The first major study of visual pigment genes and lenses in snakes has found that the reptiles match their vision to their lifestyles.
The Montpellier snake (Malpolon monspessulanus), one of the snake species studied by the team. Image credit: Alexandre Roux / CC BY-SA 2.0.
Researchers have long known that snakes have highly variable sets of rods and cones – the specialized cells in the retina that an animal uses to detect light. But until now, most modern studies of vision in vertebrates have concentrated on mammals, birds and fish.
“There are more than 3,500 living species of snakes, with very diverse lifestyles,” said study senior author Dr. David Gower, an evolutionary biologist at the Natural History Museum, London, UK.
“Most modern work on the genetics of vision has been done on mammals, birds and fish. But studying snakes’ eyes is important for a more accurate and complete understanding of how vision functions and has evolved in vertebrates more generally.”
To investigate snake visual evolution, Dr. Gower and his colleagues from the United Kingdom, India and Australia examined the genes involved in producing visual pigments in 69 different species of snakes – as the genes vary from species to species so does the exact molecular structure of the pigments and the wavelengths of light they absorb.
They discovered that most snakes express three visual opsin genes (rh1, sws1, and lws) and are likely dichromatic in daylight – seeing two primary colors rather than the three that most humans see.
However, the team also discovered that the genes have undergone a great amount of evolution, including many changes to the wavelengths of light that the pigments are sensitive to, in order to suit the diversity of lifestyles that snakes have evolved.
“These opsin genes (especially rh1 and sws1) have undergone much evolutionary change, including modifications of amino acid residues at sites of known importance for spectral tuning, with several tuning site combinations unknown elsewhere among vertebrates,” the researchers said.
“These changes are particularly common among dipsadine and colubrine ‘higher’ snakes.”
Most snakes examined in the new study are sensitive to UV light, which likely allows them to see well in low light conditions.
For light to reach the retina and be absorbed by the pigments, it first travels through the lens of the eye. Snakes with UV-sensitive visual pigments therefore have lenses that let UV light though.
In contrast, the research showed that those snakes that rely on their eyesight to hunt in the daytime, such as the gliding golden tree snake (Chrysopelea ornata) and the Montpellier snake (Malpolon monspessulanus), have lenses that block UV light.
As well as perhaps helping to protect their eyes from damage, this likely helps sharpen their sight – in the same way that skiers’ yellow goggles cut out some blue light and improve contrast.
Moreover, these snakes with UV-filtering lenses have tuned the pigments in their retina so that they are no longer sensitive to the short UV light, but absorb longer wavelengths.
All nocturnal species examined — such as the glossy snake (Arizona elegans) — were found to have lenses that do not filter UV.
Some snake species active in daylight also lack a UV-filtering lens, perhaps because they are less reliant on very sharp vision or live in places without very bright light.
The study concluded also that the most recent ancestor of all living snakes had UV-sensitive vision.
“The precise nature of the ancestral snake is contentious, but the evidence from vision is consistent with the idea that it was adapted to living in low light conditions on land,” Dr. Gower said.
The team’s findings were published in the October 2016 issue of the journal Molecular Biology and Evolution.
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Bruno F. Simões et al. 2016. Visual Pigments, Ocular Filters and the Evolution of Snake Vision. Mol Biol Evol 33 (10): 2483-2495; doi: 10.1093/molbev/msw148
