For a century, scientists have watched European gypsy moth caterpillars infected with a virus use their last strength to do something that a healthy gypsy moth caterpillar would never do in daylight hours -- climb high into a tree and onto a leaf. This behavior in infected caterpillars was so consistent that it inspired the term "Wipfelkrankheit," or "tree top disease," to describe the virus that caused it. For scientists, the question has been how does a virus change its host's behavior?
A team of researchers that included a U.S. Forest Service scientist has answered that question. An article in Science published on Sept. 9 identifies a specific viral gene that drives infected caterpillars to die in a way that offers the best potential for spreading the virus that killed it.
The research team was led by Dr. Kelli Hoover of Pennsylvania State University and included Dr. Jim Slavicek, a research biologist with the U.S. Forest Service, Northern Research Station. Team members included other Pennsylvania State University researchers and scientists with the Harvard Medical School.
The discovery provides evidence of a genetic basis for a parasite's ability to have an effect on the host behavior, which scientists call the extended phenotype. "The identification of a specific viral gene that causes the extended phenotype allows other researchers working in this area to narrow their search for the genes that may be responsible in their system," according to Slavicek.
Baculoviruses are viruses that are used to infect and kill the caterpillars of insect pests of trees and crops. One of these viruses kills larvae of the gypsy moth, an urban and forest tree pest in northeastern states that defoliates trees. This virus is specific for the gypsy moth, and consequently will not impact any other insect, animal, or plant in the treatment area. The Northern Research Station and the USDA Animal and Plant Health Inspection Service (APHIS) are partners in producing the virus for states, the National Park Service and other agencies to use to control gypsy moths.
A healthy gypsy moth caterpillar feeds at night and either hides in a tree's bark crevices during the day or climbs down the tree to the soil to avoid predators. For the virus behind tree top disease, there is a significant advantage to a caterpillar dying in the middle of a leaf within the canopy of the tree rather than in a crevice. The dead caterpillar liquefies, releasing millions of virus particles into the environment where they can spread throughout the tree and contaminate other gypsy moth larvae.
Other instances have been identified where parasites and pathogens manipulate host behavior to enhance transmission to new victims. For example, Ophiocordyceps fungus-infected arboreal ants are manipulated into "zombies," inducing them to position themselves in the tree canopy where the microenvironment is ideal for the release of fungal spores and dispersal to new hosts. The doomed "zombie" ants hold themselves in the canopy by clamping down their mighty jaws on a leaf vein.
"Who knew that a virus could change the behavior of its host?" Slavicek said. "Maybe this is why we go to work when we have a cold."
- Kelli Hoover, Michael Grove, Matthew Gardner, David P. Hughes, James McNeil, and James Slavicek. A Gene for an Extended Phenotype. Science, 9 September 2011 DOI: 10.1126/science.1209199
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