Insights into the evolution of photoreceptor oil droplets in frogs and toads.

Photoreceptor oil droplets (ODs) are spherical organelles placed most commonly within the inner segment of the cone photoreceptors. Comprising neutral lipids, ODs can be either non-pigmented or pigmented and have been considered optically functional in various studies. Among living amphibians, ODs were only reported to occur in frogs and toads (Anura), while they are absent in salamanders and caecilians. Nonetheless, the limited understanding of their taxonomic distribution in anurans impedes a comprehensive assessment of their evolution and relationship with visual ecology. We studied the retinae of 134 anuran species, extending the knowledge of the distribution of ODs to 46 of the 58 currently recognized families, and providing a new perspective on this group that complements the available information from other vertebrates. The occurrence of ODs in anurans shows a strong phylogenetic signal, and our findings revealed that ODs evolved at least six times during the evolutionary history of the group, independently from other vertebrates. Although no evident correlation was found between OD occurrence, adult habits and diel activity, it is inferred that each independent origin involves distinct scenarios in the evolution of ODs concerning photic habits. Furthermore, our results revealed significant differences in the size of the ODs between nocturnal and arrhythmic anurans relative to the length of the cones' outer segment.

Simultaneous Expression of UV and Violet SWS1 Opsins Expands the Visual Palette in a Group of Freshwater Snakes.

Snakes are known to express a rod visual opsin and two cone opsins, only (SWS1, LWS), a reduced palette resulting from their supposedly fossorial origins. Dipsadid snakes in the genus Helicops are highly visual predators that successfully invaded freshwater habitats from ancestral terrestrial-only habitats. Here, we report the first case of multiple SWS1 visual pigments in a vertebrate, simultaneously expressed in different photoreceptors and conferring both UV and violet sensitivity to Helicops snakes. Molecular analysis and in vitro expression confirmed the presence of two functional SWS1 opsins, likely the result of recent gene duplication. Evolutionary analyses indicate that each sws1 variant has undergone different evolutionary paths with strong purifying selection acting on the UV-sensitive copy and dN/dS ∼1 on the violet-sensitive copy. Site-directed mutagenesis points to the functional role of a single amino acid substitution, Phe86Val, in the large spectral shift between UV and violet opsins. In addition, higher densities of photoreceptors and SWS1 cones in the ventral retina suggest improved acuity in the upper visual field possibly correlated with visually guided behaviors. The expanded visual opsin repertoire and specialized retinal architecture are likely to improve photon uptake in underwater and terrestrial environments, and provide the neural substrate for a gain in chromatic discrimination, potentially conferring unique color vision in the UV-violet range. Our findings highlight the innovative solutions undertaken by a highly specialized lineage to tackle the challenges imposed by the invasion of novel photic environments and the extraordinary diversity of evolutionary trajectories taken by visual opsin-based perception in vertebrates.

Simultaneous expression of UV and violet SWS1 opsins expands the visual palette in a group of freshwater snakes

Snakes are known to express a rod visual opsin and two cone opsins, only (SWS1, LWS), a reduced palette resulting from their supposedly fossorial origins. Dipsadid snakes in the genus Helicops are highly visual predators that successfully invaded freshwater habitats from ancestral terrestrial-only habitats. Here, we report the first case of multiple SWS1 visual pigments in a vertebrate, simultaneously expressed in different photoreceptors and conferring both UV and violet sensitivity to Helicops snakes. Molecular analysis and in vitro expression confirmed the presence of two functional SWS1 opsins, likely the result of recent gene duplication. Evolutionary analyses indicate that each sws1 variant has undergone different evolutionary paths with strong purifying selection acting on the UV-sensitive copy and dN/dS ∼1 on the violet-sensitive copy. Site-directed mutagenesis points to the functional role of a single amino acid substitution, Phe86Val, in the large spectral shift between UV and violet opsins. In addition, higher densities of photoreceptors and SWS1 cones in the ventral retina suggest improved acuity in the upper visual field possibly correlated with visually guided behaviors. The expanded visual opsin repertoire and specialized retinal architecture are likely to improve photon uptake in underwater and terrestrial environments, and provide the neural substrate for a gain in chromatic discrimination, potentially conferring unique color vision in the UV–violet range. Our findings highlight the innovative solutions undertaken by a highly specialized lineage to tackle the challenges imposed by the invasion of novel photic environments and the extraordinary diversity of evolutionary trajectories taken by visual opsin-based perception in vertebrates.

