The tiny roundworm Caenorhabditis elegans is one of Earth’s most-studied creatures. Transparent and easily grown in the lab, it’s a favorite of geneticists and other researchers who experiment on it to derive broad lessons about animals’ inner workings. It can be frozen and then thawed back to life, and manipulated to model a panoply of human diseases. Its genome was first sequenced in 1998—five years before the same was completed for humans.
Yet C. elegans still harbors secrets, and a big one is unveiled today (March 4) in Science: this eyeless worm can, in a way, see, using color to help it discriminate between toxic and harmless bacteria when searching out food.
Researchers have previously shown that C. elegans can sense some types of light, notes study coauthor Dipon Ghosh, a biology postdoc at MIT who started the project when he was a graduate student at Yale University. The new results show that the worms are “actually comparing ratios of wavelengths, and using that information to make decisions,” he says. “And that, I think, was completely surprising and unexpected.”
In the wild, C. elegans favors environments such as rich soil and decomposing food heaps, where it feeds on bacteria. The worm is known to avoid munching on the poisonous species Pseudomonas aeruginosa, and the study came about, says Ghosh, because he was curious about how C. elegans does this. In reading studies relevant to the question, he learned that one of the toxins P. aeruginosa secretes is blue. In addition, the worm isn’t always foraging in dark, subterranean niches, Ghosh notes. “Compost heaps, of course, are above ground, suggesting that worm environments might be more illuminated than we once thought.” Given all this, he says, “I wondered whether the worm avoidance of this colorful, pathogenic bacteria could be informed, at least in part, by the pigmentation or the color of the bacteria.”
Given that several sensory perceptions were found in this simple model organism, this tiny worm may be much smarter than we think.—Jie Liu, Monash University
Finding the answer wasn’t straightforward. In one experiment, Ghosh tried to tease out the effect of color on the worms by replacing the blue toxin in one batch of P. aeruginosa with a harmless blue dye. In another batch, he replaced it with a colorless toxin. While C. elegans avoided ordinary, unaltered P. aeruginosa, it didn’t shy away from either the nontoxic blue or the toxic, colorless version, leaving Ghosh confused. Ultimately, he found he could get the worms to avoid the toxic, colorless bacteria by shining blue-filtered light on their dishes—suggesting that color did indeed influence C. elegans’s foraging behavior.
In further experiments, Ghosh discovered that he could affect the worms’ foraging behavior by varying the ratio of blue to amber light shining on their dishes. But when he ran the same test on dozens of wild strains of C. elegans, not all responded in the same way to the same ratios.
Through genetic analyses, Ghosh and his colleagues identified two genes, jkk-1 and lec-3, that appear to be involved in the responses to color. Neither code for opsins, the class of light-sensitive proteins needed for vision in the eye; rather, the study’s authors suggest, they may be involved in light-influenced stress-response pathways.
The study “helps us to understand the interaction between microbes and their foraging hosts,” writes Jie Liu, a neuroscientist at Monash University in Australia who was not involved in the study, in an email to The Scientist. “Given that several sensory perceptions were found in this simple model organism, this tiny worm may be much smarter than we think.”
Anne Hart, a neuroscientist at Brown University who also was not involved in the work, echoes that reaction. “I think the biggest implication is probably: don’t underestimate the invertebrates,” she says. Hart calls the study’s results “surprising and fascinating,” but says they make sense given that bacteria are thought to produce pigments to aid them in infecting 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.”
Sensory neuroscientist Piali Sengupta of Brandeis University says the finding that C. elegans relies on multiple cues to detect toxic bacteria aligns with discoveries of other such signals it uses, including nitric oxide emitted by P. aeruginosa. She speculates that the worms may rely on different cues in different circumstances, such as when light is or isn’t available. “Maybe under a certain context they use combination A, and then [in] a different context they use combination B,” says Sengupta, who was not involved in Ghosh’s work. “I think that would be pretty cool to figure out going forward.”
Connie Cepko, a biologist and Howard Hughes Medical Institute investigator at Harvard Medical School who studies cells in the retina, says the work “showed, I think, a level of interpretation of environmental signals that are dependent on light that lead one to appreciate just how important light is as an environmental cue.” Cepko, who also did not participate in the research, points out that “light is extremely ubiquitous on Earth, and it has an incredible amount of information. . . . I think the fact that we see that there are all these different kinds of proteins that have evolved to capture light speaks to the importance of light as an environmental signal.”
D.D. Ghosh et al., “C. elegans discriminates colors to guide foraging,” Science, 371:1059–63, 2021.