Blue and ultraviolet light induced damage to the Drosophila retina: ultrastructure

Intense ultraviolet (UV) and blue stimulation photolyses rhodopsin through a fluorescent metarhodopsin (M') in the predominant photoreceptor type, R1-6, of the compound eye of white eyed Drosophila melanogaster. We investigated the associated retinal degeneration using High Voltage Electron Microscopy (HVEM). The threshold for UV induced damage was about 19 log quanta/cm2 while for blue, the threshold was about 20. These intensities are toward the upper level of the dynamic range for rhodopsin photolysis. Thus, there is a sensitization for near UV induced degeneration as had been found for photolysis of the visual pigment. Vitamin A deprivation protects against light elicited retinal degeneration, particularly in the UV. Since vitamin A deprivation eliminates the blue absorbing rhodopsin and a UV sensitizing pigment in R1-6, the degeneration is likely mediated through quantal absorption through these photoexcitation pigments. Intense light converts the microvilli of the rhabdomeres (the photopigment containing organelles) into dense strands and the cytoplasm fills with a dense reticulum. Such damage is elicited shortly after stimulation and is permanent. Under most conditions, the second order interneurons are spared. These results are discussed in the context of other animal models of intense light retinal degeneration.