Bengaluru-based researchers have found that flatworms, 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.
The two light-sensing responses — brain-mediated network and eye-independent response — seen in flatworms ( Schmidtea mediterranea ) show “a typical hierarchy of dominance”.
While the light sensors spread across the body (eye-independent response) helped the decapitated worms 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
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Flatworms have similar simple eye structures and neural networks to those observed in other animals, the authors say, and the 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. 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. Akash Gulyani, the corresponding author of the paper.
Flatworms 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 between them.
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For instance, when exposed to blue and green light of the same intensity, the worms always tended to move towards green. “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. Gulyani.
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.
Sensing light intensity
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 and trying to discriminate between two wavelengths,” says Nishan Shettigar, the first author of the paper.
To confirm if the brain was involved in sensing and processing a very small difference in wavelength, the researchers 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 again, and the worms were able to sense light and move to darkness. But the worms were not able to differentiate between two colours. Ability to do fine discrimination of colours with 25 nm difference was restored after 12 days. The researchers also found that decapitated worms could respond to a small amount of long wavelength UV light. “They seem to have sensors for long UV light all over the body, enabling decapitated worms to move after sensing UV light,” Dr. Gulyani says.