Fig. 3. R8, R7, and L1-L5 axons target to distinct medulla layers independently at the first layer-selection stage. (A-D') Layer-specific targeting of R8, R7, and L1-L5 growth cones was assessed in various mutant backgrounds at 17% (A,A',C-D') and 40% APF (B,B'). The R7 axons (green) were labeled using PM181-Gal4, UAS-mCD8-GFP (A,A') or PM181-lacZ marker (C,C'), and visualized with anti-GFP or anti-LacZ antibodies, respectively. The L1-L5 axons (blue) were labeled using Gcm-Gal4, UAS-lacZ and visualized with anti-LacZ antibodies (C-D'). R8 and older R7 axons were visualized with Mab24B10 (red). Glia were visualized with anti-Repo antibody (green in B,B'). (A-B') In hh¹ mutants, L1-L5 fail to differentiate while 11-13 rows of R-cells still develop (Huang and Kunes, 1996). In the absence of L1-L5, the R7 axons project past R8 growth cones and terminate at the layer below (A', arrowheads). However, the R7 growth cones do not separate from R8 growth cones until 40% APF (A,B'). The presumptive R7 and R8 layers were marked with dotted lines. The R-cell growth cones appear disorganized at 40% APF. (C,C') Expressing EGFR^DN in the young lamina blocks the development of L1-L5 in this region and recapitulates the hh¹ phenotypes in the younger part of medulla (bracket). (D,D') In sevenless mutants, R7 neurons fail to develop. However, the R8 (red) and L1-L5 (blue) axons (arrow and arrowhead) target correctly to their temporary layers. Scale bars: in A, 30 µm for A-D; in A',B', 10 µm for A'-D'. (E) A schematic diagram summarizing A,C,D.