Abstract
Pinopsin is the opsin most closely related to vertebrate visual pigments on the phylogenetic tree. This opsin has been discovered among many vertebrates, except mammals and teleosts, and was thought to exclusively function in their brain for extraocular photoreception. Here, we show the possibility that pinopsin also contributes to scotopic vision in some vertebrate species. Pinopsin is distributed in the retina of non-teleost fishes and frogs, especially in their rod photoreceptor cells, in addition to their brain. Moreover, the retinal chromophore of pinopsin exhibits a thermal isomerization rate considerably lower than those of cone visual pigments, but comparable to that of rhodopsin. Therefore, pinopsin can function as a rhodopsin-like visual pigment in the retinas of these lower vertebrates. Since pinopsin diversified before the branching of rhodopsin on the phylogenetic tree, two-step adaptation to scotopic vision would have occurred through the independent acquisition of pinopsin and rhodopsin by the vertebrate lineage.
Highlights
Pinopsin is the opsin most closely related to vertebrate visual pigments on the phylogenetic tree
These data suggest that fish and anuran pinopsin can function as a visual pigment in the retina for dim- light vision
To compare the expression patterns of pinopsin with those of visual pigments, we investigated the distribution of visual pigment transcripts in the retina of spotted gar and X. tropicalis
Summary
Pinopsin is the opsin most closely related to vertebrate visual pigments on the phylogenetic tree This opsin has been discovered among many vertebrates, except mammals and teleosts, and was thought to exclusively function in their brain for extraocular photoreception. The opsin phylogenetic tree shows that pinopsin is the non-visual opsin most closely related to visual pigments (Supplementary Figure 1). Pinopsin converts to MII intermediate, whose absorption maximum (λmax) lies in the UV region, to couple with Gt11 These properties are not observed in other closely related non-visual opsins, such as VA opsin and parapinopsin, found in vertebrates[12,13]. The retinal chromophore of pinopsins shows a low thermal isomerization rate similar to that of rhodopsin, which is important for functioning under dim-light conditions These data suggest that fish and anuran pinopsin can function as a visual pigment in the retina for dim- light vision. We revisit the evolutionary position of pinopsin in the context of color vision and dim-light vision acquisition
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