It's Raining Caterpillars!

An exciting 1-page paper was published in this week's issue of Science. Hoover et al. discovered a gene whose effect is an extended phenotype, a phenomenon Richard Dawkins introduced his book named, appropriately, The Extended Phenotype. We usually think of phenotypes as confined to the organism containing the gene. The typical high school biology examples of genetically encoded phenotypes are things like ear lobe attachment/detachment, eye color, and hitchhiker's thumb. However, there is nothing stopping genes from having effects on other organisms or other parts of the external environment. One (of many) examples Dawkins uses is the beaver. If its dam building behaviors are products of evolution (as they certainly are), then we must concede that it has genes with extended phenotypes.

What Hoover et al. describe is a trait found in some pathogens. That is to modify the behavior of the host to improve your (the pathogen's) reproductive success. Dawkins devotes a whole chapter to the topic in his book. My favorite example is the fluke Dicrocoelium dendriticum, which must end up in a grazing animal like a sheep via an ant intermediate. The fluke accomplishes this by invading the ant's brain and inducing it to climb to the tips of blades of grass. There it is more likely to be eaten by a grazing sheep and continue its parasitic lifecycle. Hoover et al. report on a different parasite-host system, involving the caterpillars of the European gypsy moth—Lymantria dispar—and a virus (called Lymantria dispar nucleopolyhedrovirus, or LdMNPV).

The caterpillars that are unfortunate enough to be infected with this virus have an odd way of dying. Instead of simply falling to the ground they make a long and expensive climb to the top of the tree in which they feed. Following death, their corpses liquefy and release millions of viruses, each posed to infect another unlucky caterpillar (note the distinction between healthy and dead and dripping caterpillar in the photos). The virus spreads through the tree habitat by hitching a ride on rain as it falls, and since the caterpillar died at the top of the tree, the virus can spread all the way down the length of the tree.

This is a pretty cool trick, but is it just coincidence? It seems hard to believe that a simple virus could influence caterpillar behavior in such a dramatic way. Amazingly, the authors of the paper were able to identify the viral gene responsible for making the caterpillars climb to their death. Called egt, the gene codes for an enzyme that inactivates a molting hormone. The investigators showed that, in the lab, caterpillars infected by viruses containing egt died at higher elevations within the test environments than those infected by viruses lacking egt. Thus, egt is a gene with an extended phenotype. It resides within a virus, but its effect can only be described as a host phenotype.

This post doesn't have much to do with synthetic biology, but I think it's important to take a step back to look at our broader subject. Biology is full of curiosities like the European gypsy moth caterpillar and its behavior-modifying virus. Maybe this system will lead to a useful application down the road, or perhaps not. Just take a minute to appreciate the wonder and exhilaration nature has provided us.

Image Credit: Healthy caterpillar: IronChris (via Wikimedia); dead caterpillar: Michael Grove

References:

Dawkins, R. The Extended Phenotype. New York: Oxford University Press, 1999.

Hoover, K. et al. A Gene for an Extended Phenotype. Science 333, 1401 (2011).