Fig. 3. Eph and Ephrin can act as a functional receptor/ligand pair in vivo. View of dissected stage 15-16 embryos labeled with anti-HRP to visualize the axon tracts within the CNS, including the anterior (AC) and posterior (PC) commissures. (A) Wild type. In sim-Gal4/+;UAS-Ephrin:myc/+ embryos (B) where Ephrin:myc is ectopically expressed by midline glia, commissural axon tracts are disrupted. In addition, the CNS often appears less compact. This is consistent with an axon repellent activity for Ephrin. (C) Similar disruptions in midline axon crossing are seen in sim-Gal4/+;UAS-Ephrin^IC:myc/+ embryos expressing an Ephrin:myc variant deleted for intracellular sequences, demonstrating that forward signaling through Eph is responsible for producing the guidance defects. (D) Ectopic Ephrin expression in Eph mutant embryos (genotype=sim-Gal4/+;UAS-Ephrin:myc/+;Eph^x652) does not disrupt commissural axons, demonstrating the Ephrin repellent activity is mediated by Eph.