Studies on the structure of ocellar photoreceptor cells of Drosophila melanogaster with special reference to subrhabdomeric cisternae.

We studied the structure of ocellar photoreceptor cells of Drosophila melanogaster, particularly the subrhabdomeric cisternae which our previous studies have shown to be essential structures for turnover of photoreceptive membranes in compound eyes. Each ocellus contained elongated photoreceptor cells with rhabdomeres positioned distally. In the subrhabdomeric regions, endocytotic invaginations were frequently observed, suggesting active turnover of photoreceptive membranes. In the vicinity of the photoreceptive microvilli, membranous structures similar to the subrhabdomeric cisternae in compound eyes were observed. These membranous structures were immunopositive for the rdgB protein, a phosphatidylinositol transfer protein that is localized to the subrhabdomeric cisternae in compound eyes. The ocellar photoreceptor cells of the retinal degeneration mutants (rdgA,B) were also studied. In these mutants, retinal degeneration has been reported to start, in compound eyes, with the disappearance of the subrhabdomeric cisternae. We found that the ocellar subrhabdomeric cisternae also disappear during the initial stage of retinal degeneration. From these observations, we conclude that the mechanism of photoreceptive membrane turnover in ocellar photoreceptor cells involves the rdgB and probably the rdgA proteins which are associated with subrhabdomeric cisternae, as is the case for photoreceptive membrane turnover in compound eyes.