The biology and pathogenesis of Deformed Wing Virus, the major virus pathogen of honeybees

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Honeybees are the most important managed pollinating insects globally. Both honey production and pollination depend upon strong, healthy colonies, a situation that has been threatened over the last century by the global spread of the ectoparasitic mite Varroa destructor. All colonies in the UK (excepting parts of Scotland and some islands) have the mite. Varroa acts as a vector for a range of viruses of honeybees which are transferred when the mite feeds on haemolymph (blood) from the developing pupa. The most important of these viruses is Deformed Wing Virus (DWV). In studies over the last 4 years we have demonstrated that mite infestation preferentially leads to the amplification of a specific virulent form of DWV. In mite-exposed developing pupae this particular virus reaches levels almost 10,000 times higher than seen in the absence of the mite. This is despite the development of an immune response to the infecting virus.We want to understand why the virulent form of DWV observed in mite-infested colonies or mite-exposed pupae replicates to such elevated levels. Is it due to the route by which the virus is acquired - directly into the haemolymph - rather than the usual route which is orally during feeding? Does this route allow the virus to infect tissues or organs it normally does not have access to? Does it allow the virus to bypass the immune response of the honeybee? Does DWV, like many others viruses, carry genes that allow it to suppress the host immune response? Can we 'vaccinate' honeybees before they are exposed to prevent subsequent replication of the virulent virus? Finally we want to understand which host genes, expression of which are suppressed upon DWV infection, contribute to the development of overt disease.Varroa-free honeybee colonies (from Andrew Abrahams, Colonsay) maintained in isolated flight cages or Varroa-infested colonies from our research apiary will be used as a source of individual honeybee larvae which will be harvested and maintained in the laboratory under carefully controlled conditions. We are perhaps the only lab in the UK with expertise in this methodology.Individual larvae will be either fed or injected with a characterised virus population. The tissues and organs in which the virus replicates will be determined using exquisitely sensitive hybridisation techniques on either dissected pupae or sections. We are particularly keen to determine whether virulent forms of the virus cause systemic infections, or whether they preferentially replicate in particular tissues or organs, for example the abdomen and wing buds, both of which exhibit developmental deformities during overt DWV infection.We will repeat these studies in larvae in which we have deliberately suppressed key components of the immune response by inhibiting expression of the genes Dicer and Argonaute. Are these larvae now susceptible to all forms of DWV, not just the virulent variant? We will additionally pre-expose larvae (via feeding; a technique we have acquired from collaborators in Louisiana, USA) to short RNA molecules designed to inhibit DWV replication. Are these larvae protected from infection and disease caused by virulent DWV? We will additionally test whether DWV carries a gene that inhibits the effectiveness of RNA-based immune responses using well-established techniques.The third component of our study is to investigate the role of specific host genes implicated in components of the immune response or development to enhanced susceptibility to DWV-mediated disease. These genes were identified in our recent studies but their importance has yet to be determined. We will suppress individual genes of interest and then challenge larvae with known virus populations.These studies will not only determine why a particular variant of DWV is associated with overt disease in honeybees but will also demonstrate whether it is possible to develop therapies to protect developing honeybees from infection.