A virus gene creates zombie caterpillars

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.

19 Comments

  1. Posted September 13, 2011 at 5:48 am | Permalink

    It ain’t just malaria.

    The cold virus causes and is spread by sneezing.

    Herpes causes and is spread by open sores on the genitals.

    You get the idea….

    Cheers,

    b&

    • ChasCPeterson
      Posted September 13, 2011 at 6:30 am | Permalink

      the idea is manipuilation of host behavior.
      Sneezing, sort of. Herpes, nah.

      • Posted September 13, 2011 at 6:53 am | Permalink

        I wonder whether veneral diseases make people hornier.

        • Posted September 13, 2011 at 6:54 am | Permalink

          Awk — ‘venereal’

          • Stavana
            Posted September 13, 2011 at 10:23 am | Permalink

            I heard from a former professor that 3rd stage syphilis patients have increased libido but haven’t found any pubs to validate the statement.

    • Marlene Zuk
      Posted September 14, 2011 at 2:53 pm | Permalink

      There is, however, evidence that syphilis became less virulent over time, possibly because making its hosts less attractive with oozing pustules, etc., reduced the transmission of the pathogen. I actually wrote about this for the NYT a few years back . . .

  2. Posted September 13, 2011 at 5:52 am | Permalink

    And what do directed evolutionists aver? That He had that intent for the parasites to evolve to trouble their hosts?
    This is why the teleonomic argument figures high amongst arguments against supernaturalism! And Carneades’s argument notes that supernaturalists beg the question of directed outcomes-vitalism-teleology- in all teleological arguments, which contain other fallacies.
    “Logic is the bane of theists.’ Fr.Griggs

  3. Ralph
    Posted September 13, 2011 at 6:09 am | Permalink

    It still boggles my mind how an evolutionary mechanism for these strategies could come by.

  4. Dominic
    Posted September 13, 2011 at 6:24 am | Permalink

    What is interesting to me is that these catapillars have urticating hair and that is a feature that makes them less attractive to birds, so the fungus has ‘chosen’ a host that will have less chance of being eaten before it has dispersed the fungal spores.

    Fascinating!

  5. Steve Smith
    Posted September 13, 2011 at 6:42 am | Permalink

    This sounds like Ubaldo Ragona’s Last Man on Earth, starring Vincent Price, in which zombie-like vampires (or vampire-like zombies) are spread by some plague. Zombie geeks can correct me, but I believe that this is the film that introduced the concept of zombification by disease—a concept ripped off or borrowed by Night of the Living Dead, and now the virus LdMNPV.

    I rembember staying up way too late to watch this on tv on one school night, and have always loved this film.

    • Posted September 13, 2011 at 9:42 pm | Permalink

      From a short story by Richrd Matheson also filmed as “I Am Legend” and “The Omega Man”.

  6. Posted September 13, 2011 at 6:49 am | Permalink

    I bet if you could ask the caterpillar what it was doing, it would reply that it is climbing this branch and staying out all day of its own free will.

    • Nicolas Perrault
      Posted September 13, 2011 at 7:30 am | Permalink

      Yes, but the virus would protest of its innocence by stressing that free will does not exist.

  7. KP
    Posted September 13, 2011 at 8:41 am | Permalink

    Make sure G. Dwyer sees this paper…He will be interested. We collaborate on a similar baculovirus in another lymantriid moth species.

    • KP
      Posted September 13, 2011 at 8:56 am | Permalink

      Nevermind, already sent it and it sounds like he’s seen it…

  8. Jim Thomerson
    Posted September 13, 2011 at 9:57 am | Permalink

    In years gone by, I have read a couple of papers which showed that low level exposure to heavy metals, or pesticides, changed fish behavior and made them more vulnerable to predators. I’ve also read papers about parasites having the same effect on fish. One wonders what the mechanism is, and whether it is the same in instances where similar results are caused by different influences.

  9. Marella
    Posted September 14, 2011 at 12:52 am | Permalink

    Viruses are where it’s at, that’s for sure. If I were a young scientist I would be studying viruses, they are truly amazing and I feel sure that endless extraordinary things about them are yet to be dreamt of.

  10. eveysolara
    Posted September 14, 2011 at 7:06 am | Permalink

    I wonder if the loss of active ecdysone, because it keeps the insect in a feeding state, keeps them up in the tree or if it is the accumulation of conjugated ecdysone (egt transfers a sugar group to ecdysone, which inactivates it) that causes an exaggerated climbing behavior.

  11. Diane G.
    Posted September 15, 2011 at 11:44 pm | Permalink

    (subscribing)


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