Studies of the Drosophila norpA phototransduction mutant. II. Photoreceptor degeneration and rhodopsin maintenance.

The norpAH44 phototransduction mutant of Drosophila melanogaster, an allele that, on eclosion, does not exhibit a receptor potential was found, at later ages, to undergo light and temperature dependent degeneration of its photoreceptors as well as decreases in rhodopsin concentration. Pseudopupil measurements and light and electron microscopy were used to monitor the structure of the photoreceptors. When norpAH44 flies were maintained exclusively in the dark, no changes in structure or rhodopsin concentration were observed. When maintained on a 12 h light-12 h dark cycle, structural changes were first observed at 6 days of age for flies maintained at 24 degrees C or at 12 days of age for flies maintained at 19 degrees C. When the light-dark cycle was initiated after 10 days in the dark there was a more rapid loss of rhodopsin concentration and pseudopupil. The data suggest that even in the dark, although no obvious changes in structure or rhodopsin concentration were observed, certain processes that support these components had been affected. NorpAP12, an allele that exhibits small receptor potential amplitudes, also displayed age- and light-dependent photoreceptor degeneration and decreases in rhodopsin concentration, whereas no degeneration or decreases in rhodopsin were observed in norpAP16, an allele that exhibits receptor potential amplitudes similar to those of wild-type. The data suggest that the processes that affect phototransduction, such as the phosphatidylinositol cycle, have a long-term role in the maintenance of rhodopsin concentration and photoreceptor integrity.