CAROTENOID REPLACEMENT IN DROSOPHILA - FREEZE-FRACTURE ELECTRON-MICROSCOPY

Because of the consequent lack of photopigment chromophore, carotenoid /retinoid (vitamin A) deprivation during the larval period of Drosophi la leads to decreased rhodopsin in adult photoreceptors. Decreased den sity of P-face particles in photoreceptor membrane (rhabdomeric microv illi) is a prominent ultrastructural feature of this rhodopsin deficie ncy. When adults are fed carotenoid, the rhabdomeric P-face particle d ensity - which reflects the concentration of rhodopsin - increases hal fway to the replete control level during the first 12 hows, and is ful ly restored by 2 days. Based on freeze-fracture replicas, there is a c ontinuity of membrane between rhabdomeric microvilli and the parent re tinula cell. That confluence is relevant to turnover of photoreceptive membrane. Microvillar and retinula cell P-face particle densities cov ary. The relevance of the demonstration of rapid recovery from chromop hore depletion is discussed in relation to hypotheses that the chromop hore and/or related retinoids regulate opsin gene transcription, and/o r post-translational processing and deployment from the endoplasmic re ticulum to the rhabdomere.