Wednesday, 23 May 2012

Zombie Caterpillars!

For the common Gypsy moth caterpillar of Europe, Asia, and America, a normal day consists of nocturnal tree climbs, leafy feasts, and liquefied zombies. A particular virus that has been infecting these insects for decades has been discovered to possess a gene entirely responsible for inducing their zombie-like behavior. This gene has been identified as “egt” and is found to interfere with the caterpillar’s moulting cycle. It disturbs the process in such a way, that it causes completely uncharacteristic behavior,  the infected  insects climbing in broad daylight to their liquid deaths.
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The lethal “Zombie” virus infecting these caterpillars is a baculovirus called Lymantria dispar nucleopolyhedrovirus (LdMNPV) which only infects invertebrates and is able to manipulate it’s caterpillar-host’s behavior through the gene egt. It is possible that egt inactivates the caterpillar’s natural moulting hormone and alters it, so that the insect is “stuck” in the feeding state of it’s moulting cycle, consuming leaves and climbing constantly instead of returning to hidden safety at sunrise. By not producing enough of the moulting hormone to allow the caterpillar to complete the moulting process, the baculovirus can spread within it’s host’s growing body, as the insect grows larger, the number of viral cells within it increases. Scientists have only recently discovered that egt was to blame for the zombie-like behavior, although, it is still not yet determined exactly how the gene works.

Once the virus-filled zombie-caterpillar has climbed to the very tops of the trees, it’s body liquefies. The insect’s entire insides are melted into a runny viral goop from other genes found within the baculovirus, which uses nearly every part of the infected insect to make more of itself. The goop drips down from the highest parts of the tree, where it had so strategically positioned it’s host, onto the foliage below, the food of other Gypsy moth caterpillars. The virus can even survive on these leaves for long periods of time until it is consumed, to infect another Gypsy moth caterpillar.

There are many different types of baculoviruses, they occur naturally and nearly every caterpillar species is infected by one of them. Although they result in the gory death of thousands of caterpillars, they do not affect the species numbers of the Gypsy moth as a whole. An infestation of Gypsy moth caterpillars usually results in an outbreak of the virus within that population, it has been described by Kelli Hoover (2011) to be a “natural control mechanism” for their population booms, the “natural enemy of the Gypsy moth”. Viruses with zombie-like affects on their hosts can be found quite extensively throughout nature, but it has only been recently that the gene responsible for the behavioral manipulation has been identified.

Thanks to the egt gene, baculovirus LdMNPV is able to infect its host, create an optimal environment for growth, and then initiate its dispersal from a position where its potential to spread is greatest. The discovery of egt is just the beginning in scientists’ pursuit to understand these incredible viruses. 
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