Figure 2. Neurogenesis in Drosophila larval central brain neuroblast lineages.

A. (Aa) The larval central brain harbors roughly 100 neuroblasts in addition to the optic lobe neuroblasts (green). These are divided into type I (blue) and type II (red) neuroblasts. (Ab) Type I neuroblasts of the central brain and ventral nerve cord (blue) divide similarly to embryonic ventral nerve cord neuroblasts shown in Figure 1 by generating a ganglion mother cell (GMC) that divides to produce two neural progeny. (Ac) Type II neuroblasts divide to give rise to an intermediate progenitor (INP) that undergoes additional “neuroblast-like” asymmetric divisions, thus greatly amplifying the number of neural progeny.

B. Type I neuroblast lineages at the larval stages express Cas and typically give rise to a series of Chinmo-expressing neurons followed by a series of Broad-Complex-expressing neurons. Chinmo expression is downregulated in the postmitotic progeny in a gradient, with early-born neurons expressing the highest Chinmo levels. (Ba) While Svp regulates the Hb-to-Kr transition in the embryo, (Bb) it is re-expressed in larval neuroblasts to regulate the temporal transition from Chinmo expression to Broad-Complex expression.

C. INPs generated from a Type II neuroblast sequentially express Dichaete (D, blue), Grainyhead (Grh, orange), and Eyeless (Ey, purple) to temporally specify distinct neural progeny. Given that neural progeny born early in the neuroblast lineage are different from those born later, it is likely that the neuroblast itself undergoes temporal transitions that are inherited by the INPs (hypothetical NB temporal identity factors are depicted by colored circle outlines).