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American Association for the Advancement of Science

Virus gene leads moths to tree top doom

Healthy gypsy moth caterpillar on a leaf. [Image courtesy of Michael Grove]

A gene found in a virus turns gypsy moth caterpillars into tree-climbing zombies, reports a new study in the journal Science on September 9 2011. These moth caterpillars infected by a virus known as a baculovirus, are hypnotized into climbing to the top of trees to die, liquefy and rain viral particles on the foliage below to infect new hosts.

In contrast, healthy caterpillars hide in bark crevices or soil during the day to avoid being eaten by birds. These larvae climb back onto leaves at night to feed. Many documented cases of pathogens controlling the behavior of their hosts to spread to new hosts can be found (one of the most recent being the discovery of parasitic fungi that turn ants into leaf-gnawing zombies), but the genetic basis for these behavioral changes remains a mystery.

Now Kelli Hoover at Penn State University and colleagues unravel some of the mystery by identifying the gene -- called egt -- that allows the baculovirus to control infected caterpillars' climbing behavior. In the study, the researchers tested six viruses for their effect on insect climbing behavior.

The researchers placed individual moth larvae in tall plastic bottles lined with a fiberglass screen and containing food at the bottom. Viruses harboring the egt gene coaxed larvae to climb to the top of the container and remain there to die (instead of staying at the bottom), while unaffected caterpillars showed no particular pattern in their position in the container.

Deleting egt eliminated this behavior, while reinsertion of the gene restored the bizarre climbing behavior in the insects, the team found. The results offer a genetic explanation for why some viruses are able to control the behavior of their hosts in a way that benefits the virus.