Fig. 1

(A) Development of locomotor behaviors in zebrafish. By the time of birth (48–72 hpf), in response to strong stimuli, zebrafish are capable of generating escapes and struggles that are characterized by strong whole body bends. During these behaviors fish can assume a C-shaped body posture as a result of simultaneous activation of ipsilateral axial muscles spanning most body segments (arrows). By 120 hpf, a fish also spontaneously exhibits weaker swimming (Spontaneous swim), which is characterized by bending of mostly the caudal portion of the tail by a relatively small amount. This indicates that the axial muscles in the caudal part of the tail are activated relatively weakly. (B) Emergence of hindbrain V2a neurons expressing EGFP. (i) Timing of experiments. (ii) Regions displayed (cyan patch) in subsequent panels. (iii) Dorsal and side views of hindbrain V2a neurons from 24 hpf to 120 hpf. The caudal end of the volume roughly corresponds to the boundary between hindbrain and spinal cord. r, rostral; c, caudal; d, dorsal; v, ventral. Scale bars, 30 μm.