Serial specification of diverse neuroblast identities from a neurogenic placode by Notch and Egfr signaling

We used the brain insulin-producing cell (IPC) lineage and its identified neuroblast (IPC NB) as a model to understand a novel example of serial specification of NB identities in the Drosophila dorsomedial protocerebral neuroectoderm. The IPC NB was specified from a small, molecularly identified group of cells comprising an invaginated epithelial placode. By progressive delamination of cells, the placode generated a series of NB identities, including the single IPC NB, a number of other canonical Type I NBs, and a single Type II NB that generates large lineages by transient amplification of neural progenitor cells. Loss of Notch function caused all cells of the placode to form as supernumerary IPC NBs, indicating that the placode is initially a fate equivalence group for the IPC NB fate. Loss of Egfr function caused all placodal cells to apoptose, except for the IPC NB, indicating a requirement of Egfr signaling for specification of alternative NB identities. Indeed, both derepressed Egfr activity in yan mutants and ectopic EGF activity produced supernumerary Type II NBs from the placode. Loss of both Notch and Egfr function caused all placode cells to become IPC NBs and survive, indicating that commitment to NB fate nullified the requirement of Egfr activity for placode cell survival. We discuss the surprising parallels between the serial specification of neural fates from this neurogenic placode and the fly retina.