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How a Virus Transforms Caterpillars into Zombies Destined to Die by Climbing

Baculoviruses
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Initial infection of the virus

Infection of Baculoviruses

Image : Williams et al (2017) [1]

The cotton bollworm moth caterpillar climbs higher and higher, its tiny body constantly mounting leaf after leaf. When an insect reaches the top of a plant, it will die, allowing the virus that guided it there to spread. One of the viruses responsible for this lethal ascent manipulates genes involved in caterpillar eyesight. As a result, researchers report online March 8 in the Journal of Molecular Ecology that the insects are more attracted to sunlight than usual [2].

Baculovirus can infect various insects

A baculovirus is the virus responsible for the caterpillar conquest. According to entomologist Xiaoxia Liu of China Agricultural University in Beijing, these viruses have been developing alongside their insect hosts for 200 million to 300 million years. Baculoviruses can infect over 800 insect species, the majority of which are caterpillars and butterflies. Once infected, the hosts develop “tree-top illness,” which causes them to be impelled to climb before dying and leaving their elevated, infected cadavers for scavengers to feast on.

According to Liu, the smart trick of these viruses has been known for more than a century. But it’s unclear how they turn caterpillars into zombies bound to soar to their own deaths. Infected caterpillars have more “phototaxis,” which means they are more drawn to light than uninfected insects, previous study said. Cotton bollworm moth caterpillars (Helicoverpa armigera) infected with a baculovirus termed HearNPV were used by Liu and her colleagues to confirm this effect in the lab.

Impact of LED light on infected caterpillars

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source : Boyes et al (2021)

Under an LED light, the researchers compared the positions of infected and uninfected caterpillars in glass tubes around a climbing mesh. Uninfected caterpillars would roam up and down the mesh before pupating, but they would always return to the bottom. Because this species develops into adults underground in the wild, this behavior makes logical. Infected hosts, on the other hand, would die at the top of the mesh. The higher the light source, the higher the infected hosts rose.

The number of hosts attracted to the light in the box was roughly cut in half when the team employed the gene-editing tool CRISPR/Cas9 to turn off the opsin genes and TRPL in infected caterpillars. On the mesh, their height at death was likewise lowered.

Baculoviruses capable of commandeering the genetic architecture of insects

Baculoviruses appear capable of commandeering the genetic architecture of caterpillar vision, exploiting an ancient importance of light for insects, Liu says. Light can cue crucial biological processes in insects, from directing their developmental timing, to setting their migration routes.

There’s still a lot to learn about this visual hijacking, Passarelli says. It’s unknown, for instance, which of the virus’s genes are responsible for turning caterpillars into sunlight-chasing zombies in the first place.

References

[1] Williams T, Virto C, Murillo R and Caballero P (2017) Covert Infection of Insects by Baculoviruses. Front. Microbiol. 8:1337. doi: 10.3389/fmicb.2017.01337

[2] X. Liu et al. Baculoviruses hijack the visual perception of their caterpillar hosts to induce climbing behaviour thus promoting virus dispersal. Molecular Ecology. Published online March 8, 2022. doi: 10.1111/mec.16425.

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