Fig. 1

The NB7-1 lineage, temporal transcription factors, and models for Hunchback function in specifying early-born neuronal identity. a The neuroblast 7–1 (NB7-1) sequentially expresses the temporal transcription factors Hunchback (Hb), Kruppel (Kr), Pou domain proteins Nubbin/Pdm2 (Pdm), and Castor (Cas). Each factor is maintained into the post-mitotic neurons, although Pdm neuronal expression is transient. The U1-U5 motor neurons are Eve+; all but the first-born U1 express the late-born neuronal marker Zfh2. b Models for Hb specification of early-born neuronal identity. (i) Hb may be required in the neuroblast, GMC, and post-mitotic neurons, matching its expression pattern. (ii) Hb expression in post-mitotic neurons may be sufficient to maintain their identity; in this case the transient neuroblast expression may be a mechanism to limit Hb to early-born neurons. (iii) Hb expression in neuroblasts may be sufficient to heritably specify identity of their neuronal progeny, perhaps by initiating a transcriptional cascade or creating epigenetic modifications