Caterpillars Vision

[Garoa Wolff]

Higherand higher still, the cotton bollworm moth caterpillar climbs, its tiny body ceaselessly scaling leaf after leaf. Reaching the top of a plant, it will die, facilitating the spread of the virus that steered the insect there.

The team moved to the horizontal plane to confirm that the hosts were responding to light rather than gravity, placing caterpillars in a hexagonal box with one of the side panels illuminated. By the second day after infection, host caterpillars crawled to the light about four times as often as the uninfected.

The team then compared how active certain genes were in various caterpillar body parts in infected and uninfected larvae. Detected mostly in the eyes, two genes for opsins, the light-sensitive proteins that are fundamental for vision, were more active after an infection with the virus, and so was another gene associated with vision called TRPL. It encodes for a channel in cell membranes involved in the conversion of light into electrical signals.

When the team used the gene-editing tool CRISPR/Cas9 to shut off the opsin genes and TRPL in infected caterpillars, the number of hosts attracted to the light in the box was cut roughly in half. Their height at death on the mesh was also reduced.

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.

Jake Buehler is a freelance science writer, covering natural history, wildlife conservation and Earth's splendid biodiversity, from salamanders to sequoias. He has a master's degree in zoology from the University of Hawaii at Manoa.

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To test this, China Agricultural University (CAU) researcher Xiaoxia Liu and her colleagues fed moth larvae either virally contaminated food or an uninfected control diet and then placed them at the bottom of glass tubes or potted cotton plants. The glass tubes had an LED light source installed at either their top, middle, or bottom of the tube, while the plants had the light source at either the top or bottom, and the team recorded the distances traversed by each larva hourly.

When lights were placed at the top of the glass tubes, infected individuals climbed significantly higher 48 hours after infection and onward than did uninfected animals. They also tended to die at the same level where the light was placed, regardless of whether it was high or low. Using assays to estimate phototaxis and the retinal response to light, the team further reported higher reaction levels to light stimuli in infected larvae compared with the healthy controls.

Caterpillars detect light using simple eyes called stemmata. These organs could be mediating the phototaxic response in infected larvae, the team speculated, so they removed them surgically. They found that without stemmata, infected caterpillars were no longer phototaxic, and the height at which they died when light was above was significantly lower than that of infected larvae that received a sham operation where they were wounded near the stemmata but the organ was left intact.

The researchers placed infected and healthy caterpillars inside glass tubes under an LED light. Each tube contained a mesh that caterpillars could climb. Healthy caterpillars wandered up and down the mesh. But the crawlers returned to the bottom before eventually wrapping themselves in cocoons. That behavior makes sense, since in the wild this species grows into adults underground. Infected caterpillars, on the other hand, died at the top of the mesh. The higher the LED light, the higher the infected critters climbed.

Here, viruses seem to hijack the genes related to caterpillar vision, Liu says. This tactic exploits the vital role of light for most insects. Light directs their aging, for example. Light also guides insect migration.

caterpillar: The larval stage of moths and butterflies. Somewhat wormy-shaped crawlers, caterpillars tend to eat leaves and other plant bits. Some will, however, dine on other insects.

cell: (in biology) The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

develop: To emerge or to make come into being, either naturally or through human intervention, such as by manufacturing. (in biology) To grow as an organism from conception through adulthood, often undergoing changes in chemistry, size, mental maturity or sometimes even shape.

developmental: (in biology) An adjective that refers to the changes an organism undergoes from conception through adulthood. Those changes often involve chemistry, size and sometimes even shape.

genetic: Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

host: (in biology and medicine) The organism (or environment) in which some other thing resides. Humans may be a temporary host for food-poisoning germs or other infective agents. (v.) The act of providing a home or environment for something.

insect: A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.

larvae: Immature insects that have a distinctly different form (body shape) than when they are adults. For instance, caterpillars are larval butterflies and maggots are larval flies. (Sometimes this term also is used to describe such a stage in the development of fish, frogs and other animals.)

membrane: A barrier which blocks the passage (or flow through) of some materials depending on their size or other features. Membranes are an integral part of filtration systems. Many serve that same function as the outer covering of cells or organs of a body.

migration: (v. migrate) Movement from one region or habitat to another, especially regularly (and according to the seasons) or to cope with some driving force (such as climate or war). An individual that makes this move is known as a migrant.

physiological: Having to do with the branch of biology that deals with the everyday functions of living organisms and how their parts function. Scientists who work in this field are known as physiologists.

protein: A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.

virus: Tiny infectious particles consisting of genetic material (RNA or DNA) surrounded by protein. Viruses can reproduce only by injecting their genetic material into the cells of living creatures. Although scientists frequently refer to viruses as live or dead, in fact many scientists argue that no virus is truly alive. It survives and reproduces by hijacking the cellular machinery of a living cell. Someone who studies viruses and the diseases they cause is known as a virologist.

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