Morphological correlates of visual pigment turnover in photoreceptors of the fly, Calliphora erythrocephala.

The content of visual pigment in one eye of Calliphora 'chalky' was measured spectrophotometrically and related to the ultrastructure of the other eye and the density of membrane particles as revealed by the freeze-fracture technique. Electron microscopy of thin-sectioned material showed that the manifestations of synthesis and breakdown of photoreceptor membrane were most prominent in flies kept in blue light, in comparison to flies kept in green light or darkness, in which only a moderate breakdown of membrane was evident. By subjecting flies to different light regimes it was found that the density of membrane particles was related to the content of visual pigment, but not on a one-to-one basis. In particular, the particle density in flies with a low (less than 10%) rhodopsin content, produced by raising flies on a vitamin A-deficient diet or by exposure to green light, was still about 35% of that of normal flies. Taken together the results indicate that all rhodopsin molecules are particles but that some particles represent another protein that most probably serves to maintain the structural integrity of the photoreceptor membrane. Furthermore, membrane synthesis can take place in the absence of rhodopsin synthesis.