Neuronal cell fate diversification controlled by sub-temporal action of .

During embryonic nervous system development, neuroblasts express a programmed cascade of five temporal transcription factors that govern the identity of cells generated at different time-points. However, these five temporal genes fall short of accounting for the many distinct cell types generated in large lineages. Here, we find that the late temporal gene sub-divides its large window in neuroblast 5-6 by simultaneously activating two cell fate determination cascades and a sub-temporal regulatory program. The sub-temporal program acts both upon itself and upon the determination cascades to diversify the window. Surprisingly, the early temporal gene acts as one of the sub-temporal genes within the late window. Intriguingly, while the temporal gene activates the two determination cascades and the sub-temporal program, spatial cues controlling cell fate in the latter part of the 5-6 lineage exclusively act upon the determination cascades.