There are lots of studies showing how insect parasites affect the insect’s behavior in a way to facilitate the parasite’s transmission. Some fungi, for example, affect the behavior of ants, causing them to climb trees or blades of grass and then die, making them easy prey for the next host (ruminants or birds), who poop out the fungal spores which are then eaten again by ants. In some cases the fungi even turn the ant’s abdomen bright red, like a berry, making the dead, fungus-filled ant a tempting target for hungry birds.
The latest issue of Science contains an even more amazing and nefarious parasite—more amazing because it is a virus, and viruses have very few genes with which to manipulate their hosts. In this case, in a paper by Kelli Hoover et al., they found that a baculovirus called LdMNPV (large, rod shaped DNA viruses that are insect pathogens) manipulated its host, the gypsy moth (Lymantria dispar). Infected caterpillars of the moth climb to the top of host trees to die, where they then liquify and “release millions of infective virus particles, with dispersal facilitated by rainfall.” Here’s a photo from the paper of a liquifying, dead caterpillar on a tree:
For reasons that aren’t explained in the short paper, the authors hypothesized that one of the genes in the virus, ecdysteroid uridine 5′-diphosphate (UDP) glucosyltransferase (shortened to egt), caused the caterpillar’s behavior by inactivating its molting hormone, 20-hydroxyecdysone (20E).
They tested that idea by injecting caterpillars with genetically modified viruses that disrupted the egt gene, asb well as a virus in which this disruptive element, after being added, was removed, restoring the normal egt gene. Finally, they created controls by injecting caterpillars, but with no virus at all.
In all cases, disruption of the egt gene removed the climbing behavior (although the disrupted-gene caterpillars still died and liquified, but did so at the bottom of their containers). When the disrupted gene was restored to normal, the caterpillars climbed up their containers before death.
The exact mechanism of how this works isn’t known, but the authors suggest that the disruption of the 20E hormone in caterpillars enables them to remain viable longer, so that they can actually climb up and feed while infected. What is pretty clear, though, is that this is a genetic adaptation on the part of the virus that creates its “extended phenotype”— the behavior of the caterpillar that facilitates its spread.
It always amazes me that what we consider “simple” organisms nevertheless have the genetic repertoire to affect the behavior of their hosts. egt truly is a “selfish gene,” turning caterpillars into zombies to facilitate its own transmission. (Malarial parasites in humans are sometimes thought to do the same thing, making us sick enough to lie prostrate, a tempting—and non-swatting—target for the mosquito whose bites carry the protozoan.)
Without a doubt, there are many yet-unknown cases of behavioral modification by parasites that are equally intriguing, and equally nefarious. Faye Flam at the Philadelphia Inquirer, who has also described this paper, recounts some other chilling stories of parsite manipulation of behavior.
Hoover, K., M. Grove, M. Gardner, D. P. Hughes, J. McNeil, and J. Slavicek. 2011. A gene for an extended phenotype. Science 333:1401.