Vnd and En are expressed in orthogonal stripes and act in a brief competence window to combinatorially specify NB7-1 and its early lineage

Understanding how neuronal diversity is generated is a major goal of neuroscience. Here we characterize the first step in generating neuronal diversity in the Drosophila embryo: spatial transcription factors (STFs) expressed in orthogonal rows and columns of neural progenitors. These factors give spatial identity to neural progenitors (neuroblasts, NBs), and are highly conserved in mammals. Here we investigate the roles of Engrailed (En+; posterior row) and Vnd+ (medial column) in specifying the well-characterized progenitor: neuroblast 7-1 (NB7-1). We show that NB7-1 is located at the intersection of Vnd and En, and we identify NB7-1 using a newly characterized gene, fd4 , that we show is specifically expressed in NB7-1 and its progeny, giving us a specific assay for NB7-1 identity. We show that En and Vnd are both required for Fd4 expression, and that Vnd and En co-expression is sufficient to induce ectopic Fd4 expression in other NBs and their lineages. Finally, we show that NBs gradually lose competence to respond to En or Vnd. We conclude that En and Vnd are STFs that act combinatorially to specify the identity of an individual progenitor, NB7-1.