When early mammals began scurrying around, they had one major threat to contend with: dinosaurs. To get around this tricky situation and avoid ending up on the dinosaurs’ plate, the tiny animals are thought to have taken to foraging at night time, seeking out the insects on which they likely preyed. But this then raises the question of how the early mammals evolved the night vision needed to find their food and survive.
New research, published in the journal Developmental Cell, suggests that the rod cells that are responsible for sensing light, and thus giving good night vision, originally developed from the color-detecting cone cells that enabled their ancestors to see ultraviolet light. They found that there has been a genetic shift that has allowed one to develop into the other, which in turn gave the early mammals the ability to see in low light conditions, and thus avoid their dinosaur predators that were active mainly during the day.
“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?’” explains 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.”
Early mammals, such as this rendering of Purgatorius unio, were likely nocturnal insectivores. Nobu Tamura
Considering that the mammal ancestors only had these cone receptors, the researchers looked into how the short-wave versions of the cones, which allow the animals to see UV light, were converted into the rod cells, which are able to detect just a single photon of light. Previously, the research group has found that a specific transcription factor known as NRL directs cells in the retina to mature into rod cells by suppressing the genes that are involved in the development of cone cells.
By turning to developing mouse embryos, the researchers were able to get a glimpse into the evolutionary changes that occurred to develop the night vision. They found that in early embryos, two days after they were born, the developing rod cells expressed genes normally associated with the cone cells, but that by the time the mice were 10 days old, these aspects had been repressed by histone and DNA methylations.
The researchers then looked to see if they could use genetics to see if they could figure out when this shift from cone to rod cells occurred in mammals. They found that in placental mammals, the gene responsible for the regulation of NRL was far more developed, and has been lost in several other non-mammalian groups.