The iris in Red-Eared Slider Turtles (Trachemys scripta elegans) is intrinsically photosensitive, slowly constricting the pupil when exposed to light. We hypothesized that one of the photopigments involved in the photomechanical response (PMR) is melanopsin (Opn4). The purpose of this project was to determine whether mRNA of Opn4 is present in their iris. After eyes were dissected from animals, we extracted iris tissues to carry out fluorescent in situ hybridization. Comparison of fluorescence from images revealed significant labeling in the iris for two melanopsin isoforms (Opn4x and Opn4m). We then carried out quantitative real-time PCR to identify relative expression levels of mRNA. Expressions by both isoforms in the iris were significant. In contrast to the retina, which had higher levels of Opn4m than Opn4x, the iris had higher levels of Opn4x than Opn4m. We also carried out Carr–Price reactions on tissue extracts, which were stabilized by hydroxylamine, and detected retinal oxime in the iris. Possession of retinal oxime indicates prior presence of retinal, a vitamin A-type chromophore, necessary for a functional protein. Evidence of a chromophore along with both Opn4 mRNA isoforms in the iris of this species supports presence of the photopigment and a possible role in the turtles' slow PMR.

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