Fig. 1.

Models: intrinsic temporal identity or time of differentiation determines U1-U5 motor neuron morphology. (A) NB7-1 (top ) sequentially expresses the temporal transcription factors Hb, Kr, Pdm and Cas. The U1-U2 neurons (bottom) born during the Hb window have an ‘early-born’ identity (green) characterized by contralateral dendrites, an axon projection to dorsal body wall muscles DO1 and DO2, and little or no nuclear Zfh2. The U3-U5 neurons born after the Hb window have a ‘late-born’ identity (red) characterized by ipsilateral dendrites, an axon projection to more ventral muscles DA3/LL1 and high nuclear Zfh2. All U1-U5 neurons have nuclear Eve. (B) Models for specification of U1-U5 axon and dendrite targeting. (i) Intrinsic temporal identity could determine axon and dendrite targeting. (ii) Neuronal time of differentiation could determine axon and dendrite targeting. (iii) Misexpression of Hb can generate late-differentiating neurons with an early intrinsic temporal identity; this mismatch reveals which mechanism is more important for axon and dendrite targeting.