By Yosef Scher, Science & Technology Editor
For quite some time, scientists have known about the infamous Nuclear Polyhedrosis Virus and its effects on caterpillars, such as forcing the caterpillar to climb to increasingly higher heights where it will be positioned until the virus liquifies the caterpillar. It wasn’t until only recently, however, thanks to Xiaoxia Liu and her colleagues at the China Agricultural University in Beijing, that it was discovered how the virus works so efficiently and effectively.
The Nuclear Polyhedrosis Virus employs a calculated and strategic plan in its hosts that allow it for maximal dissemination of the virus. Once infected, a caterpillar will crawl to the top of a plant and “hang in an inverted V from its middle legs or in a straight line from its anal prolegs.” After three days of hopelessly hanging on the plant, the caterpillar dies and turns into a dark brown or black-colored liquid. According to the International Butterfly Breeders Association, the Nuclear Polyhedrosis Virus “attacks the very cell structure of the caterpillar, rearranging it at the atomic level, forming ‘crystals’ which are, basically, inanimate and incapable of maintaining life.” The virus produces a foul smell which attracts many different insects to the deformed caterpillar. Additionally, birds and other wildlife that enjoy caterpillars will eat them, “carrying the infection over long distances.”
Once in contact with the caterpillar, the insects disperse the virus wherever they go. After having been entirely liquified, the residual elements of the caterpillar drip to the leaves lower down on the plant. The virus is further spread by rain, which washes more of the virus particles to the lowest leaves on the plant and other surrounding areas. Butterflies that land on leaves infected with the virus pick up the virus particles and spread the virus to other nectar plants. When the butterfly lays its eggs on the infected plant that it touched, “the hatchling caterpillars eat the leaves and immediately are infected with the virus,” causing the cycle to restart.
In order to understand how the Nuclear Polyhedrosis Virus caused the caterpillar to behave in such a bizarre fashion, Xiaoxia Liu and her colleagues conducted an experiment that revealed that the virus “[cranked] up the expression of genes involved in the larvae’s visual system—specifically, ones involved in perceiving light.” Liu’s team started their investigation into the caterpillar’s uncanny behavior by identifying the genes responsible for opsins, “light-sensitive proteins key to vision.” Although those findings were significant, the researchers’ critical discovery came when they found a third overactive gene, TRPL, in the infected caterpillars, which was not nearly as active in uninfected caterpillars. A normal TRPL gene assists cell membranes in converting light into electrical signals. However, when there is an unusual amount of activity with the TRPL gene, an organism begins to “crave more light than usual.” Using CRISPR technology, a gene-editing tool, Liu and her team temporarily shut off the opsin genes and TRPL gene in the infected caterpillars to determine how this would affect them. They found that these caterpillars were less attracted to the light, and the “number of insects that [even] moved towards the light in the box fell by roughly half.” As a result of the experiment, Liu’s team proved that the Nuclear Polyhedrosis Virus seems to “hijack the genes related to caterpillar vision,” which ultimately begins the caterpillar’s demise.
Although this virus seems to be a terrible thing plaguing caterpillars, some organizations, like the United States Government, are glad that it is around. In the United States, an invasive caterpillar species called the Gypsy Moth Caterpillar damages “roughly a million acres of forest … each year by devouring tree leaves.” Scientists believe that a lot more damage could have been done to the forests without the virus controlling the number of Gypsy Moth Caterpillars. According to Jon Hamilton, a correspondent for NPR‘s Science Desk, “[t]he virus is so effective that the government actually sprays it on trees to help control gypsy moth outbreaks.”
While Liu and her colleagues certainly made an incredible breakthrough in discovering how the virus manipulates the caterpillar’s genes to change its behavior, there is still a lot more for scientists to uncover.
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Sources:
https://butterfly-fun-facts.com/npv-nuclear-polyhedrosis-virus/
https://www.npr.org/2011/09/12/140226986/how-a-clever-virus-kills-a-very-hungry-caterpillar
https://www.internationalbutterflybreeders.org/nuclear-polyhedrosis-virus/
https://www.eekwi.org/explore/species-status/alien-invasion-gypsy-moth-time-line-north-america