Genetic Weapons

Virus gene engineer sends caterpillars to a sticky end

http://www.newscientist.com/article/dn20886-virus-gene-engineer-sends-caterpillars-to-a-sticky-end.html

It takes just one gene to rule them all. With that gene, a voodoo virus compels its caterpillar hosts to emerge from their shady hideaways, climb en masse to the tops of trees, deliquesce and fall as a rosy rain of viral particles on their fellow healthy caterpillars. Soon, they too will make the climb of doom.

The virus is known as baculovirus and its unsuspecting host the gypsy moth caterpillar. Like many other mind-controlling viruses, fungi and bacteria that, for instance, turn ants into zombies and make rats unusually fond of cats, baculovirus can selfishly change the behaviour of its host.

These bizarre relationships have long been documented but are poorly explained: suspicions that many of the behavioural changes reflect changes in gene expression have rarely led to genetic culprits.

Now researchers have found that the baculovirus uses a single gene to force gypsy moth caterpillars to meet their sticky treetop end and infect their relatives below.

Zombies by day

During the day, healthy gypsy moth caterpillars hide from birds and other predators by staying near the soil, sticking to the underside of branches or wedging themselves into crevices in tree trunks. Under the cloak of night, they come out to feed.

In contrast, caterpillars that are infected with baculovirus climb up to treetops in broad daylight, where they die because the virus has commanded every cell in their body to make copies of itself.

As if that wasn't bad enough, the virus also produces enzymes that break down the caterpillar's cell walls, liquefying it (see picture) into oozing pink puddles that swim with new viruses. The next time it rains, drops of water pelt the puddles and carry viruses down the tree, where they infect other caterpillars.

No more moulting

Kelli Hoover of Pennsylvania State University in University Park and her colleagues suspected that the baculovirus uses a gene called egt to change caterpillars' climbing behaviour. Egt codes for an enzyme that inactivates an insect moulting hormone called 20-hydroxyecdysone. This is useful because when caterpillars moult, they hunker down and largely stop feeding – so are much less likely to climb trees, foiling the baculovirus's plans.

Hoover infected gypsy moth caterpillars with six different strains of baculovirus: two of the viruses were unmodified, two were missing egt and two had had the gene removed and reinserted. Hoover placed the caterpillars in tall plastic soda bottles with food at the bottom and a fibreglass screen that the caterpillars could climb to reach the top.

Caterpillars infected with the normal baculovirus and the baculovirus with the returned egt gene died at the top of the bottles, in the manner typical of treetop disease. Caterpillars infected with the baculovirus missing the gene did not demonstrate unusual climbing behaviour.

Vince D'Amico, an entomologist with the US Department of Agriculture Forest Service, says the work is a first and key to advancing understanding of how baculovirus is transmitted through gypsy moths.

Genetic weapon

Exactly how egt forces caterpillars to climb to the tops of trees is not clear, but Hoover has some ideas. She notes that infected caterpillars initially act like healthy ones: they only sprint to the treetops about 24 to 36 hours before the virus overwhelms all the cells in their body.

She thinks that once the virus has inserted its genes into the caterpillar's cells, the enzyme for which egt codes gradually increases the amount of inactivated moulting hormone in the caterpillar's body – and that this build-up somehow triggers the unusual climbing behaviour.

In future, forest ecologists may be able to tweak baculovirus's genes to control the gypsy moth, which is an invasive species in North America and a major pest of hardwood trees, defoliating vast swathes of forest. Researchers have tested baculoviruses as biopesticides in the past, but found it worked too slowly compared with other pesticides. Genetically modifying the virus – perhaps by duplicating egt – could turn it into the gypsy moth's worst nightmare.

Journal reference: Science, DOI: 10.1126/science.1209199

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