How early mammals evolved night vision to avoid predators
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 published 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.
section of a mouse retina showing rod photoreceptors (green) and cone
photoreceptors (magenta). |
Cell nuclei are stained in blue [Credit: Jessica Gumerson]
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.
photoreceptors (green) in a slice from a mouse retina. A majority of
the photoreceptors (97%) in the retina |
are rods (black). The black layer on top of the photoreceptors is retinal pigment epithelium
[Credit: Jung-Woong Kim]
The researchers saw that in early stages, two days after the mice were born, developing rod cells expressed genes normally seen in mature short-wavelength cones (which are used in other animals to detect ultraviolet light). When the researchers examined the epigenetics of purified rod cells from mice, they saw that these aspects became repressed by histone and DNA methylations later in development, ten days after the mice were born.
The team concluded that in mammals, the transcription factor NRL became restricted to the photoreceptors in the eye, forcing the cells to change from cones to rods and giving early mammals the edge they needed to take up an active nighttime lifestyle. (Counter-intuitively, humans depend more on cones for our vision, but that's because our ancestors later evolved to take advantage of the daylight hours again.)
"These rod photoreceptors retain the molecular footprint of short-wavelength cones," says Swaroop. "We've provided evidence that by acquiring the regulatory elements for NRL to shift short-wavelength cones into rods, early mammals changed one type of cell from capturing UV light--which isn't necessary at night--to something that is just extremely sensitive to light."
Source: Cell Press [June 20, 2016]