The zebrafish space cadet gene controls axonal pathfinding of neurons thatmodulate fast turning movements

All vertebrates depend on neural circuits to produce propulsive movements; however, the contribution of individual neural cell types to control such m ovements are not well understood. We report that zebrafish space cadet muta nt larvae fail to initiate fast turning movements properly, and we show tha t this motor phenotype correlates with axonal defects in a small population of commissural hindbrain neurons, which we identify as spiral fiber neuron s. Moreover, we demonstrate that severing spiral fiber axons produces space cadet-like locomotor defects, thereby providing compelling evidence that t he space cadet gene plays an essential role in integrating these neurons in to the circuitry that modulates fast turning movements. Finally, we show th at axonal defects are restricted to a small set of commissural trajectories , including retinal ganglion cell axons and spiral fiber axons, and that th e space cadet gene functions in axonal pathfinding. Together, our results p rovide a rare example in vertebrates of an individual neuronal cell type th at contributes to the expression of a defined motor behavior.