Fig. 1. smo mutant clones are not recovered between R axon bundles. Unless otherwise noted, all images are late third instar, oriented with anterior left and dorsal up. (A) Developing visual system of the third instar larva. R axons (white) and Dac-positive neural cell precursors (green) are labeled; the developing retina, optic stalk and lamina are shown. (B) A schematic drawing of the R axon projection. Photoreceptor cells differentiate as a row in a synchronized stepwise fashion and in a posterior to anterior order, and extend their axons to the lamina via the optic stalk. R axon projection induces lamina neuron differentiation; hence lamina neurons differentiate coordinately in the same manner. Colored bars represent rows of developing photoreceptors and lamina neurons; the same colors roughly represent axonal connections. (C) Schematic illustration of lamina column assembly from the horizontal perspective (left) and lateral perspective (right). As R axon innervations trigger the differentiation of lamina precursor cells in a posterior-to-anterior direction in a stepwise manner, the lamina furrow progresses anteriorly. The G1-phase lamina precursor cells (yellow) contact the most recently arriving R axons, receive Hh provided by the axons, and differentiate into the lamina neurons (light blue) at the trough of the lamina furrow. In older columns (at the posterior of the lamina), a subset of lamina neurons express neuronal markers (pink) as they become mature lamina neurons L1-L5. The R1-R6 axons (orange lines in horizontal and dots in lateral) stop between rows of glial cells (light green), whereas the R7 and R8 axons proceed to deeper target regions in the medulla. We have divided the lamina into two domains, one anterior to the newly arriving R axons and one posterior, and refer to the domains as pre-assembling and assembling, respectively. (D-I) Clones were induced by NP6099-GAL4 UAS-flp and marked by the absence of GFP. The optic lobe with wild-type (D-F) and smo (G-I) clones is shown. The developing lamina was demarcated by white dots (D,G). Developing lamina neurons express Dac (cyan in E,H; blue in F,I). R axons are visualized by the anti-HRP antibody (white in E,F,H,I). (G) smo clones are recovered in OPC (arrow) and in the pre-assembling domains (arrowhead). GFP-negative clones are outlined in F and I (yellow). (G,H) Dac expression is abolished in smo¹ clones cell autonomously. (I) In contrast to control (F), smo¹ clones were not recovered in the assembling domain.