When flatworms (planarians) choose one colour over the other, they are not truly sensing colour. They are comparing the two lights and converting the difference in effective intensity of light into clear behavioural choices.
In a seminal work, Bengaluru-based researchers have found that flatworms (Schmidtea mediterranea), which are highly averse to light and move away from it, have the ability to discern different colours despite being colour-blind. Unlike humans who have three colour-sensitive photoreceptors for red, blue and green, flatworms have just one type. The study also found that flatworms were able to move in response to long wavelength ultraviolet light even when the head was cut — proof that the worms could sense light even without their eyes and brain. Similar eye-independent light-sensing is seen in fruit fly, C. elegans and Platynereis.
The two light-sensing responses — brain-mediated network and eye-independent response — seen in flatworms show a typical hierarchy of dominance. While the light sensors spread across the body (eye-independent response) helped the decapitated worms to move away from UV light in a reflex-like response, the brain-mediated response to light became predominant when the brain regenerated, the researchers from the Institute for Stem Cell Biology and Regenerative Medicine (inStem) at the National Centre for Biological Sciences (NCBS) found. The results were published in the journal Science Advances.
Colour-blind but can choose colours
Flatworms (planarians) cannot tolerate any colour or amount of light and tend to move away from it. But when exposed to two colours, the worms were able to fully discriminate different colours. For instance, when exposed to blue and green light of same intensity, the worms always tend to move towards green. In the case of red and green lights, the worms moved away from green light.
“Individually, the worms hate both the colour lights used. Since they are forced to make a choice, the worms were running away from a colour they hate more,” says Dr. Akash Gulyani, from inStem and the corresponding author of the paper. “Since they hate light they can move towards any colour. But we found all the worms were always moving to a specific colour.”
Surprisingly, the worms were able to efficiently differentiate light with just 25 nanometre wavelength difference. The worms showed maximum avoidance to light with 450-500 nanometre wavelength, with avoidance dropping off on either side of the peak values. So they preferred 425 nm over 450 nm, and 525 nm over 500 nm, 545 nm over 525 nm and so on.
How flatworms see colours
The researchers proved that the choice of colour was not based on wavelength but on light intensity. The worms that initially preferred 545 nm over 500 nm reversed their choice when the intensity of the 545 nm light alone was increased. “This proves that worms are converting the different wavelength information into intensity information (which is the number of photons absorbed) and trying to discriminate between two wavelengths,” says Nishan Shettigar from inStem and the first author of the paper.
“Though they were choosing one colour over the other, they were not truly sensing colour. They were comparing the two lights and converting the difference in effective intensity of light into clear behavioural choices. They have acute sensing and could convert even small difference in the amount of light absorbed,” Dr. Gulyani says.
And light sensing returns
Since the worms were able to sense very small difference in wavelength and process the information, the researchers concluded that the brain was involved. To confirm this, they took advantage of the worms’ ability to regenerate any body part and cut the head to study how light-sensing returns.
After five days, a basic eye structure and part of the brain were formed and the worms were able to sense light and move to darkness. But the worms were not able to differentiate between two colours. “Light sensing gets restored by day five but ability to discriminate colours based on intensity is lacking,” says Dr. Gulyani. Ability to do fine discrimination of colours with 25 nm difference gets restored only after 12 days.
The whole body senses UV light
The researchers also found that decapitated worms can respond to small amount of long wavelength UV light. “They seem to have light sensors for long UV light all over the body which allows the decapitated worms to move after sensing UV light,” Dr. Gulyani says.
The worms’ ability to sense UV light was a chance discovery. “One night I was discussing our work with a few summer interns. Unintentionally I used a UV filter in the microscope while studying a cut tail piece and found the tail piece moving. But when I used a blue filter the tail did not move. The next day we verified the experiment. Other researchers had observed this behaviour since 1900 but nobody had probed it further,” recalls Shettigar.
The researchers found the brain-mediated light-sensing network can override the eye-independent response in the case of intact worms, while the eye-independent response dominates when the head is cut or when the brain is damaged.
“We initiated this work once I met Dr. Dasaradhi Palakodeti, a scientist studying regeneration in planarians at inStem. This is a beautiful example of how curiosity-driven experiments, initiated by people from different backgrounds working together, can lead to scientific breakthroughs,” says Dr. Gulyani.
As flatworms have similar simple eye structures and neural networks to those observed in other animals, the authors say these complex light sensing and processing abilities may be much more widespread in nature than previously believed.
“Our results now make it possible for us to use flatworms as a model for eye-brain regeneration, where the exact function of the eye and brain can be studied as regeneration takes place and link it to the recovery of a specific function. We can identify the molecules and genes that are responsible for functional eye regeneration in flatworms and test for their importance in human disease models and human stem cells,” says Dr. Gulyani. “Researchers are already using human stem cells to make specific eye cells and eye tissues. So we can see if the genes identified in flatworms are also important in human eye tissues. And if they are important then the genes and molecules would have potential applications in humans.
The labs of Dr. Gulyani and Dr. Palakodeti are actively collaborating to find genes and molecules that control regeneration.