Fig. 1. babo mutants exhibit a severe defect in photoreceptor axon targeting. (A) Wild-type yw photoreceptor axon projections in a late third-instar Drosophila larva are highlighted by staining with antibody 24B10. The growth cones of R1-R6 form a neural plexus (arrowhead) at the lamina. R7 and R8 axons project to the medulla with individual growth cones forming a lattice-like array (arrow). Structures of individual growth cones of R7/R8 are illustrated with higher magnification in the inset. (B) An early white prepupa of babo^32/52 mutant (same magnification as A) showing a smaller brain lobe, reduced lamina plexus (arrowhead), abnormal R7/R8 photoreceptor axon projections (arrow) and bundled growth cones (inset). (C) A late third-instar Drosophila Smad2 (dSmad2^mb388) mutant larva displaying photoreceptor axon (green, 24B10) targeting defects similar to those of babo mutants. Glia cells are stained by an anti-Repo antibody (red), and Dachshund antibody labeled the lamina neuron precursor cells (green, arrowhead) in the brain lobe and photoreceptor precursor cells in eye discs (also green, arrowhead). (D) A wild-type day 3 pupa showed normal turning (arrow) of R7/R8 axons (stained with 24B10) between lamina and medulla and a very well spaced array of R7/R8 axons in the medulla. (E) A day 3 babo^32/52 pupa showing lack of turning (arrow) and highly disorganized photoreceptor axons. (F) A schematic graph shows Drosophila central nervous system of a late third-instar larva with eye disc. Most images in this paper are horizontal confocal optic sections unless otherwise stated. br, brain lobes; ed, eye disc; la, lamina; md, medulla; os, optic stalk.