The Diversity and Adaptive Evolution of Visual Photopigments in Reptiles

Abstract

Reptiles are a highly diverse class that consists of snakes, geckos, iguanid lizards, and chameleons among others. Given their unique phylogenetic position in relation to both birds and mammals, reptiles are interesting animal models with which to decipher the evolution of vertebrate photopigments (opsin protein plus a light-sensitive retinal chromophore) and their contribution to vision. Reptiles possess different types of retinae that are defined primarily by variations in photoreceptor morphology, which range from pure-cone to rod-dominated retinae with many species possessing duplex (rods and cones) retinae. In most cases, the type of retina is thought to reflect both the lifestyle and the behavior of the animal, which can vary between diurnal, nocturnal, or crepuscular behavioral activities. Reptiles, and in particular geckos and snakes, have been used as prime examples for the "transmutation" hypothesis proposed by Walls in the 1930s-1940s, which postulates that some reptilian species have migrated from diurnality to nocturnality, before subsequently returning to diurnal activities once again. This theory further states that these behavioral changes are reflected in subsequent changes in photoreceptor morphology and function from cones to rods, with a return to cone-like photoreceptors once again. Modern sequencing techniques have further investigated the "transmutation" hypothesis by using molecular biology to study the phototransduction cascades of rod-and cone-like photoreceptors in the reptilian retina. This review will discuss what is currently known about the evolution of opsin-based photopigments in reptiles, relating habitat to photoreceptor morphology, as well as opsin and phototransduction cascade gene expression.

Short-wavelength-sensitive 2 (Sws2) visual photopigment models combined with atomistic molecular simulations to predict spectral peaks of absorbance

“…In some vertebrates (e.g. lampreys [ 31 , 44 , 45 ], many freshwater teleosts [ 46 ], lungfishes [ 47 , 48 ], the green anole lizard Anolis carolinensis [ 49 , 50 ]; reviewed in [ 1 – 3 , 51 ], the visual system is based on porphyropsins that incorporate a vitamin-A 2 -derived chromophore or a combination of both rhodopsins and porphyropsins. Therefore, within the context of biological relevance, the predicted λ max values that result using our approach may have to be converted, where appropriate, to longer wavelengths to account for the possession of a vitamin-A 2 -derived chromophore in native photopigments.…”

Size of Snake Eyes Correlates With Habitat Types and Diel Activity Patterns

“…There might be up to five visual opsins present in the ancestral vertebrate (i.e., RH1, RH2, SWS1, SWS2, and LWS; Simões et al, 2015). While the snakes that belong to the basal lineage, scolecophidians, only express RH1 genes likely as an adaptation of fossorial habits, most of the stem snakes express the additional two opsin genes that enable dichromatic color vision (i.e., SWS1 and LWS) (Tu, 2004;Davies et al, 2009;Simões et al, 2015;Katti et al, 2019). The better visual ability, as the result of the development of larger eyes (Corral-López et al, 2017), could have further advanced the performance of how they explore and utilize resources in the environment.…”

“…Currently, the evolution of eye size has been studied thoroughly in many taxa [e.g., fishes (Caves et al, 2017;Vinterstare et al, 2020), frogs (Shrimpton et al, 2021), geckos (Werner and Seifan, 2006), birds (Thomas et al, 2006;Burton, 2008) and mammals (Kirk, 2006;Heard-Booth and Kirk, 2012;Nummela et al, 2013)]. However, it is relatively underexplored how the eye size across different snake species is shaped by the environment, facilitating their demands for behavioral activities and foraging strategies (Katti et al, 2019). For example, it is unclear whether the size of eyes is different in the snakes adopting active and sedentary foraging strategies.…”

Not just shades of grey: life is full of colour for the ocellate river stingray (Potamotrygon motoro)

“…The activity of rods and cones under these mid-illumination conditions are likely to be species-specific, so further study is required to determine the potential for any mesopic-based activity of P. motoro photoreceptors. If deemed to be correct, however, mesopia may result in the potential for increased wavelength discrimination and the presence of 'conditional trichromacy' in P. motoro (as hypotheised for others vertebrates in Davies et al (2012), Davies et al (2009b), Katti et al (2019), Katti et al (2018) and Theiss et al (2012)), with an rh2/rh1 overlap at 479 nm at a normalised absorbance level at 92% (Fig. 5, line (i)) and an rh1/lws overlap at 529 nm at a normalised absorbance level at 71% (Fig.…”