Lens transmittance shapes ultraviolet sensitivity in the eyes of frogs from diverse ecological and phylogenetic backgrounds.

The amount of short wavelength (ultraviolet (UV), violet and blue) light that reaches the retina depends on the transmittance properties of the ocular media, especially the lens, and varies greatly across species in all vertebrate groups studied previously. We measured the lens transmittance in 32 anuran amphibians with different habits, geographical distributions and phylogenetic positions and used them together with eye size and pupil shape to evaluate the relationship with diel activity pattern, elevation and latitude. We found an unusually high lens UV transmittance in the most basal species, and a cut-off range that extends into the visible spectrum for the rest of the sample, with lenses even absorbing violet light in some diurnal species. However, other diurnal frogs had lenses that transmit UV light like the nocturnal species. This unclear pattern in the segregation of ocular media transmittance and diel activity is shared with other vertebrates and is consistent with the absence of significant correlations in our statistical analyses. Although we did not detect a significant phylogenetic effect, closely related species tend to have similar transmittances, irrespective of whether they share the same diel pattern or not, suggesting that anuran ocular media transmittance properties might be related to phylogeny.

A dune with a view: the eyes of a neotropical fossorial lizard.

BACKGROUND: Lizards are excellent models to study the adaptations of the visual system to different scenarios, and surface-dwelling representatives have been relatively well studied. In contrast, very little is known about the functional anatomy of the eyes of fossorial lineages, and properties such as the light transmission by the ocular media have never been characterised in any fossorial species. Some lizards in the family Gymnophthalmidae endemic to the sand dunes of North Eastern Brazil have evolved sand-burrowing habits and nocturnal activity. Lizards in the sister group to Gymnophthalmidae, the family Teiidae, have decidedly diurnal and epigeal lifestyles, yet they are equally poorly known in terms of visual systems. We focussed on the eye anatomy, photoreceptor morphology and light transmittance properties of the ocular media and oil droplets in the gymnophthalmid Calyptommatus nicterus and the teiid Ameivula ocellifera. RESULTS: The general organisation of the eyes of the fossorial nocturnal C. nicterus and the epigeal diurnal A. ocellifera is remarkably similar. The lenses are highly transmissive to light well into the ultraviolet part of the spectrum. The photoreceptors have the typical cone morphology, with narrow short outer segments and oil droplets. The main difference between the two species is that C. nicterus has only colourless oil droplets, whereas A. ocellifera has colourless as well as green-yellow and pale-orange droplets. CONCLUSIONS: Our results challenge the assumption that fossorial lizards undergo loss of visual function, a claim that is usually guided by the reduced size and external morphology of their eyes. In the case of C. nicterus, the visual system is well suited for vision in bright light and shows specialisations that improve sensitivity in dim light, suggesting that they might perform some visually-guided behaviour above the surface at the beginning or the end of their daily activity period, when light levels are relatively high in their open dunes habitat. This work highlights how studies on the functional anatomy of sensory systems can provide insights into the habits of secretive species.

Differences in ocular media transmittance in classical frog and toad model species and its impact on visual sensitivity.

The transmittance properties of the cornea, lens and humours of vertebrates determine how much light across the visible spectrum reaches the retina, influencing sensitivity to visual stimuli. Amphibians are the only vertebrate class in which the light transmittance of these ocular media has not been thoroughly characterised, preventing large-scale comparative studies and precise quantification of visual stimuli in physiological and behavioural experiments. We measured the ocular media transmittance in some commonly used species of amphibians (the bufonids Bufo bufo and Rhinella ornate, and the ranids Lithobates catesbeianus and Rana temporaria) and found low transmittance of short wavelength light, with ranids having less transmissive ocular media than bufonids. Our analyses also show that these transmittance properties have a considerable impact on spectral sensitivity, highlighting the need to incorporate this type of measurement into the design of stimuli for experiments on visual function.