevalence changes over time. Understanding the patterns of virus 'host-shifts' after host range change could help us better predict the success of particular biological invasion events and further informs our understanding of emerging viral diseases in both humans and livestock. The ability of a virus to shift host ultimately comes down to its ability to overcome its host's immune system. In chapter 4 I investigate the comparative genome-wide transcriptomal immune responses of D. suzukii and its congener D. melanogaster after treatment with two highly divergent viruses. The relative responses of these flies was shown to be highly dissimilar as was the response of males and females of the same species. Few model species allow comparative expression studies of this depth granting us unprecedented insights into the evolution of insect innate immune systems.