Lizard skin detects and blocks light: A study of non-visual opsin expression

Vertebrate skin is the organ with the most exposure to sunlight, and emerging literature has shown that nonvisual opsins (a group of light-sensitive proteins) are expressed in dermal tissues in many taxa. Light detection is particularly important in terrestrial reptiles for physiological and behavioral regulation, yet opsin expression is understudied in this group. In this study, we examine the green anole lizard, Anolis carolinensis, to map opsin expression in the skin and internal organs, and to quantify the exposure to sunlight experienced by the exterior and interior of the lizard. Using qPCR, we quantified the expression of the four major non-visual opsins and found that a series of them are more highly expressed in dorsal skin, which is directly exposed to sunlight, than in ventral skin, which is generally sheltered from direct sun. We also found that nonvisual opsins are abundantly expressed in internal organs. Yet, our measures of light transmittance through tissues reveal that very little light penetrates the body cavity wall to reach the internal organs. The light that does get through is noticeably shifted to wavelengths longer than 700 nm, outside the conventional opsin sensitivity range. These preliminary findings suggest that tissue-specific expression of dermal opsins may be critical for light detection, yet opsins in internal tissues may perform a different primary function.