Viral Gene alters Bug Behaviour

05 September 2011
Posted by Emma Stoye.

Scientists have identified the gene which allows a virus to manipulate its insect host's behaviour for its own benefit.

Dead gypsy moth caterpillarSome pathogens, such as baculoviruses, use behavioural manipulation of their hosts to improve their chances of transmission.  Gypsy moth caterpillars infected with one such baculovirus climb to the tops of trees where they die, liquefy and release a shower of infectious viral particles.  Their elevated position helps the virus spread, particularly when it is raining, as the particles dissolve in raindrops and are carried to lower branches and leaves.  Healthy, uninfected, caterpillars tend to hide in bark crevices or soil in the daytime in order to avoid predators.

Now, thanks to researchers at Harvard and Pennsylvania State Universities, the genetic basis for this behavioural control is no longer a mystery.

Dr. Kelli Hoover and her colleagues suspected one baculovirus gene - known as egt - was involved in the process.  They observed the behaviour of captive gypsy moth larvae when infected with the wild type virus, when infected with a version of the virus with the egt gene deleted, or when left uninfected.

Gypsy moth egt experimentCaterpillars exposed to the unchanged baculovirus showed similar behaviour to infected wild caterpillars: they climbed up the sides of the containers they were kept in and died in an elevated position.  However, neither control caterpillars nor caterpillars exposed to the modified virus showed this abnormal climbing behaviour.  These findings, reported this week in the journal Science, suggest that the egt gene is responsible for the behavioural changes induced by the virus.

This is the first time the genetic basis for viral manipulation of host behaviour has been identified.  The exact mechanism by which the behavioural changes take place is still poorly understood, although the action of the gene itself may provide a clue: egt codes for an enzyme which inactivates one of the caterpillar's hormones.  Hormonal changes are known to influence behaviour in larvae, so this could be important.

The next step will be to investigate the action of egt in more detail so that the whole process can be better understood.

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