Evolutionary transformation of rod photoreceptors in the all-cone retina of a diurnal garter snake

Abstract

Vertebrate retinas are generally composed of rod (dim-light) and cone (bright-light) photoreceptors with distinct morphologies that evolved as adaptations to nocturnal/crepuscular and diurnal light environments. Over 70 years ago, the "transmutation" theory was proposed to explain some of the rare exceptions in which a photoreceptor type is missing, suggesting that photoreceptors could evolutionarily transition between cell types. Although studies have shown support for this theory in nocturnal geckos, the origins of allcone retinas, such as those found in diurnal colubrid snakes, remain a mystery. Here we investigate the evolutionary fate of the rods in a diurnal garter snake and test two competing hypotheses: (i) that the rods, and their corresponding molecular machinery, were lost or (ii) that the rods were evolutionarily modified to resemble, and function, as cones. Using multiple approaches, we find evidence for a functional and unusually blue-shifted rhodopsin that is expressed in small single "cones." Moreover, these cones express rod transducin and have rod ultrastructural features, providing strong support for the hypothesis that they are not true cones, as previously thought, but rather are modified rods. Several intriguing features of garter snake rhodopsin are suggestive of a more cone-like function. We propose that these cone-like rods may have evolved to regain spectral sensitivity and chromatic discrimination as a result of ancestral losses of middle-wavelength cone opsins in early snake evolution. This study illustrates how sensory evolution can be shaped not only by environmental constraints but also by historical contingency in forming new cell types with convergent functionality. rhodopsin evolution | visual evolution | reptile vision | snake photoreceptors | visual pigment H ow complex structures can arise has long fascinated evolutionary biologists, and the evolution of the eye, as noted by Charles Darwin (1), is perhaps the most famous example. Within the vertebrate eye, the light-sensing photoreceptors are complex, highly specialized cellular structures that can be divided into two general types based on their distinct morphologies and functions: cones, which are active during the day and contain cone opsin pigments, and rods, which mediate dim-light vision and contain rhodopsin (RH1) (2-4). The visual pigments contained in cone photoreceptors are classified into four different subtypes that mediate vision across the visible spectrum from the UV to the red (2). Although most vertebrate retinas are duplex, containing both cones and rods, squamate reptiles (lizards and snakes) are unusual, not only in having highly variable photoreceptor morphologies, but also for several instances of the absence of an entire class of photoreceptors, resulting in simplex retinas composed of only cones or rods (4).In a seminal book published in 1942, Walls (4) hypothesized that, during evolution, vertebrate photoreceptors could transform from one type to another, a process that he termed photoreceptor "transm...

Cone-like rhodopsin expressed in the all cone retina of the colubrid pine snake as a potential adaptation to diurnality

“…Pituophis melanoleucus inhabits the eastern half of the United States and Canada (Stull, 1940) and spends relatively short intervals on the surface during the day to forage for prey such as small mammals and birds, and to create new burrows (Diller and Wallace, 1996;Himes, 2001). While P. melanoleucus has been found to possess an all-cone retina (Caprette, 2005), similar to previous diurnal colubrid snakes studied (Schott et al, 2016;Sillman et al, 1997), unlike other strongly diurnal colubrids such as the garter snake, P. melanoleucus is more secretive and is thought to spend a considerable amount of time burrowing (Gerald et al, 2006).…”

“…Early studies of the colubrid visual system found a green-sensitive visual pigment in addition to a red and a blue pigment (Sillman et al, 1997) in the simplex retina, but were unable to distinguish between a spectrally shifted rhodopsin in a transmuted rod or a potentially resurrected RH2 cone opsin (Cortesi et al, 2015). More recently, a study from our group identified a functional blue-shifted RH1 pigment in the all-cone retina of the ribbon snake (Thamnophis proximus), and proposed that this resulted from a rod to cone evolutionary transmutation in colubrid snakes that may have allowed for enhanced spectral discrimination and even trichromatic colour vision (Schott et al, 2016). A recent study that sequenced the opsins of several other colubrid snake species discovered the widespread presence of full-length rhodopsin genes in species with supposed simplex retinas that were previously presumed to have lost rods or rhodopsins (Simões et al, 2016).…”

“…The family Colubridae is the most speciose family of snakes and encompasses a diverse range of lifestyles and ecologies. Colubrid snakes have recently emerged as a compelling group in which to study visual system evolution and adaptation (Schott et al, 2016;Simões et al, 2015Simões et al, , 2016.…”

Multiple rod–cone and cone–rod photoreceptor transmutations in snakes: evidence from visual opsin gene expression

“…(reported in [19,20]) detected likely photopic (typical cone) visual pigments identified from their spectral sensitivities by microspectrophotometry (MSP). (iii) Chang and co-workers [21,22] l max values, we suggest the former two are likely LWSand SWS1-based visual pigments, respectively. The l max of the 458 nm pigment suggests either an SWS2-or RH2-based pigment but because neither of these have been found in snakes [10][11][12] we consider it more likely to be an RH1-based pigment (with a lower l max value than is typical for vertebrate RH1-based pigments).…”

A second visual rhodopsin gene,rh1-2, is expressed in zebrafish photoreceptors and found in other ray-finned fishes

“…In order to function as a visual rhodopsin, rh1-2 would likely have to be expressed in rods; however, expression in cones is also possible as there are rare cases where rhodopsins and cone opsins are expressed in the opposing photoreceptor type in reptiles and amphibians (Kojima et al, 1992;McDevitt et al, 1993;Schott et al, 2016). While a ∼496 nm peak corresponding to Rh1-2 was not detected in previous microspectrophotometry studies, which could have helped to localize expression at the cellular level (Nawrocki et al, 1985;Robinson et al, 1993;Cameron, 2002), this is likely the result of either a lack of sensitivity to detect the poorly expressing opsin or confounding signals with other opsins that have similar λ max values, such as rhodopsin, Rh2-3 or Rh2-4 (Chinen et al, 2003).…”