Fig. 1. The commissureless mutant phenotype is not significantly rescued by expression of Commissureless at the midline. Axon tracts within the ventral nerve cord of (A) wild-type and (B) comm mutant embryos labelled with mAb BP102 at stage 15. In the wild-type embryo, the axons are organised into two longitudinal tracts, which are connected via two commissural tracts per segment. In comm mutants, the commissures do not form. (C) A wild-type embryo stained with a polyclonal Comm antibody. Comm protein accumulates at the commissure and is absent from the longitudinals. (D-F) Comm expression is driven at the midline using either the sim- (D) or slit- (E,F) GAL4 drivers and the resultant axon scaffold visualised with mAb BP102. (D,E) Driving wild-type Comm expression at the midline leads to a partial rescue of comm^A490 loss-of-function mutants. However there is no rescue of the null allele, comm^E39 (F). All panels are a view of the dorsal surface of the CNS and are shown anterior upwards.