One of the main goals of the natural world is to reproduce. Viruses are no different. And there is one of them who executes a brutal strategy against the caterpillars just to achieve his goal. The virus manipulates the genes associated with the caterpillars’ vision to make them feel more attracted to sunlight than usual, causing a completely literal climb, leaf after leaf, aiming, as if it were Icarus, to get as close to the Sun as possible.
After this deadly climb in which the tiny body incessantly climbs the plant, the caterpillar will die when the scavengers feast on it , facilitating, from above, the spread of the virus that was inside the insect.
It looks like something out of a scary movie, but it’s more common than it seems in the world of insects.
Unveiling secrets of entomology
Now a new study reveals exactly how this process happens. Specifically, with a group of viruses that infect insects known as nucleopolyhedroviruses (NPV) and caterpillars of the cotton bollworm ( Helicoverpa armigera ).
Known as “tree canopy disease,” it’s all related to phototaxis, or the way organisms are attracted to a light source (in this case, the Sun).
“The mechanisms by which parasites and pathogens manipulate host behavior are of great interest, but few studies have definitively characterized them,” the authors write in their paper published in the journal Molecular Ecology . “Here, we illustrate how HearNPV induces enhanced phototaxis in H. armigera larvae by hijacking the host’s visual perception and triggering climbing behavior, causing infected larvae to die at a high altitude.”
Nucleopolyhedroviruses belong to a larger group of viruses called baculoviruses that usually proceed by altering the locomotion of their hosts. Baculoviruses can infect more than 800 species of insects, mainly caterpillars of moths and butterflies.
Although the macabre trick of these viruses to achieve their goal was known, scientists were not entirely clear how the viruses turned the caterpillars into zombie organisms.
For the study, China Agricultural University researcher Xiaoxia Liu and her colleagues fed moth larvae either virus-contaminated food or an uninfected control diet and then placed them at the bottom of glass tubes or cotton plants in pots. The glass tubes had an LED light source installed at the top, middle, or bottom of the tube, while the plants had the light source at the top or bottom, and the team recorded the distances traveled by each larva each hour. .
The researchers compared the positions of the infected and uninfected caterpillars. Those in the control group wandered around the mesh, but eventually came back, but the infected hosts ended up dead on top of the mesh . The higher the light source, the higher the infected caterpillars climbed . For comparison, caterpillars with the virus crawled toward the light about four times longer than uninfected ones. It was clear that the virus was using the caterpillar’s vision against itself.
Taking a look at the genes, they found that two genes for opsins, the light-sensitive proteins that are critical for vision, were more active after being infected by the virus, as was another gene associated with vision called TRPL. .
“Since sunlight shines on plants from above, positive phototaxis is likely a reliable mechanism to ensure that infected larvae are killed at high altitudes on host plants,” the researchers write.
The new research shows that viruses manipulate “yet another host physiological process: visual perception.” Of course, how the virus alters the expression of these genes remains a mystery.
“More research is needed on how viral infection is affecting gene expression,” the experts conclude.
Referencia: 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.