TEM revealed retinal degeneration in ktub mutant. Transmission electron micrographs indicated the photoreceptor cells of wild type and ktub flies raised under constant light for 6 days. Figures show one ommatidium of the wild type (A) and the ktub mutant retina (C). Higher magnification shows single rhabdomere in the wild type (B) and the ktub mutant (D). In the wild type, the rhabdomeres were organized as a regular array of microvilli. The plasma membrane at the base of the rhabdomere appeared as a catacomb-like extracellular space (B, arrows). In the ktub mutant, the rhabdomeres displayed short microvilli, distorted catacomb-like structures at the base, and curtains of microvillar membranes involuting into the photoreceptor cell (C, D). The photoreceptor R7 as not affected, and remained as in the wild type. The scale bar is 2 μm (A, C) and 1 μm (B, D).