Photoreceptor physiology and evolution: cellular and molecular basis of rod and cone phototransduction.

The detection of light in the vertebrate retina utilizes a duplex system of closely related rod and cone photoreceptors: cones respond extremely rapidly, and operate at 'photopic' levels of illumination, from moonlight upwards; rods respond much more slowly, thereby obtaining greater sensitivity, and function effectively only at 'scotopic' levels of moonlight and lower. Rods and cones employ distinct isoforms of many of the proteins in the phototransduction cascade, and they thereby represent a unique evolutionary system, whereby the same process (the detection of light) uses a distinct set of genes in two classes of cell. The molecular mechanisms of phototransduction activation are described, and the classical quantitative predictions for the onset phase of the electrical response to light are developed. Recent work predicting the recovery phase of the rod's response to intense flashes is then presented, that provides an accurate account of the time that the response spends in saturation. Importantly, this also provides a new estimate for the rate at which a single rhodopsin activates molecules of the G-protein, transducin, that is substantially higher than other estimates in the literature. Finally, the evolutionary origin of the phototransduction proteins in rods and cones is examined, and it is shown that most of the rod/cone differences were established at the first of the two rounds of whole-genome duplication more than 500 million years ago.