Researchers discover extraocular light sensing system in flatworms

Hyderabad: In what could be the light at the end of the proverbial tunnel, a research team from Department of Biochemistry, School of Life Sciences at University of Hyderabad (UoH), has demonstrated that an eye-independent system (extraocular) lining the periphery of the worm's body allows even a headless flatworm to move like an intact worm with incredible coordination.

The discovery of such sensitive natural light sensing proteins has potential applications in equipping visually challenged people with the ability to 'sense' light and to control inner working of cells/tissues with light (optogenetics), according to the authors.

In their studies, the team headed by Dr Akash Gulyani, found that the worm body is dotted with a whole array of very unique light sensing cells that are patterned all over the worm, especially at the periphery of the worm.

Cells within this eye-independent system produced light-sensitive proteins called opsins which help the flatworm to respond to light even in the absence of eyes. However, the eye-independent system only responds to a limited range of ultraviolet light at 365 to 395 nm, whereas the flatworm eyes can detect a broad wavelength of visible light (365 to 625 nm).

"Interestingly, the eye-independent system only arises in the adult organisms, unlike the standard set of eyes that develop in the embryo", notes the study published in the Proceedings of the National Academy of Sciences (USA).

This discovery of such an array of body-wide light sensing cells that link to the body-wide nervous system and allow headless worms to move, may constitute the uncovering of a new kind of body-wide organ system for sensing light. This discovery is likely amongst one of the most sensitive eye-independent light sensing systems in nature, a UoH statement read.

Intriguingly, these newly discovered light sensing cells appear unique as they do not resemble any neuron-like cells but are more similar to a distinct cellular class (parenchymal cells) that includes glia-like cells, which are generally thought to have a supporting rather than a sensory role. This seems very distinct from all light sensing systems known so far across the animal kingdom.

Planarian flatworms are light-aversive and are known to rely on two sensitive eyes connected to a simple brain to sense light and play a crucial role in controlling their behavior and movement.

The team comprising Nishan Shettigar, Anirudh Chakravarthy, Suchitta Umashankar, Vairavan Lakshmanan, Dasaradhi Palakodeti, followed up on previous research that shows planarians can survive decapitation (removal of the head) and retain the ability to move away from the light source when exposed to low doses of ultraviolet light.