ACCUMULATION OF CALCIUM IN DEGENERATING PHOTORECEPTORS OF SEVERAL DROSOPHILA MUTANTS

The hypothesis that a large, possibly toxic, increase in cellular calc ium accompanies photoreceptor cell degeneration in several different D rosophila mutants was tested. The calcium content of wild type and mut ant photoreceptors of Drosophila was measured using rapid freezing of the eyes and energy-dispersive x-ray analysis (e.d.x.) of cryosections and semithin sections of cryosubstituted material. Light- and dark-ra ised mutants of the following strains were studied: retinal degenerati on B (rdgB); retinal degeneration C (rdgC); neither inactivation nor a fterpotential C (ninaC), and no receptor potential A (norpA). These ar e light-dependent retinal degeneration mutants in which the affected g ene products had been previously shown as myosin-kinase (ninaC), calci um-dependent phosphoprotein phosphatase (rdgC), phosphoinositide trans fer protein (rdgB), and phospholipase C (norpA). In light-raised mutan ts, ommatidia of variable degrees of degeneration were observed. Mass- dense globular bodies of 200-500 nm diameter in relatively large quant ities were found in the degenerating photoreceptor of all the mutants tested. These subcellular globules were found to have a very high calc ium content, which was not found in wild type or in nondegenerating ph otoreceptors of the mutants. Nondegenerating photoreceptors were found not only in dark-raised mutants, but in smaller quantities also in li ght-raised mutants. Usually these globular structures contained high l evels of phosphorus, indicating that at least part of the calcium in t he mutant photoreceptors is precipitated as calcium phosphate. The res ults indicate that a large increase in cellular calcium accompanies li ght-induced photoreceptor degeneration in degenerating Drosophila muta nts even when induced by very different mutations, suggesting that the calcium accumulation is a secondary rather than a primary effect in t he degeneration process.