Pigment Mixtures and other Determinants of Spectral Sensitivity of Vertebrate Retinal Photoreceptors (pp. 1-32)
Authors: (T. Isayama, C.L. Makino, Department of Ophthalmology, Massachusetts Eye & Ear Infirmary, Harvard Medical School, Boston, Massachusetts, USA)
Abstract: Vertebrates see the world through their rod and cone photoreceptors. The wavelengths supporting vision are determined by structural features in the eye that modify the transmission of light and the visual pigments contained within the photoreceptors. A visual pigment is formed by the combination of an opsin protein and an 11-cis retinal (A1) chromophore or its 3-dehydro derivative (A2). Some photoreceptors contain almost exclusively A1 pigment, some contain predominantly A2 pigment and some contain mixtures of A1 and A2 pigments based on the same type of opsin. Although it was originally thought that a given rod or cone expressed only a single type of opsin, it is now evident that many photoreceptors express more than one. Expression of multiple opsins may arise in several different ways. There may be cross breeding in closely related species. Gene duplication followed by genetic drift might create similar alleles encoding opsins that give rise to pigments absorbing maximally at different wavelengths. Some photoreceptors undergo a developmental changeover in opsin expression and contain two different opsins during the transition. The changeover is presumably adaptive because it often accompanies a significant change in lifestyle. But the most intriguing cases are those in which the expression of two or more opsins is stable and apparently purposeful, although the physiological significance is not well understood. A related phenomenon is the use of an antenna pigment in which a fluorophore, that is not opsin based, absorbs light and transfers the photic energy to an opsin based visual pigment by Förster resonance energy transfer, thus mimicking the presence of a different visual pigment. One wonders what other mechanisms for tuning a photoreceptor‘s spectral sensitivity remain to be discovered!