The first solid genetic proof of an extended phenotype, a concept postulated by Richard Dawkins in 1982, has just been reported by Kelli Hoover and colleagues at Pennsylvania State University (PSU) in University Park, as well as from the U.S. Department of Agriculture Forest Service in Delaware, Ohio. These researchers have identified a viral gene that induces its caterpillar host to climb up to the tree tops, hang onto leaves or bark with their prolegs, die, liquefy, and rain millions of infective viral particles down where they can infect other larvae. In contrast, healthy caterpillars climb down the tree during the day and hide in bark crevices or soil to avoid being eaten by birds.
“Baculoviruses have been known to induce abnormal climbing behavior, referred to as “tree top disease” or “Wipfelkrankheit” in gypsy moths (Lymantria dispar) caterpillars for more than 100 years,” says Hoover, “but until now no one knew how that was achieved.”
“Discovery of the genetic mechanisms behind the process, linking the behavioral shift to a single locus in the viral genome is very neat,” says Garret Suen, from the University of Wisconsin in Madison, who did not participate in the study. “It’s a very important finding and contributes substantially to our understanding of how viruses can co-opt a host’s genetics to ensure their survival. What is astounding is that this behavioral change in the caterpillars can be induced by stopping the action of a single pheromonal cue,” he adds.
Viruses like chubby caterpillars
Knowing that fat caterpillars enable the virus to make more infective particles than skinny ones, and that gypsy moth caterpillars fast for 12 to 24 hours while preparing to molt, Hoover and her collaborators reasoned that keeping the larvae feeding would be in the best interest of baculoviruses. Thus, the scientists focused on a viral gene called ‘ecdysteroid uridine 5’ –diphosphate (UDP)-glucosyltransferase (egt) which encodes an enzyme that inactivates the molting hormone 20-hydroxyecdysone (20E). They tested this idea by inoculating age-matched L. disparwith six different versions of baculoviruses, and placing them, along with mock-inoculated larvae, in tall plastic bottles lined with climbing screens.
The natural (wild type) viruses caused the laboratory version of tree top disease –the larvae infected with intact baculoviral genes climbed to the top of the container and died. Deletion of the viral egt gene eliminated the abnormal climbing behavior and re-insertion of the gene restored it. “All the infected caterpillars had the same symptoms during the initial stage of the infection, but only those inoculated with egt-containing viruses climbed to die, proving that egt triggers the climbing behavior,” Hoover explains.
Why L. dispar larvae climb and stay aloft remains a mystery
By climbing to, and staying in, the tree top during the day, baculovirus-infected L. dispar caterpillars enter a world fraught with danger. Exactly why they climb, despite inactivated molting hormone, and remain in the open isn’t yet known, says Hoover. “One obvious possibility is that even without the molting cue, the caterpillars simply want to eat continuously and tree tops are usually where the leaves are so they stay there.” But while these mysteries haven’t as yet been solved, “the important thing is that we’ve found a gene that somehow induces caterpillars up to the exactly right location to enhance transmission of the virus to new hosts,” says Hoover.
Gypsy moths not the lone victims of microbial exploitation strategies
“Many parasites and pathogens manipulate host behavior to enhance their transmission to new victims,” says Hoover. Among the most famous is Ophiocordyceps unilateralis, a fungus that turns hapless carpenter ants called Camponotus leonardi into zombies, making them climb down from their nests high in the forest canopy, bite down on the underside of a leaf and die exactly where the parasite wants them to be. Then, the fungus grows a spore-dispersal stalk from the base of the dead ant’s head, continuously infecting ants that stray into the spore dispersal zone.
David P. Hughes, who first described zombie ants after spending countless uncomfortable hours searching for dead C. leonardi on rainforest floors, is now at PSU and participated in the gypsy moth research. The Hughes lab is now in active pursuit of the fungal gene or genes that manipulate ants into doing their bidding before turning them into spore factories, he says, “an excellent example of an adaptive extended phenotype described by Dawkins almost 30 years ago.”
In addition to investigating the genomics and transcriptomics of behavioral manipulation, the Hughes lab is examining plant responses to biting ants; how individual ant colonies, which scientists consider ‘superorganisms,’ respond to the spread of fungal disease, and how the enemies of O. unilateralis (and there are a lot of them) impact the disease dynamics between ants and fungi.
Sources: Kelli Hoover, PhD; David P. Hughes, PhD; and Garret Suen, PhD.
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Dawkins, R. 1982. The extended phenotype. Oxford University press, Oxford.
Hoover, K., M. Grove, M. Gardner, D.P. Hughes, J. McNeil and J. Slavicek. 2011. A gene for an extended phenotype. Science 333: 1401.
Hughes, D.P., Andersen, S. Hywel-Jones, N.L, Himaman, W., Bilen, J and J.J. Boomsma. Behavioral mechanisms and morphological symptoms of zombie ants dying from fungal infection. BMC Ecology 11:13doi:10.1186/1472-6785-11-1.
McNeil, J., J. Slavicek, S. Thiem, M. Gardner, D. Cox-Foster, and K. Hoover. 2010. Pathogenesis of Lymantria dispar multiple nucleopolyhedrovirus (LdMNPV) in L. dispar and mechanisms of developmental resistance. J Gen. Virol. 91: 1590-1600.