Hunchback is required for the specification of the early sublineage of neuroblast 7-3 in the Drosophila central nervous system

The Drosophila ventral nerve cord (VNC) derives from neuroblasts (NBs), which mostly divide in a stem cell mode and give rise to defined NB lineages characterized by specific sets of sequentially generated neurons and/or glia cells. To understand how different cell types are generated within a NB lineage, we have focused on the NB7-3 lineage as a model system. This NB gives rise to four individually identifiable neurons and we show that these cells are generated from three different ganglion mother cells (GMCs). The finding that the transcription factor Hunchback (Hb) is expressed in the early sublineage of NB7-3, which consists of the early NB and the first GMC (GMC7-3a) and its progeny (EW1 and GW), prompted us to investigate its possible role in NB7-3 lineage development. Our analysis revealed that loss of hb results in a lack of the normally Hb-positive neurons, while the later-born neurons (designated as EW2 and EW3) are still present. However, overexpression of hb in the whole lineage leads to additional cells with the characteristics of GMC7-3a-derived neurons, at the cost of EW2 and EW3. Thus, hb is an important determinant in specifying early sublineage identity in the NB7-3 lineage. Using Even-skipped (Eve) as a marker, we have additionally shown that hb is also needed for the determination and/or differentiation of several other early-born neurons, indicating that this gene is an important player in sequential cell fate specification within the Drosophila CNS.