Photoreceptor-specific efficiencies of beta-carotene, zeaxanthin and lutein for photopigment formation deduced from receptor mutant Drosophila melanogaster.

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.