Fig. 2. Wnt-Drl/Ryk mediates axon repulsion in the Drosophila nerve cord and vertebrate cortico-spinal tract. (A-C) Schematics of a Drosophila nerve cord (NC) segment, illustrating anterior (ac) and posterior (pc) commissures. The axons of anterior commissural neurons (ACN) express derailed (drl; shown in blue) and are repelled by Wnt5 derived from midline cells (red circles). (A) In wild-type Drosophila, axons of posterior commissural neurons (PCN) do not express drl and enter the pc. (B) The misexpression of drl in PCN forces their axons through the ac. (C) Wnt5 loss of function (grey circles) suppresses the phenotype induced by drl ectopic expression, demonstrating that Wnt5, through Drl, is an essential cue for proper commissural axon projection. (D) Schematic of the cortico-spinal tracts (cst) in the dorsal portion of the postnatal mouse spinal cord. In vertebrates, a Ryk-mediated repulsive gradient of Wnt proteins (Wnt1 or Wnt5a) guides cortico-spinal axons in the AP direction. (E) Injections of anti-Ryk antibodies into the cervical spinal cord stall cortico-spinal axons posterior to the site of injection, with a consequent shortening of the cst. D, dorsal; V, ventral.