Life

A parasitic worm that makes the grasshopper it invades jump into water and commit suicide does so by chemically influencing its brain, a study of the insects' proteins reveal.

The parasitic Nematomorph hairworm (Spinochordodes tellinii) develops inside land-dwelling grasshoppers and crickets until the time comes for the worm to transform into an aquatic adult. Somehow mature hairworms brainwash their hosts into behaving in way they never usually would - causing them to seek out and plunge into water.

Once in the water the mature hairworms - which are three to four times longer that their hosts when extended - emerge and swim away to find a mate, leaving their host dead or dying in the water. David Biron, one of the study team at IRD in Montpellier, France, notes that other parasites can also manipulate their hosts' behaviour: "'Enslaver' fungi make their insect hosts die perched in a position that favours the dispersal of spores by the wind, for example."

But the "mechanisms underlying this intriguing parasitic strategy remain poorly understood, generally", he says.

Now Biron and his colleagues have shown that the worm brainwashes the grasshopper by producing proteins which directly and indirectly affect the grasshopper's central nervous system.

To view a video of the parasite and grasshopper in action, which includes a brief interview, in French, with lead researcher Frederic Thomas, visit the Canal IRD website.

Selective manipulation

"It's a very novel study, because there are very, very few papers on how behaviour actually changes," says Shelley Adamo at Dalhousie University in Halifax, Canada, an expert in insect behavioural physiology who is familiar with Biron's work.

"One of the reasons they are interesting is that parasites are often able to get in there and selectively manipulate behaviour," she told New Scientist. She says the eventual hope is that understanding how parasites manipulate their hosts' behaviour - by affecting the nervous and endocrine systems - might further the understanding of how human behaviour-systems link.

Biron and colleagues found that the adult worms - those ready to prime their hosts for a watery death - altered the central nervous system function of their hapless hosts by producing certain molecules mimicking the grasshoppers' own proteins.

Gravity response

And grasshoppers housing the parasitic worm expressed different proteins in their brains than uninfected grasshoppers. Some of these proteins were linked to neurotransmitter activities. Others included those linked to geotactic behaviour - the oriented movement of an organism in response to gravity.

The team used an approach called "proteomics" to study the hijacking of the grasshopper's behaviour. This technique analyses all the proteins expressed in a cell or tissue.

Biron and colleagues collected and analysed the proteins of grasshoppers (Meconema thalassinum) with and without parasitic hairworms before, during and after the grasshoppers' suicidal plunges into a swimming pool at night-time.

"This is a unique approach and a very exciting one," says Adamo. "This is the first time it's been used to address this issue."

Journal reference: Proceedings of the Royal Society B (DOI: 10.1098/rspb.2005.3213)

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