Ectopic R1–R6 photoreceptors in the medulla (M) visualized with Rh1-lacZ. A, B, Photoreceptors express Rh1-lacZ (R1–R6: β-gal, magenta) and anti-chaoptin immunolabeling (R1-R8, green). A, C, E, R1–R6 photoreceptors terminate exclusively in the lamina (L) of Rh1-lacZ flies. B, D, F, In Rh1-lacZ/UAS-runt; MT14-GAL4/+ flies, ectopic R1–R6 axons innervate the medulla where they extend as deep as stratum M6, forming bleb-like swellings along their length (B, arrows). C, D, G, H, Areas expressing the vesicle-associated protein, CSP, are candidate sites of synaptic release visualized in single image planes (0.8 μm) from a Vibratome slice. C, CSP (green) colocalized (white) to β-gal immunoreactivity (magenta) of R1–R6 photoreceptor terminals in the wild-type lamina. D, G, H, CSP also colocalized (arrows) to β-gal around the perimeter of R1–R6 photoreceptor terminals in the mutant lamina (D, G) and ectopic terminals in the medulla (D, H). E, F, I, J, The T-bar ribbon-associated protein Bruchpilot is immunolocalized to presynaptic sites with the antibody nc82 (green), and was detected in the lamina of both Rh1-lacZ flies (E) and mutant (F, I) flies, as well as in ectopic R1–R6 terminals (magenta, white) in the medulla (F, J, arrow). In the hsFLP/UAS-mCD8::GFP; NeoFRT40A actin-GAL80/NeoFRT40A, GMR-GAL4, UAS-runt (MARCM) visual system (K–P) subsets of Runt overexpressing photoreceptors (green cells in K) and their axons (projection images from confocal stacks in L–P) are labeled by mCD8::GFP (green), and the neuropils are colabeled with nc82 (magenta). In the lamina (L–N), most photoreceptor axons have a structure resembling that of wild-type R1–R6 terminals (L), are through-going like wild-type R7–R8 axons (M, arrow), or have an abnormal terminal with varicosities (arrows) along its length (N). Occasional photoreceptor axons appear to form synaptic swellings in both optic neuropils (O, P). These axons expand in both the lamina (enlarged in O′, P′) and medulla (enlarged in O″, P″), and occasionally along the axon during its passage in the chiasma (O, arrow). Scale bars: A–F, 10 μm; G–K, 5 μm; (in L) L, M, N, 5 μm; (in O) O, P, 5 μm.