Vitamin A2-based photopigments within the pineal gland of a fully terrestrial vertebrate.

Fully terrestrial vertebrates were previously thought to exclusively employ vitamin A1 to generate visual pigments. However, recent studies on the visual system of the lizard Anolis carolinensis have shown that its visual pigments are vitamin A2-based. This unexpected result prompted an investigation of the pineal photopigments in this species [13]. HPLC analysis has shown that this extraretinal photoreceptor also exclusively utilizes a vitamin A2-derived chromophore. The adaptive significance of this chromophore within the pineal is unclear. The extended long wavelength sensitivity characteristic of vitamin A2-based visual pigment systems may enhance important visual tasks such as prey detection or mate selection [13]. A similar argument cannot be made for the pineal, whose role is not image formation, but rather detection of the irradiance changes associated with dawn and dusk. We suggest that the pineal may passively utilize whatever retinoids have been adaptively selected by the visual system.