Fig. 4. Wnt5 repels commissural axons from the posterior commissure. During Drosophila development, the axons composing the CNS have a ladder-like structure: each segment comprises an anterior and a posterior commissural tract that cross the midline and join the longitudinal tracts. (A) In wild-type animals, the Wnt5 ligand, which is restricted to the posterior commissure (red), repels axons expressing Derailed (Drl⁺; light green) from the posterior commissure into the anterior commissure. (B) In wnt5 mutants (as well as in drl mutants, not shown), the Drl⁺ neurons that normally project into the anterior commissure project into the posterior commissure. A, anterior; P, posterior.