These Worms Have No Eyes, but They Avoid the Color Blue

When a scientist noticed that blind nematodes avoid bacteria that make blue toxin, he wondered if they took color into account

A photo of nematodes viewed under a microscope where they appear purple
C. elegans are roundworms that are about one millimeter long and commonly used in scientific experiments as model organisms. Image by Eugene Lee

Tiny roundworms called Caenorhabditis elegans don’t have eyes, but that won’t stop them from picking a least-favorite color. New research shows that C. elegans can sense and avoid the color blue even though they lack even the most basic light-sensing systems, Veronique Greenwood reports for the New York Times.

When the worms forage for microbes in compost piles, they need to avoid landmines like the deadly, bright blue toxin produced by Pseudomonas aeruginosa bacteria. Biologist Dipon Ghosh, then working at Yale, wanted to figure out whether the worms used the toxin’s color as a clue to avoid it. A series of experiments tested the worms’ ability to avoid P. aeruginosa that made beige toxins or worked under different-colored lights. The results were in the journal Science on Thursday.

"We wanted to understand which component of the blue pigment toxin pyocyanin – its color or its toxicity – was informing the worms’ avoidance," says Ghosh, now at MIT, to Inverse’s Tara Yarlagadda. "We could reproduce the effects of pyocyanin on bacterial avoidance with both a harmless blue dye and a clear, colorless toxin, but not either separately."

The researchers compared the C. elegans’ reactions to toxic blue bacteria, toxic beige bacteria and safe blue bacteria, and found that the combination of blue color and toxicity has the greatest impact on the worms’ decision to wiggle away. In another experiment, the scientists put the C. elegans in a dish with toxic blue bacteria and then turned out the lights. Without lights, the worms didn’t escape the bacteria as quickly as they did with the lights on.

“When he showed me the results of the first experiments, I was shocked,” says Yale neuroscientist Michael Nitabach, a co-author of the study, to the New York Times.

In additional tests of the nematodes’ surprising color-sensing abilities, Ghosh created an apparatus that shone blue and amber-colored light on either side of a lab dish holding nematodes and the colorless version of the toxic bacteria. Under amber light, the nematodes struggled to suss out the dangerous microbes; under blue light, they steered clear.

“We were able to definitively show that worms aren't sensing the world in grayscale and simply evaluating the levels of brightness and darkness,” says Ghosh in a statement. “They're actually comparing ratios of wavelengths and using that information to make decisions — which was thoroughly unexpected.”

When the researchers expanded their pool of worms to include wild, varying versions, they realized that the standard lab strain of C. elegans actually isn’t very good at sensing blue. DNA sequencing showed that blue-sensitive C. elegans have a couple of genetic mutations that blue-insensitive worms lack. Future work could figure out what exactly those mutations do to help the worms respond to blue light.

The findings build on previous research that C. elegans avoid ultraviolet light, and other results that show they can sense smell, taste, touch and temperature.

The new results are “surprising and fascinating,” says Brown University neuroscientist Anne Hart, who was not involved in the study, to Shawna Williams at the Scientist magazine, but adds that they make sense because pigments play such a role in microbes’ toxicity and ability to infect hosts.

“There’s every reason for other organisms like C. elegans who have to deal with them to cue in on color and pigment as something to be avoided in some scenarios,” says Hart to the Scientist. “I think the biggest implication is probably: don’t underestimate the invertebrates.”

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