News: Virus benefits insect hosts
Posted by Victoria Stern
 
Bacteria-infecting viruses, generally thought to be harmful to their hosts, can also be surprisingly beneficial.

A pea aphid
Image: Wikipedia

A bacteriophage infecting a bacterium living in the cells and body cavity of a small, plant-eating insect protects the insect from attack by a deadly wasp predator, reports a study published online in Science today (August 20). The discovery may help create more effective methods of pest control, the authors say.

"This is a wonderful paper," Christoph Vorburger, an ecologist at the University of Zurich, told The Scientist. "It is important because it shows clearly that genetic elements such as bacteriophages are vehicles for transmitting resistance to predators," added Vorburger, who was not involved in the study.

Small insects called pea aphids are parasitized by a species of wasp, Aphidus ervi. The wasp injects its eggs into the aphids, and the eggs develop, pupating and eventually killing the insects. Earlier work has shown that the symbiotic relationship between aphids and a species of bacteria called Hamiltonella defensa can keep the insect safe, but no one understood exactly how the bacterium offered protection.

Using genetic sequencing tests, Kerry Oliver from the University of Georgia and his colleagues determined that Hamiltonella harbors a bacteriophage. To find out whether the phage is what safeguards the insect, they then generated three aphid--lines one without the bacterium, one with the bacterium infected by the phage, and one with the uninfected bacterium. The team found that only aphids with the phage were protected from the wasps' egg.

"Approaching 100% of aphids were able to survive parasitism with the phage present," said Nancy Moran, an author on the paper and a professor of evolutionary biology at the University of Arizona. "This study is the first time it has been shown that the phage is the source of the resistance," added Moran.

Conversely, the scientists saw that eliminating the phage from the aphid made the insect vulnerable to its natural enemy.

The team believes that the phage defends its bacterial host by making toxins. The toxins, which the bacterium then passes to the aphid, may cause the aphid to become resistant to the wasp or may help strengthen the aphid's immune response, making it impervious to its predator, explained Seth Bordenstein, an evolutionary geneticist at Vanderbilt University in Nashville, TN., who was not involved in the study.

"This study adds a new dimension to our understanding of insect ecology," said Angela Douglas, a professor of insect physiology and toxicology at Cornell University in Ithaca, NY, who was not involved in the research. Also, she added, it suggests "opportunities to enhance the efficacy of biological control strategies for pest insects."

Aphids are among the most destructive insect pests, ravaging crops around the world, and the parasitic wasp is an important agent used to control aphid pests. Field entomologists have observed that strains and populations of aphids seem to differ in their susceptibility to biological control efforts. "Assessing the presence of the phage in aphid populations could help better target control efforts," Moran noted.

Douglas warns, however, that such protective relationships are quite complex and need to be investigated further before attempting to manipulate them for agricultural purposes.