In the hot, greasy environment of a compost heap, small roundworms feed on bacteria. But some of these microbes produce toxins, and the worms avoid them. In the lab, scientists who are curious about how the roundworms can tell what the food is and what is dangerous are on top of mats of different bacteria to see if they unwind. A microbial species, Pseudomonas aeruginosa, send them reliably scurrying.
But how do the worms, common laboratory animals of the species Caenorhabditis elegans, do this? Dipon Ghosh, then a graduate student in cellular and molecular physiology at Yale University, asked him if it was because they could sense the toxins produced by the bacteria. Or maybe it has something to do with the fact that carpets from P. aeruginosa is a bright blue shade?
Since roundworms do not have eyes, cells that naturally detect light, or even any of the known genes for light-sensitive proteins, it seems slightly far-fetched. However, it was not difficult to set up an experiment to test the hypothesis: dr. Ghosh, who is now a postdoctoral researcher at the Massachusetts Institute of Technology, has placed some worms on P. aeruginosa. Then he turned off the lights.
To the surprise of his adviser, Michael Nitabach, the worms’ flight against the bacteria in the dark was considerably slower, as if the roundworms could not realize that they were in danger.
“When he showed me the results of the first experiments, I was shocked,” said dr. Nitabach said, studying the molecular basis of neural circuits that guide behavior at the Yale School of Medicine.
In a series of follow-up experiments, set out in an article published in Science on Thursday, dr. Ghosh, dr. Nitabach and their colleagues fast that some roundworms clearly respond to that characteristic pigment and perceive it – and flee from it – without the benefit of any known visual system.
How they accomplish this perceptual achievement remains a mystery, but the findings suggest that the worms hacked other cellular warning systems to gain some sort of color vision.
Nematodes such as C. elegans do have an aversion to ultraviolet light and certain wavelengths of visible light, which were worked on earlier, and too much light can affect worms’ lifespan. Researchers generally view this behavior as a way to avoid stressful exposure to sunlight.
But using color to control their feeding behaviors – that was a new idea. To see if the change in the color of the bacterium would have an effect, Dr. Ghosh placed worms on a mutated strain of P. aeruginosa that was beige rather than blue.
This time the worms did not move away faster, whether it was light or dark in the laboratory. This indicates that they do not have extra signs of the bacteria’s color.
He also put the blue pigment – a toxin called pyocyanin – on E. coli, a common food source for the worms. But rather than feasting on the bacteria, he found that they quickly fled from the microbes when they were well lit.
In other experiments it was found that although the worms can feel something unpleasant to the toxin without the color being there, they really move when blue is visible.
The researchers tested dozens of roundworm strains and found that while some did not respond to blue, others were extremely sensitive to it, leaving behind a carpet of harmless E. coli. if the right colored light is shining.
The researchers tried to understand how the eyeless creatures perceive it, and compared the genomes of worms that react strongly to color with those that ignore them. They were able to determine several regions of the genome associated with the behavior.
Thereafter, they designed worms with mutations in genes in those regions to see if the creative abilities of the creatures were affected. They have indeed discovered two genes, jkk-1 and lec-3, which appear to affect the worms’ behavior when mutated.
It is still unknown how these two genes, which encode proteins without a clear connection to vision, join the worms’ enigmatic talent. They can be part of a long bucket brigade with proteins, which carry the message from one to the other that something is blue in the environment, until it reaches the worm’s neurons and gets the creature moving.
The proteins have been labeled in the past in cellular responses to stressors such as ultraviolet light in human and mouse cells, says Dr. Ghosh.
If researchers can discover how the roundworms detect color, they have new insight into surprising behavior and can see how organisms without a traditional visual system can still perceive visible light. It could also be that an evolutionarily ancient way to avoid stressors is set on the color blue.