Have you ever been sure that you were being watched? Do you wake up at the time you’re supposed to without having to set your alarm? Can you tell when a Republican is lying? (OK, that last one is too easy.)
We humans have some weird senses we can’t explain, but there are also a whole lot of them out in nature that we don’t understand or even know about. What’s your cat learning when he smells the air before going outside? What’s the thing a dog senses before an earthquake? What does ultraviolet look like to a bee? How does a bird follow the Earth’s magnetic field to migrate? We’ll never really experience those things. All we can try to do is academically explain them.
Well, another bizarro sense has popped up in a March 5 report in the journal Science. The nematode — a very small worm that has been a favorite in labs because we’ve mapped every cell and neuron in its body — has a sense that we would never have thought possible. Surely an animal with no eyes can’t perceive color, right?
Ha! The nematode is amused at our naïveté.
Nematodes really like to eat bacteria, and usually that’s not a problem. But there is a toxic bacterium called Pseudomonas aeruginosa that they sometimes run into that can kill them within hours. The distinguishing feature of this bacterium is that it secretes a toxin called pyocyanin that is deep blue in color:
Pseudomonas aeruginosa secreting pyocyanin
The toxic blue pigment pyocyanin
Pseudomonas aeruginosa is toxic to humans as well. It’s an opportunistic infectious agent that affects especially cystic fibrosis and burn patients. Not only is it resistant to many antibiotics, but the toxicity of pyocyanin makes matters much worse. Pyocyanin can catalyze the formation of superoxide and hydrogen peroxide, which can damage almost anything in their path (DNA, proteins, you name it), and it can also interfere with respiration (the ability to breathe oxygen). That sounds nasty.
So how does the nematode so deftly avoid this bacterium? You and I can see its blue color with our eyes. (Not that we’d be tempted to slurp it up, no matter what color it was.) But nematodes don’t have eyes. In fact, they don’t even have opsins, the light-responsive pigments that are the basis of light perception in bacteria all the way up to humans.
Are nematodes still somehow perceiving the blue color of the toxin anyway? Researchers from MIT and Yale (D. Dipon Ghosh, Dongyeop Lee, Xin Jin, H. Robert Horvitz, and Michael Nitabach) have determined that the answer is yes, meaning nematodes are detecting color with a means we did not know existed.
Horvitz said:
"It's amazing to me that a tiny worm—with neither eyes nor the molecular machinery used by eyes to detect colors—can identify and avoid a toxic bacterium based, in part, on its blue color. One of the joys of being a biologist is the opportunity to discover things about nature that no one has ever imagined before."
They found that nematodes will flee from these blue bacteria, and even from the pigment itself, but only in the presence of white light. That is, only when blue appears blue. Nematodes won’t flee from many other colorless toxins, but if you add blue pigment, they will. Interestingly, they won’t flee from nontoxic blue pigments alone. So somehow they can tell if something is toxic AND blue.
These researchers wanted to find out what the nematodes were sensing. When white light gets filtered through a blue pigment, the longer wavelengths (red, orange, yellow) are absorbed, so the light you see looks blue.
Wavelengths of visible light range from about 400 nanometers (nm) for violet to about 665 nm for red
So they put the incoming light through filters that allowed only long or short wavelengths through, and they found that the nematodes weren’t sensing “blue” in either case. Even in blue light, the “blue” signal from pyocyanin wasn’t getting to the nematodes.
They found that the incoming light needed to include both blue and yellow. They could eliminate red light and it would still work, but the filters couldn’t get any tighter than that. So it seemed the nematodes were maybe sensing the ratio of yellow light to blue light. A blue pigment would interrupt the yellow but leave the blue, and maybe that’s what the nematodes needed to “see”.
So then they came up with a clever test: They’d put a colorless toxin (octanol) out for the nematodes, and then shine light on it with different ratios of yellow to blue, but with the same total light intensity. The more yellow you add to blue, the closer it gets to white light:
Mixing blue and yellow light gives white light
They found that the nematodes would only avoid the octanol if the blue:yellow light ratio was between 4:1 and 12:1; in other words, if the yellow light were reduced but not eliminated.
They also found that it worked the other way, too: when the blue light was reduced in a similar way, the nematodes would avoid the octanol. To you and me, that color would look orangish or yellowish. Well, it just so happens that Pseudomonas makes oxidizing pigments that are orangish and yellowish, too:
Different strains of Pseudomonas aeruginosa make yellow, blue, and orange pigments
That suggests that nematodes would avoid these as well, by knowing that something was “off” in the ratio of blue to yellow that they could “see”.
Dipon Ghosh remarked:
“[Nematodes are] actually comparing ratios of wavelengths, and using that information to make decisions. And that, I think, was completely surprising and unexpected.”
So far, it appears that the jkk-1 and lec-3 genes are needed for this response, and neither has anything to do with conventional vision or opsins. A quick search on BLAST shows that human beings have genes that resemble both of these, but who knows exactly what we do with them?
If we can figure out how the nematode is sensing color, the fact that it’s an entirely new mechanism makes me think there will be practical applications. But more than that, it reminds us that there’s much, much more going on in nature than we’ll ever comprehend, even within ourselves.
But one thing we do know now: if you’re a bacterium, a nematode knows when you’re blue.
