Overexpression of slit results in ectopic lumen formation. All embryos are stage 17. (A–D) Dorsal view of embryos stained for Mef2 (magenta), which labels CB nuclei, and Slit or Robo (green). (A) A wild-type embryo showing the normal pattern of Slit. (B) slit embryo with one copy each of Mef-Gal4 and UAS-slit (slit GOF) in which Slit is no longer restricted to the CB apical domains. (C) Wild type pattern of Robo. (D) Robo is mislocalized to CB basal surfaces (arrow) in a slit GOF embryo. (E) EM of a wild-type embryo in XS showing the lumen (arrow) that forms between two CBs. (F) EM of a slit GOF embryo showing the two-lumen phenotype. Arrowheads indicate cell membranes. (G and H) Close up views of E and F. Arrowheads indicate the extracellular matrix. (I and J) XSs of embryos stained for Mef2, which labels CB and somatic muscle nuclei (brown). (I) The lumen is visible between CBs (arrowhead) in a wild-type embryo. (J) slit GOF embryo with two lumens (arrowheads). Bars: (A) 6 μm; (E) 2 μm; (G) 0.5 μm.