Evolution of snake vision

A new study by researchers from the Natural History Museum and City, University of London has analysed the genes involved in snake vision to better understand how their sight evolved.

The research highlights the incredible diversity present in their visual systems – many of which are not known in other vertebrates – and could improve our knowledge of snake origins and the evolution of the vertebrate visual system.

There are approximately 3,500 species of snake spread across all continents, with the exception of Antarctica. Snakes also display a greater ecological diversity than other vertebrates and include species that burrow, glide, are fully aquatic and also those which are active only during the day or night.

This is also reflected in their visual systems, as over time snakes have evolved major differences in how their eyes detect and transmit light. In particular this is seen in the light-sensitive proteins – known as opsins - found in the photoreceptors present in their eyes.

To find out more about how the gene sequences of these opsin proteins and other parts of the visual system have changed over evolutionary time, the team analysed the gene sequences of 69 species of snake from around the world, sampling a wide variety of anatomically distinct species.

Led by researchers at the Natural History Museum, the study – which included Professor Ron Douglas from City, University of London - also looked specifically at how light passed through the lenses in their eyes to further investigate how snake visual evolution occurred.

The researchers found that while the vast majority of snakes have retained three of the visual opsin genes likely to have been present in the ancestral snake - known as rh1, sws1, lws - these have undergone considerable diversification through changes to the their genetic code.

The team also found that there were changes in the way that some snake eyes transmit light. In particular some species filtered the shorter wavelengths of light towards the blue and ultraviolet ends of the spectrum. This affects the sensitivity of the retina and could be useful for species active at night as it could aid vision in low light conditions. The paper was published in the journal for Molecular Biology and Evolution.

Professor Ron Douglas, from the Department of Optometry and Visual Sciences at City, University of London and an expert in visual systems who contributed data to the study, said:

“The eyes of snakes are one of the most interesting cases of visual adaptation among vertebrates, but for many years they have remained overlooked. What we found in the study is that snakes display remarkable diversity when it comes to their visual systems, and in particular the way that their eyes detect and transmit light. As a result, snakes are clearly an important system for understanding of the evolution of the vertebrate visual system and further research is needed to clarify the role they have played.”