How early mammals evolved night vision to avoid predators

Date: June 20, 2016

Source: Cell Press

Early mammals evolved in a burst during the Jurassic period, adapting a nocturnal lifestyle when dinosaurs were the dominant daytime predator. How these early mammals evolved night vision to find food and survive has been a mystery, but a new study publishing June 20 in Developmental Cell suggests that rods in the mammalian eye, extremely sensitive to light, developed from color-detecting cone cells during this time to give mammals an edge in low-light conditions.

Cone cells are specialized for certain wavelengths of light to help animals detect color, while rods can detect even a single photon and are specialized for low-light vision. "The majority of mammals have rod-dominant retinas, but if you look at fish, frogs, or birds, the vast majority are cone-dominated--so the evolutionary question has always been, 'What happened?'" says Anand Swaroop, a retina biologist at the National Eye Institute, part of the National Institutes of Health. "We've been working for a long time to understand the fundamental mechanisms behind rod and cone development."

Previous work done by Swaroop and his colleagues showed that a transcription factor called NRL pushes cells in the retina toward maturing into rods by suppressing genes involved in cone development. "We began to wonder if, somehow, the short-wavelength cones were converted into rods during evolution," says Swaroop.

To investigate the origin of rods in mammals, Swaroop and his team examined rod and cone cells taken from mice at different stages of development. Details of an organism's embryonic development often reveal traits carried by its evolutionary ancestors; consider, for instance, how human embryos initially develop gill-like slits and a tail.

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