From temporal patterning to neuronal connectivity in Drosophila type I neuroblast lineages

During Drosophila neurogenesis, a small pool of neural progenitor cells generates a diverse population of neurons. Initially, embryonic neural progenitors (called neuroblasts; NBs) are diversified by spatially restricted expression of early transcription factors (reviewed in [58]. Drosophila NBs undergo type I or type II lineages. In type I lineages, the NB generates a series of ganglion mother cells (GMCs) that each produce a pair of sibling neurons; in type II NB lineages, the NB generates a series of intermediate neural progenitors (INPs) which each divide asymmetrically to generate 4–6 GMCs and their subsequent sibling neurons. In this review we focus on temporal patterning in type I NBs; type II lineages will be covered by another review in this issue.

Diversity within type I clonally related neurons is achieved through temporal patterning, in which each NB undergoes a series of asymmetric divisions, sequentially expressing a cascade of key temporal transcription factors (TTFs) [23]. Recent work in the ventral nerve cord (VNC) and central complex (CX) has demonstrated the ability of TTFs to regulate high-order features of neuronal identity in post-mitotic neurons, including molecular identity, morphology, and axon and dendrite targeting [35], [38], [39], [55], [56], [59]. These results define temporal patterning as a powerful mechanism for generating neuronal diversity and determining terminal features. While this phenomenon has been well characterized in the VNC, temporal patterning is employed in other key brain regions as well, including the central brain and visual processing centers (optic lobes). Here we review the recent advances in understanding the role of temporal patterning and TTFs in circuit assembly and neural function in the Drosophila CNS.

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