Disentangling the Relationship Between Deformed wing virus, the Honey Bee Host (Apis mellifera) and the Viral Vector, the Ectoparasitic Mite Varroa destructor

Abstract

The ectoparasitic mite Varroa destructor is indisputably the most significant driver of global colony losses of the Western honeybee, Apis mellifera. Colony deaths are frequently attributed to Deformed wing virus (DWV), which is vectored by the mite. In this thesis I attempt to disentangle the tripartite relationship between DWV, A. mellifera and V. destructor, by investigating whether the two major DWV genotypes, A and B, differ from the point of view of the virus, the honey bee and the mite. First, I assessed the viral accumulation dynamics of multiple DWV genotypes during single or co-infection in Australian pupae (naïve to both DWV and Varroa). I found that DWV-B accumulated to higher levels than DWV-A when singly and co-injected, suggesting that DWV-B is able to outcompete DWV-A. Yet despite higher viral loads, DWV-B was associated with the lowest level of mortality. Therefore, I next investigated if the bees’ immune system reacted differently to the two DWV genotypes. I examined the expression of 19 immune genes and analysed the small RNA response of pupae exposed to DWV-A and DWV-B. Overall, I found little evidence to indicate that A. mellifera responds differently to either genotype. Finally, to uncover what role vector transmission by V. destructor plays in DWV genotype prevalence at the colony level, I experimentally increased and decreased the number of mites within A. mellifera colonies and analysed viral loads over a period of ten months. I found that DWV-A was strongly affected by mite numbers, whereas DWV-B persisted in the presence and absence of V. destructor. Overall, my thesis furthers our understanding of the intricate relationship between DWV, A. mellifera and V. destructor, and provides insight into some of the factors that may be contributing to the increasing prevalence of DWV-B.