In the hot, foul environment of a compost heap, tiny roundworms feast on bacteria. But some of these microbes produce toxins and the worms avoid them. In the lab, scientists curious about how roundworms can tell what’s dinner and what’s dangerous often place them on mats of various bacteria to see if they move away. A species of microbe, Pseudomonas aeruginosa, reliably sends them rushing.
But how do 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, wondered if it was because they could detect toxins produced by bacteria. Or could it have something to do with the fact that the carpets of P. aeruginosa are a brilliant shade of blue?
Considering that roundworms don’t have eyes, cells that obviously sense light, or even one of the known genes for light-sensitive proteins, it seemed a bit 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, put worms on plaques of P. aeruginosa. Then he turned off the lights.
To the surprise of his advisor, Michael Nitabach, the flight of the bacteria’s worms was significantly slower in the dark, as if not being able to see prevented the roundworms from realizing that they were in danger.
“When he showed me the results of the first experiments, I was shocked,” said Dr. Nitabach, who studies the molecular basis of neural circuits that guide behavior at the Yale School of Medicine.
In a series of follow-up experiments detailed in an article published Thursday in Science, Dr Ghosh, Dr Nitabach and their colleagues establish that certain roundworms clearly respond to this distinctive pigment, perceiving it – and running away – without the benefit. of any known visual system.
How they accomplish this perceptual feat remains a mystery, but the results suggest that the worms may have hacked into other cellular warning systems to gain some sort of color vision.
Nematodes like C. elegans have an aversion to ultraviolet light and certain wavelengths of visible light, previous work has shown, and too much light can affect the lifespan of worms. Researchers generally viewed this behavior as a way to avoid stressful exposure to sunlight.
But using color to guide their foraging behavior – it was a new idea. To see if the bacteria’s color change would have an effect, Dr Ghosh then put worms on a mutated strain of P. aeruginosa that was beige rather than blue.
This time the worms didn’t move any faster, whether it was light or dark in the lab. This suggested that they were missing additional clues as to the color of the bacteria.
He also put the blue pigment – a toxin called pyocyanin – on E. coli, a common food source for worms. But rather than feasting on the bacteria, he found that they quickly flee germs when they are well lit.
Other experiments established that although the worms could feel something unpleasant about the toxin without the presence of the color, they were actually moving when the blue was visible.
Researchers tested dozens of roundworm strains and found that while some did not respond to blue, others were extremely sensitive to it, leaving a mat of E. Harmless coli. if the right colored light was shining.
Trying to understand how the eyeless creatures felt it, the researchers compared the genomes of worms that reacted strongly to color with those that ignored it. They were able to identify several regions of the genomes that correlate with behavior.
Then they engineered worms with mutations in the genes in those regions to see if the creatures’ color-sensing abilities were affected. Indeed, they discovered two genes, jkk-1 and lec-3, which seemed to affect the behavior of the worms when mutated.
It’s still unclear how these two genes, which code for proteins with no obvious connection to vision, connect to the enigmatic talent of worms. They can be part of a long protein brigade, conveying the message from one to the other that there is something blue in the area, until it reaches the neurons of the worm and makes move the creature.
However, proteins have been reported in the past in cellular responses to stressors such as ultraviolet light in human and mouse cells, explains Dr. Ghosh.
If researchers can find out how roundworms detect color, they will gain new insight into surprising behavior and an idea of how organisms without a traditional visual system can still perceive visible light. It may also be that an evolutionary way of avoiding stressors has been adapted to the color blue.