Ultrastructure of the compound eye and first optic neuropile of the photoreceptor mutant oraJK84 of Drosophila.

The developmental mutant of Drosophila (oraJK84) is characterized by nonfunctional photoreceptor cells (R1-6), while the R7/R8 cells are normal. A fundamental question is: Does the near absence of photosensitive membranes inhibit development of the R1-6 axons and their synapses at the other end of the cell? The retina and first optic neuropile (lamina ganglionaris) were examined with freeze-fracture technique and high voltage electron microscopy. R1-6 have reduced rhabdomere caps; rhabdomeric microvilli have about 50% of the normal diameter and 20% of the normal length. Affected cells exhibit prominent vacuoles which appear to communicate with some highly convoluted microvillar membranes. Almost no P-face particles (putative rhodopsin molecules) are present in the R1-6 rhabdomeres, and particle densities are lower in R7 than previously reported. Near the rhabdomere caps, microvilli of R1-6 are fairly normal, but at more proximal levels they are greatly diminished in length and changed in orientation, while at still more proximal levels they are lost. R1-6, R7, and R8 axons from each ommatidium are bundled into normal pseudocartridges beneath the basement membrane. No abnormalities are found in the lamina ganglionaris, and all synaptic associations as well as the presumed "virgin" synapses (of R1-6) appear normal. No glial anomalies are present, and R7/R8 axons project through the lamina in the usual fashion. These fine structural findings are correlated with known electrophysiological, biochemical, and behavioral correlates of both sets of photoreceptors (R1-6, and R7/R8).