Mutations in the stumpy gene reveal intermediate targets for zebrafish motor axons

Primary motoneurons, the earliest developing spinal motoneurons in zebrafis h, have highly stereotyped axon projections. Although much is known about t he development of these neurons, the molecular cues guiding their axons hav e not been identified. In a screen designed to reveal mutations affecting m otor axons, we isolated two mutations in the stumpy gene that dramatically affect pathfinding by the primary motoneuron, CaP, In stumpy mutants, CaP a xons extend along the common pathway, a region shared by other primary moto r axons, but stall at an intermediate target, the horizontal myoseptum, and fail to extend along their axon-specific pathway during the first day of d evelopment. Later, most CaP axons progress a short distance beyond the hori zontal myoseptum, but tend to stall at another intermediate target. Mosaic analysis revealed that stumpy function is needed both autonomously in CaP a nd non-autonomously in other cells. stumpy function is also required for ax ons of other primary and secondary motoneurons to progress properly past in termediate targets and to branch. These results reveal a series of intermed iate targets involved in motor axon guidance and suggest that stumpy functi on is required for motor axone to progress from proximally located intermed iate targets to distally located ones.