Sensing the light, but not to see: Study offers insight on the evolution of photosensitive cells

In a primitive marine organism, MBL scientists find photosensitive cells that may be ancestral to the “circadian receptors” in the mammalian retina.

Among the animals that are appealing “cover models” for scientific journals, lancelets don’t spring readily to mind. Slender, limbless, primitive blobs that look pretty much the same end to end, lancelets “are extremely boring. I wouldn’t recommend them for a home aquarium,” says Enrico Nasi, adjunct senior scientist at the Marine Biological Laboratory (MBL). Yet Nasi and his collaborators managed to land a lancelet on the cover of the Journal of Neuroscience last December. These simple chordates, they discovered, offer insight into our own biological clocks.

Nasi and his wife, MBL adjunct scientist Maria del Pilar Gomez, are interested in phototransduction, the conversion of light by light-sensitive cells into electrical signals that are sent to the brain. The lancelet, also called amphioxus, doesn’t have eyes or a true brain. But what it does have in surprising abundance is melanopsin, a photopigment that is also produced by the third class of light-sensitive cells in the mammalian retina, besides the rods and cones. This third class of cells, called “intrinsically photosensitive retinal ganglion cells” (ipRGCs), were discovered in 2002 by Brown University’s David Berson and colleagues. Now sometimes called “circadian receptors,” they are involved in non-visual, light-dependent functions, such as adjustment of the animal’s circadian rhythms.

"It seemed like colossal overkill that amphioxus have melanopsin-producing cells," Nasi says. "These animals do nothing. If you switch on a light, they dance and float to the top of the tank, and then they drop back down to the bottom. That’s it for the day." But that mystery aside, Gomez and Nasi realized that studying amphioxus could help reveal the evolutionary history of the circadian receptors.