Boosting insight into how animals see
Biologist Belinda Chang finds secrets to animal evolution in their eyes
Humans sometimes describe eyes as the window to the soul, but for most animals, eyes are all about vision and vision is all about survival.
Evolutionary biologist Belinda Chang leads a lab that is devoted to understanding how animals see, and how their vision evolves and changes based on their needs to adapt to their environment.
One area of particular focus for the lab is the visual pigments in the retina’s photoreceptor cells that absorb light and enable vision. The activation of a visual pigment is the first step in a chain of events that sends a signal to the brain that light, and therefore an image, has been perceived. When there is a change in the properties of visual pigments, it can have profound consequences on an animal’s ability to see and survive.
“Most animals, including humans, have two types of photoreceptor cells — rods and cones — in their retinas. Rods contain a visual pigment called rhodopsin and are sensitive to dim light, while cones differ in their molecular machinery, including different opsins, that allow them to operate under bright light conditions,” said Chang, an associate professor in the Departments of Ecology & Evolutionary Biology and Cell & Systems Biology.
“This combination of visual pigments allows animals to see in both bright and dim conditions. However, there are unusual exceptions. In some animals, only one type is present.”
This is the case, for example, with the Western ribbon garter snake, the subject of a recent paper by Chang’s group published in Proceedings of the National Academy of Sciences in January. The animal, whose habitat stretches from the central United States south to Central America, is active only in daylight and has previously been found to have only cones in its retinas.
The snake debate: land or sea origins?
Chang’s group – which features a combination of undergraduate and graduate students and postdoctoral researchers – had been intrigued by a 70-year-old theory that through evolution, rods could transform into cones and vice versa in a process known as ‘transmutation.’ They wondered if such a process led to the absence of rods in the snakes, hypothesizing that somewhere along the line they sacrificed their ability to see well in dim light in order to improve their daylight vision. With the origin of all-cone retinas in this particular snake still a mystery, Chang decided to test that theory.
“At some point in its evolution, this garter snake lost two of four ancestral cones,” said Ryan Schott, a PhD candidate in Chang’s lab. “This likely happened during an early, possibly burrowing underground phase of snake evolution, long before they came to have all-cone retinas. Living in dim light, they would have been more reliant on rhodopsin found in their rod cells.”
Using a variety of experimental approaches, Chang and her team tested two competing hypotheses to determine what had happened to those rods when the snake later evolved into an above-ground creature. They found two surprising things. First, rhodopsin, which is only supposed to be found in rods, was indeed present in some of the snake’s cone cells. Second, those cone cells had structural features that were remarkably similar to those found in rods.
“The results suggest that they are not true cones, but are in fact modified rods.” said Nihar Bhattacharya, a PhD candidate in Chang’s lab.
But there was another twist: the modified cone-like rods may have enabled snakes to see colour in day light, making it another aspect of restoring a functionality that disappeared with the loss of some ancestral cones, during an early burrowing phase.
Chang says the rare transformation has implications for how complex cellular types can arise in sensory systems.
“Physiological constraints imposed by historical losses can actually be shaped by selective forces to produce remarkable evolutionary benefits.”
Windows into the past: looking for clues by recreating ancestral genes
Chang is known for being especially adept at recreating the ancestral genes of an animal. This enables her to get the actual picture of a genetic structure at a specific point in time and so better understand how evolutionary changes contribute to an animal’s overall fitness and survival.
The snake eyes study was only one in a series of successive advances by Chang and her colleagues in understanding the evolution of vision across the animal kingdom. In another study published in the February issue of Molecular Biology and Evolution, the team described how the rhodopsin protein evolved in killer whales to improve their ability to see underwater in the predominantly blue-tinted light.
Sarah Dungan, another PhD candidate in Chang’s lab and lead author of the paper elaborates.
“Whales are particularly reliant on rhodopsin because light fades very quickly as they swim deeper,” said Dungan. “But the majority of light in the ocean is also blue, so for a diving animal, the fact that rhodopsin is extra-sensitive in the blue part of the spectrum enables the eyes to make the most of the scarce light.
”And being able to see well means the difference between catching your prey and going hungry.”