Fig. 6. Immunoreactivity of the Drosophila visual system to White protein. Frontal 10 µm cryostat sections of fly heads immunolabeled with a polyclonal antibody that detects the extracellular loop of the White protein. (A) The wild-type shows a strong signal only in the lamina (La), with a much weaker signal in the photoreceptors (P). In the retina, signal is concentrated in the primary pigment cells (inset, top: arrows) and at in the basement membrane (inset, middle), probably in the base of secondary and tertiary pigment cells (Longley and Ready, 1995). In the lamina, White label is clear only in the epithelial glia (inset, bottom: asterisk) that surround the array of lamina cartridges. A ring of R1–R6 terminals is faintly visible within each cartridge, revealing weak photoreceptor expression. Terminals of the R7 and R8 photoreceptors are also faintly visible in the medulla (arrow), but label is otherwise very weak and diffuse in the deeper medulla (Me) neuropile, revealing no cellular expression site. (B–E) Matched wild-type and mutant visual systems immunolabeled in parallel, for comparison. (B) Wild-type. (C) white mutant. (D) brown mutant. (E) scarlet mutant. Relative to the wild-type, the immunosignal is essentially absent in white and brown, and greatly reduced in scarlet. Scale bars, in A, 50 µm and 10 µm (insets); and 100 µm (in D for B–D).