Adaptation und Photoregeneration im Fliegenauge

Ultraviolet and blue light induced damage to the Drosophila retina: microspectrophotometry and electrophysiology

Intense ultraviolet (UV) and blue stimulation decreases visual pigment concentration and increases long wavelength fluorescent emission in R1-6 photoreceptors in the white eyed fruit fly Drosophila melanogaster. We used microspectrophotometry to show that the threshold for visual pigment decrease is about 1 log unit lower for UV than for blue light (18.7 vs approximately 19.9 log quanta/cm2 respectively). UV and blue stimuli about 0.2 log units brighter had been shown to cause structural degeneration. Above the threshold for structural damage, visual pigment is decreased permanently while below this level, a recovery of visual pigment was achieved within several hours. Microspectrofluorometric data are partially consistent with the hypothesis that the visual pigment is converted into a fluorescent product which had been named M'. M' had been proposed to be a new form of metarhodopsin which absorbs chiefly in the yellow and which has a fluorescent emission in the red; long wavelength stimulation had been reported to regenerate the native visual pigment from M'. Our data suggest that the situation is significantly more complex than this simple model. For instance, we report that long wavelength stimulation regenerates only a small fraction of the visual pigment which had been decreased by UV or blue stimulation. Furthermore, several lines of evidence suggest that other fluorescent products are also created by intense UV and blue stimulation. We were particularly interested in the lower damage threshold for UV light because of the hypothesis that UV visual sensitivity is mediated by a sensitizing pigment which absorbs UV light and transfers its energy to the blue absorbing rhodopsin. Our data suggest that the UV light decreases the rhodopsin without preferentially decreasing the sensitizing pigment.

Evidence for a sensitising pigment in fly photoreceptors

Many photoreceptor cells in invertebrates have a dual-peak spectral sensitivity. Evidence is presented that in fly photoreceptors the ultraviolet peak is due to a photostable pigment that absorbs light quanta and transfers the energy to the blue-absorbing visual pigment.

Spectral sensitivities and photopigments in adaptation of fly visual receptors

Chromatic adaptation and receptor-deficient mutants have been used to systematically eliminate specific receptor function to provide the first complete spectral sensitivity and photopigment characteristic data of all three receptor types (R1-6, R7 and R8) in Drosophila compound eyes. Different fly genera have different adaptation responses which allow characterization of photopigment spectra. Drosophila mutants with anomalous excitation also reveal mechanisms coupling photopigment activation with receptor excitation.