Visual pigment, sensitivity, and rhabdomere size were measured throughout a 12-h light/12-h dark cycle in Drosophila. Visual pigment and sensitivity were measured during subsequent constant darkness [dark/dark (D/D)]. MSP (microspectrophotometry) and the ERG (electroretinogram) revealed a cycling of visual pigment and sensitivity, respectively. A visual pigment decrease of 40% was noted at 4 h after light onset that recovered 2-4 h later in white-eyed (otherwise wild-type, wper + ) flies. The ERG sensitivity [in wper + flies in light/dark (L/D)] decreased by 75% at 4 h after light onset, more than expected if mediated by visual pigment (MSP) changes alone. ERG sensitivity begins decreasing 8 h before light onset while decreases in visual pigment begin 2 h after light onset. These cycles continue in constant darkness (D/D), suggesting a circadian rhythm. White-eyed period (per) mutants show similar cycles of visual pigment level and sensitivity in L/D; per's alterations, if any on the D/D cycles were subtle. The cross-sectional areas of rhabdomeres in wper + were measured using electron micrographic (EM) morphometry. Area changed little through the L/D cycle.

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Lipids of Drosophila heads were extracted and separated by high-performance thin-layer chromatography. Fatty acid compositions of major phospholipids as well as of triglycerides were analyzed by gas-liquid chromatography. Proportions of the major fatty acids (14:0, 16:0, 16:1, 18:0, 18:1, 18:2, 18:3) varied depending on the lipid analyzed. Docosahexaenoic acid (22:6), common in vertebrate photoreceptors and brain, and arachidonic acid (20:4), a precursor of eicosanoids, were lacking. A comparison of the fatty acid composition of the diet vs. the head suggested that Drosophila can desaturate but may not be able to elongate fatty acid carbon chains. Fatty acid analyses were carried out after the following visual system alterations: i) the transduction mutant where no receptor potential results from a deficit in phospholipase C; ii) an allele of eyes absent; iii) the mutant outer rhabdomeres absent which lacks visual pigment and rhabdomeres in the predominant type of compound eye receptor, rhabdomeres 1 through 6; and iv) carotenoid deprivation which reduces opsin and rhabdomere size. We also evaluated aging by comparing newly-emerged vs. aged wild-type flies. Alterations in fatty acid composition based on some of these manipulations were found. Based on comparisons between flies reared on media differing in C16 and C18, there is an indication that diet readily affects tissue fatty acid composition.

Electron microscopy was used to investigate membrane turnover in the photoreceptors of Drosophila. Coated pits and vesicles, multivesicular bodies, primary lysosomes, multilamellate bodies, residual bodies and Golgi complexes are present throughout a light/dark cycle. Serial sections reveal that the membrane bounding of multivesicular bodies is only seen at an optimal plane of section. The temperature-sensitive shibire (shi(ts)) mutant has a defect in conversion of coated pits into vesicles which may also affect visual receptors. We used monoclonal antibodies to Rh1 in R1-6 receptors in the compound eye (also to Rh2 in ocellar receptors in the simple eyes) ro relate turnover processes at the visual pigment compared with membrane levels. Compound eye rhabdomeres but not rhabdomere caps stained selectively. Immunogold labelling was equivocal in multivesicular bodies. Further, early in the process of carotenoid replacement therapy, labelling is high in the rough endoplasmic reticulum, demonstrating de novo opsin synthesis.

Drosophila rearing media had only beta-carotene, zeaxanthin or lutein as precursors for photopigment chromophores. Zeaxanthin and lutein are potentially optimum sources of the 3-hydroxylated retinoids of visual and accessory photopigments. Mutants made the electroretinogram in white (w) eyes selective for compound eye photoreceptors R1-6, R7 and R8: R1-6 dominates w's electroretinogram; R7/8 generates w;ora's (ora = outer rhabdomeres absent); R8 generates w sev;- ora's (sev = sevenless). Microspectrophotometry revealed R1-6's visual pigment. In w, all 3 carotenoids yielded monotonic dose-responses for sensitivity or visual pigment. An ultraviolet sensitivity peak from R1-6's sensitizing pigment was present at high but not low doses. In w;ora, all 3 carotenoids gave similar spectra dominated by R7's high ultraviolet sensitivity. For w sev;ora, all spectra were the shape expected for R8, peaking around 510 nm. The sensitivity dose-response was at its ceiling except for low doses in w;ora and zero supplementation in w sev;ora. Hence, without R1-6, most of our dose range mediated maximal visual pigment formation. In Drosophila, beta-carotene, zeaxanthin and lutein mediate the formation of all major photopigments in R1-6, R7 and R8.