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Fig. 2. Slit is required for optic lobe development. (A,B,E-H) Third instar visual systems, photoreceptor axons visualized with anti-Chaoptin. (A) In wild type, photoreceptor axons grow into the brain through the optic stalk. The R1-R6 subset of photoreceptor axons stop in the lamina neuropil while R7 and R8 continue into the medulla neuropil. (B) In slit^dui mutants, there are gaps in the lamina neuropil (arrow) and increased numbers of axons enter the medulla (arrowhead). (C) Wild type and (D) slit^dui visual systems in which R2-R5 photoreceptor axons are visualized using Ro--lacZ (Garrity et al., 1999). (C) In wild type, all R2-R5 axons stop in the lamina neuropil. (D) In slit^dui mutants, many R2-R5 axons pass through the lamina and enter the medulla (arrowheads). (E) slit^dui/slit^l(2)k05248, (F) slit^dui/slit^E158 and (G) slit^dui/slit² animals show photoreceptor axon targeting defects indistinguishable from slit^dui homozygotes, with gaps in the lamina (arrow) and increased numbers of axons entering the medulla (arrowhead). (H) Omb-Gal4;UAS-Slit; slit^dui/slit² visual system. Slit cDNA expression controlled by Omb-Gal4 largely rescues slit targeting defects, restoring even layer of photoreceptor growth cones in the lamina (arrow). (I,J) Animals carrying loco:lacZ enhancer trap (which is strongly expressed in epithelial and marginal glia) stained with anti-lacZ. (I) In wild type, continuous layers of epithelial and marginal glia are observed in the lamina. (J) In slit mutants, there are clumps of glia (arrowhead) and gaps (arrow) in the glial layers. (K,L) Photoreceptors axons are visualized with GMR:GFP (green) and glial nuclei with Repo:lacZ (magenta). (K) In wild type, R1-R6 axons stop in the lamina between layers of glia (open arrowheads). (L) In slit mutants, there are gaps in the photoreceptor innervation of the lamina, correlated with regions of the lamina devoid of glia (asterisk) and uneven innervation in regions containing clumps of glia (arrow). The clear separation between glia at the base of the lamina and glia surrounding the medulla observed in wild type is missing in slit mutants (arrowhead). ep, epithelial glia; lamn, lamina neuropil; ma, marginal glia; medn, medulla neuropil.

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