According to an international team of scientists led by Dr. Nicholas Mundy at the University of Cambridge, a gene for red color vision that originated in the reptile lineage approximately 250 million years ago has resulted in the red bird feathers and ‘painted’ turtles, and may be evidence that dinosaurs could see as many shades of red as birds.
Torvosaurus gurneyi. Image credit: © Sergey Krasovskiy.
Earlier this year, two groups of scientists independently identified an enzyme-encoding gene that enables birds to produce and display the color red.
Now, a new study by one of these groups shows the same gene, called CYP2J19, is also found in turtles, which share an ancient common ancestor with birds. Both share a common ancestor with dinosaurs.
CYP2J19 allows birds and turtles to convert the yellow pigments in their diets into red, which they then use to heighten color vision in the red spectrum through droplets of red oil in their retinas.
Birds and turtles are the only existing land vertebrates to have these red retinal oil droplets. In some birds and a few turtle species, red pigment produced by the gene is also used for external display: red beaks and feathers, or the red neck patches and rims of shells seen in species such as the painted turtle.
Dr. Mundy, senior author on the study, and his colleagues from the United Kingdom, the United States and Sweden mined the genetic data of various bird and reptile species to reconstruct an evolutionary history of CYP2J19, and found that it dated back hundreds of millions of years in the ancient archelosaur genetic line – the ancestral lineage of turtles, birds and dinosaurs.
The findings provide evidence that CYP2J19 originated around 250 million years ago, predating the split of the turtle lineage from the archosaur line, and runs right the way through turtle and bird evolution.
“As dinosaurs split from this lineage after turtles, and were closely related to birds, this strongly suggests that they would have carried the CYP2J19 gene, and had the enhanced ‘red vision’ from the red retinal oil,” Dr. Mundy and co-authors said.
“This may have even resulted in some dinosaurs producing bright red pigment for display purposes as well as color vision, as seen in some birds and turtles today, although this is more speculative.”
“These findings are evidence that the red gene originated in the archelosaur lineage to produce red for color vision, and was much later independently deployed in both birds and turtles to be displayed in the red feathers and shells of some species, going from seeing red to being red.”
“This external redness was often sexually selected as an ‘honest signal’ of genuine high quality in a mate.”
Previous research in zebra finches showed a possible link between red beaks and the ability to break down toxins in the body, suggesting external redness signals physiological quality, and there is some evidence that coloration in red-eared terrapins is also linked to honest signaling.
“The excellent red spectrum vision provided by the CYP2J19 gene would help female birds and turtles pick the brightest red males,” said study first author Hanlu Twyman, also from the University of Cambridge.
The structure of retinas in the eye includes cone-shaped photoreceptor cells.
Unlike mammals, avian and turtle retinal cones contain a range of brightly-colored oil droplets, including green, yellow and red. These oil droplets function in a similar way to filters on a camera lens.
“By filtering the incoming light, the oil droplets lead to greater separation of the range of wavelengths that each cone responds to, creating much better color sensitivity,” Dr. Mundy said.
“Humans can distinguish between some shades of red such as scarlet and crimson. However, birds and turtles can see a host of intermediate reds between these two shades, for example.”
“Our work suggests that dinosaurs would have also had this ability to see a wide spectrum of redness.”
Over hundreds of millennia of evolution, CYP2J19 was independently deployed to generate the red pigments in the external displays of some bird species and a few turtle species. Co-option of CYP2J19 for red coloration in dinosaurs would also have been possible.
The ancestral lineage that led to scaly lizards and snakes split from the archosaur line before turtles, and, as the findings suggest, before the origin of the red gene. These reptiles either lack retinal oil droplets, or have yellow and green but not red.
However, the crocodilian lineage split from the archelosaur line after turtles, yet crocodiles appear to have lost the CYP2J19 gene, and have no oil droplets of any color in their retinal cones.
“There is some evidence that oil droplets were lost from the retinas of species that were nocturnal for long periods of their genetic past, and that this hypothesis fits for mammals and snakes, and may also be the case with crocodiles,” Dr. Mundy said.
The team’s findings were published online today in the journal Proceedings of the Royal Society B.
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Hanlu Twyman et al. 2016. Seeing red to being red: conserved genetic mechanism for red cone oil droplets and co-option for red coloration in birds and turtles. Proc. R. Soc. B 283 (1836): 20161208; doi: 10.1098/rspb.2016.1208
