A scanning electron micrograph of the photoreceptors in a garter snake retina. Photo credit: Belinda Chang.
Snakes have a remarkably diverse range of eye designs, from moveable lenses for focusing, to slit pupils in species that specialise in a nocturnal lifestyle. The light-sensitive retina at the back of the eye can be equally varied, with some retinas composed entirely of rod photoreceptors packed with highly light-sensitive rhodopsin protein, while others depend exclusively on bright colour-sensing cones that use other less-sensitive photopigments. ‘Intriguingly, retinal composition seems to shift between closely related species’, says Belinda Chang from the University of Toronto, Canada, who explains that some close relatives have retinas that are composed entirely of rod cells alone, while others depend exclusively on cones. ‘In order to explain this, Gordon Wall proposed the theory of transmutation, which hypothesised that instead of losing and re-evolving photoreceptor cell types… photoreceptors could be evolutionarily transformed between rod and cone cell types’, explains Chang. Initially, this transformation was confirmed in one species of nocturnal gecko; however, Chang and her colleagues recently discovered rhodopsin in the cone cells of day-active garter snakes. Intrigued by the possibility that other members of the colubrid snake family may also have converted rods into cones, Chang and students Benedict Darren and Nihar Bhattacharyya began investigating the retinas of northern pine snakes.
‘These snakes are a good species [to investigate] because they have an all-cone retina and they can burrow and hunt underground, which suggests that dim light vision – supplied by rods – could be useful to the species’, says Bhattacharyya. Searching for evidence of the photopigment genes that are essential for vision in the snake's cone-packed retina, Darren found three. And when he analysed the sequences of the genes, he and Ryan Schott realised that two of the genes would produce the coloured opsins that are always found in cones – one that responds to green wavelengths and a second tuned to violet tones. However, the third gene was identical to the rhodopsin genes that are usually found only in rod cells.
Wondering whether the gene was still active and capable of producing the key rod protein in the snake's cone retina, Bhattacharyya and Vince Tropepe painstakingly sliced thin sections from the back of the snake's eye then incubated them with an antibody that only binds rhodopsin proteins, revealing that the rhodopsin protein was present in the pine snake's retina. It was beginning to look as though rod cells in the pine snake retina had transmuted into cones.
However, Chang still needed to be convinced that the rhodopsin pigment was capable of responding to light. Having tackled the challenging task of artificially producing and purifying the protein, Bhattacharyya was then able to test the protein's response to light and was impressed when its light absorption pattern was essentially the same as that of other rhodopsins. However, when they checked the protein's reaction with the chemical hydroxylamine – which is traditionally used to identify cone cells – Bhattacharyya and Chang were amazed to see the chemical react as if the pine snake rhodopsin was a cone pigment.
Considering their observations, Chang says, ‘We have found striking evidence in the pine snake of the “relics” of rod photoreceptor origins in their all-cone retina’, adding, ‘This would suggest that the ability to have dim light vision might have been useful to the snake’. She also suspects that the evolutionary transformation of rods into cones may be widespread in colubrid snakes and that the rod proteins have become more cone-like over time.
