Honeybee colonies have been devastated over the last decade. And researchers think it may have to do with the mutually beneficial relationship enjoyed by a parasitic mite called Varroa destructor and the virus it transmits. The work is described in Proceedings of the National Academy of Sciences this week.
The Varroa destructor (pictured below) carries the deformed wing virus (DWV), and this parasite and its pathogenic passenger have already been implicated in the staggering losses of honeybee colonies. Previous studies have found that mites and their offspring feed on the blood-like vital fluid (or haemolymph) of immature honeybees called pupae. This helps spread the virus that suppresses the immune system of honeybees by tampering with the signaling of a protein called NF-κB, and stress makes matters worse. However, the mechanics underlying this mite-virus association and the evolutionary implications remain largely obscure.
To see if a mutualism exists between the mite vector and the virus, a team led by Francesco Nazzi of the University of Udine and Francesco Pennacchio of the University of Naples tested whether or not virus-induced immune suppression facilitates mite feeding.
Insects respond to parasite intruders by encasing them in a capsule coated in the pigment melanin to prevent them from continuing to feed. So the team implanted a 0.08-millimeter piece of nylon thread in the bodies of virus-infected Apis mellifera larvae (the developmental stage before pupae). After a day, they removed the implants and measured the degree of the thread's melanization and encapsulation using a light microscope.
The team found that melanization, encapsulation, and the levels of gene expression of related genes were negatively associated with virus levels in the larvae: Bees with higher virus loads suffered reduced immunity. Meanwhile, virus levels were positively correlated with mite reproduction on honeybee pupae. This jump in the numbers of viral genome copies and mite offspring suggests that a viral infection enhances the reproductive success of the parasitic mites.
The mutualistic symbiosis between Varroa destructor and DWV perpetuates a reciprocal loop – one that has an adverse impact on honeybee health, the authors write. And this mite-virus link may be the key to understanding at least one of the factors contributing to the collapse of honeybee colonies.
Image in the text: Manfred Ruckszio/Shutterstock
