Imp and Chinmo are required for embryonic motor neuron axon and dendrite targeting

Neural progenitors generate distinct neuronal populations over time. Drosophila larval neural progenitors, neuroblasts (NBs), generate neuronal diversity by expressing temporal gradients of transcription factors and RNA-binding proteins, including early factors Imp and Chinmo and late factors Syp, Mamo, and Broad. These factors have been well characterized in the larval central nervous system (CNS), yet nothing is known about their expression or function in the embryonic CNS. We show that embryonic Imp is expressed in a low-to-high temporal gradient, the opposite of the larval Imp gradient. Embryonic Chinmo is expressed in all post-mitotic neurons, but not in a gradient, while the late larval factors Mamo, E93, Syp, and Broad show little embryonic expression. We show that Imp is required for Chinmo expression in postmitotic neurons, and loss of Chinmo - but not Imp - derepresses Syp. Finally, we tested whether Imp and Chinmo are required for motor neuron molecular identity or morphology. Although neither is required to specify temporal or molecular neuronal identity, both are required for axon targeting to the correct body wall muscle, and downregulating dendrite outgrowth. We conclude that temporal factors are regulated differently in embryos and larvae, and that Imp and Chinmo are required for proper neuronal axon and dendrite projections.