Differential responses to DWV infection in honey bees: A case of tolerance or resistance?

Authors: Simone-Finstrom, Michael

Submitted to: Bee World
Publication Type: Proceedings
Publication Acceptance Date: 4/1/2018
Publication Date: 4/17/2018
Citation: Simone-Finstrom, M. 2018. Differential responses to DWV infection in honey bees: A case of tolerance or resistance?. Bee World. 95(2):47-72. 10.1080/0005772X.2018.1450208.
DOI: https://doi.org/10.1080/0005772X.2018.1450208

Interpretive Summary: The Varroa mite feeds on honey bee pupae, often injecting the developing bee with viruses. The injection of viruses by mites, like deformed wing virus (DWV), often results in more severe viral infections. The goal of this work is to determine if honey bees are resistant or tolerant to DWV in hopes that this can be a trait that can be selected to create a line of virus resistant or tolerant honey bees. In this regard, resistance would be indicated by bees that maintain a low level of virus despite infection whereas tolerance would be shown by bees that have high level of virus but the viral infection does not cause symptoms or increase mortality. Pupae from colonies were injected with a low, moderate or high dose of DWV. Pupae showed differential survival and development of the typical DWV symptoms (wrinkled or poorly developed wings, shortened abdomen) based on colony origin. A subset of pupae were selected to examine differences in viral titer and antiviral responses. Overall, 3 days post-injection, pupae exhibited similar viral titers based on injection dose regardless of colony of origin (e.g. low survival or high survival colonies). The relationship of antiviral responses to DWV level was different from pupae that derived from the low survival colonies as compared to those from the high survival colonies. It appears that honey bee colonies have differential tolerance to DWV infection and that this is at least in part due to increased antiviral responses. Future work will address the heritability of DWV tolerance in honey bees with the aim of initiating a breeding program.

Technical Abstract: Honey bees contend with a variety of abiotic and biotic stressors, and this has led to high and likely unsustainable annual colony mortality. The ectoparasitic mite Varroa destructor is the biggest threat affecting honey bee health in large part because of the viruses that mites vector while feeding during reproduction and development on honey bee pupae. Deformed wing virus (DWV), in particular, has been noted to be associated with colony losses. Because of the significance of Varroa-DWV dynamics, there has been much interest in the relationship between colony mite infestation and viral prevalence. In a few cases, it has been noted that colonies that have natural resistance mechanisms against Varroa have lower incidences of DWV infection. However in other populations mite-resistance seems to be correlated with tolerance to DWV, meaning that mite-resistant colonies survive with high levels of DWV and exhibit fewer symptoms. To clarify whether resistance (maintain low viral titers despite infection) or tolerance (high survival, no symptoms with high virus) to DWV appears to be driving differential effects across honey bee colonies in the absence of mites, pupae from single-drone inseminated queens were injected with a low, moderate or high dose of DWV solution containing 104, 107, 1010 viral copies respectively. Pupae showed differential survival and development of the typical DWV symptoms (wrinkled or poorly developed wings, shortened abdomen; see Figure) based on colony origin. A subset of pupae were selected from 5 colonies that displayed low survival and high symptom development at all doses and 5 colonies that displayed high survival and low symptom development at all doses to examine differences in viral titer and antiviral responses via real-time PCR. Overall, at 3 days post-injection, pupae exhibited similar viral titers based on injection dose regardless of colony of origin (e.g. low survival or high survival colonies). The relationship of antiviral responses to DWV level was different from pupae that derived from the low survival colonies as compared to those from the high survival colonies. It appears that honey bee colonies have differential tolerance to DWV infection and that this is at least in part mediated by antiviral responses. However these immune responses do not appear to be sufficient to reduce viral replication, at least at 3 days post-injection. Future work will address the heritability of DWV tolerance in honey bees.