The Hunchback temporal transcription factor determines motor neuron axon and dendrite targeting in Drosophila

The generation of neuronal diversity is essential for circuit formation and behavior. Morphological differences in sequentially born neurons could be due to intrinsic molecular identity specified by temporal transcription factors (henceforth called intrinsic temporal identity) or due to changing extrinsic cues. Here we use the Drosophila NB7-1 lineage to address this question. NB7-1 sequentially generates the U1-U5 motor neurons; each has a distinct intrinsic temporal identity due to inheritance of a different temporal transcription factor at time of birth. Here we show that the U1-U5 neurons project axons sequentially, followed by sequential dendrite extension. We misexpress the earliest temporal transcription factor, Hunchback, to create “ectopic” U1 neurons with an early intrinsic temporal identity but later birth-order. These ectopic U1 neurons have axon muscle targeting and dendrite neuropil targeting consistent with U1 intrinsic temporal identity, rather than their time of birth or differentiation. We conclude that intrinsic temporal identity plays a major role in establishing both motor axon muscle targeting and dendritic arbor targeting, which are required for proper motor circuit development.