LOF and GOF phenotypes in the heart. All embryos are stage 17. (A) Dorsal view of a wild-type embryo stained for Mef2 (magenta) and Slit (green). (B) EM of a wild-type heart in XS showing the lumen between two CBs (arrow). (C) robo embryo stained for Slit (green) and Mef2 (magenta). (D) EM of a robo embryo showing a block in lumen formation. Arrow indicates the region where the CBs remain inappropriately attached. (E) XS of a slit GOF embryo stained with anti-Mef2, which labels CB nuclei. Two lumens are visible (arrowheads). (E′) Removal of robo in a slit GOF background suppresses the two-lumen phenotype. (F) Anti–E-Cad staining in the wild-type heart. Staining at the ventral attachment point is marked with an arrow. (G) EM of shg/E-Cad mutant shows that CBs fail to attach to each other (bracket). (H and H′) Close up of EMs in G and D. In robo embryos (H′), the CBs are closely associated (arrow), as compared with shg/E-Cad mutants (H), where the cells fail to adhere, as indicated by the presence of extracellular space between CBs (arrow). (I) EM of a shg/+,robo/+ embryo showing defects in lumen formation. (J) EM of a Mef-Gal4/+;UAS-shg/+ embryo showing that lumen formation is blocked (arrow). (K) Mef-Gal4/+;UAS-robo/+ embryo stained for Slit (green) and Mef2 (magenta). Slit is localized to the apical domain of CBs but the staining is nonuniform (arrows). (L) EM of a Mef-Gal4/+;UAS-robo/+ embryo in which the CBs have lost their dorsal point of attachment (arrows). Bars: (B) 2 μm; (H) 1 μm.