Fer3 is uniquely expressed in NotchOFF hemilineages, where it promotes interneuron identity

In the Drosophila nervous system, neuroblasts (NBs) divide to produce a clone of neurons that establish distinct fates via precise timing and patterning of transcription factors (TFs). The final step in neurogenesis is a Notch/Numb asymmetric division that produces one daughter neuron that has active Notch signaling (NotchON) and one that does not (NotchOFF). NotchON neurons are well characterized, but the NotchOFF progeny are understudied due to lack of molecular markers. Here, we have identified Fer3 (forty-eight related 3) as a NotchOFF-specific transcription factor expressed in the NB7-1, NB6-1 and NB5-2 lineages. Fer3 is inhibited by Notch signaling in post-mitotic neurons, thereby restricting its expression to NotchOFF neurons. In some contexts, Fer3 is a transcriptional repressor, but we find that Fer3 misexpression generates ectopic Dbx+ neurons, and this is more penetrant when we misexpress a Fer3:activation domain fusion protein. Moreover, Fer3 is sufficient to induce Dbx expression in a neuroblast lineage that does not endogenously express Dbx or Fer3. This work presents the first known NotchOFF exclusive TF in the developing embryonic Drosophila ventral nerve cord.