Researchers at China Agricultural University, Beijing, found a virus that manipulates the activity of vision controlling genes in caterpillars and turns them into zombies. The virus is also responsible for their deaths.
|Image credit: Pexels
The cotton bollworm moth caterpillar ceaselessly climbs higher on the plant. When it reaches the top of the plant, it dies due to the spread of the virus in its body.
This virus affects the functions of the genes associated with caterpillars’ vision. As an outcome, these insects are more attracted to sunlight than normal.
The virus involved in cotton bollworm moth caterpillar is a variant of baculovirus. ” These viruses may have been evolving with their insect hosts for 200 million to 300 million years,” said Xiaoxia Liu, an entomologist at China Agricultural University in Beijing. The baculoviruses can contaminate more than 800 insect species, mostly the caterpillars of butterflies and moths. These infected insects show “tree top disease”, a changed behavioural state that is identified by scaling to a raised position before death. The scavengers feast upon the left dead bodies of these infected insects.
“The clever trick of these viruses has been known for more than a century,” said Liu. But how do these viruses result in the death of the caterpillars? The answer was not discovered.
Previous research indicated that infected caterpillars showed greater “phototaxic” behaviour, meaning they were more attracted to sunlight as compared to uninfected caterpillars. The phototaxis effect was confirmed using Helicoverpa armigera (cotton bollworm moth caterpillars) infected with HearNPV (a type of baculovirus).
The researchers compared the uninfected and infected caterpillars’ positions in glass tubes surrounded by climbing mesh in the presence of LED light. This comparison showed that the uninfected caterpillars wandered up and down the mesh, but the infected caterpillars ended up dead at the top of the mesh.
The research team confirmed that the infected hosts responded to light instead of gravity after placing the caterpillars in a hexagonal box with one of the side panels illuminated. These infected caterpillars moved to the light four times faster than the uninfected ones.
The researchers surgically removed the infected caterpillars’ eyes. After placing them in a box, they found that the number of blinded insects moving to the light became one-fourth of the total number of unaltered infected insects. This showed that the virus was affecting caterpillars’ vision.
The researchers then compared the active genes of various body parts in infected and uninfected caterpillars. They detected that two opsin genes and TRPL were highly active after the infection. The opsin genes are light-sensitive proteins important for vision, and TRPL is a gene that converts light into electrical signals.
The team used the gene-editing tool CRISPR/Cas9 to shut off the TRPL and opsin genes in the infected caterpillars and concluded that the number of the infected hosts moving to the light was reduced to half. Also, the count of caterpillar death on top of the mesh was reduced.
“Baculoviruses appear capable of commandeering the genetic architecture of caterpillar vision, exploiting an ancient importance of light for insects,” said Liu.
Light plays a crucial role in insects’ biological processes such as directing their developmental timing and setting their migration routes.
“These viruses were already known to be master manipulators in other ways, tweaking their hosts’ sense of smell, moulting patterns and the programmed death of cells,” said Lorena Passarelli, a virologist at Kansas State University in Manhattan.
More research is required to find which specific virus’s genes are responsible for turning caterpillars into sunlight-chasing zombies.