Successive specification of Drosophila neuroblasts NB 6-4 and NB 7-3 depends on interaction of the segment polarity genes wingless, gooseberry and naked cuticle

The Drosophila central nervous system derives from neural precursor cells, the neuroblasts (NBs), which are born from the neuroectoderm by the process of delamination. Each NB has a unique identity, which is revealed by the p roduction of a characteristic cell lineage and a specific set of molecular markers it expresses. These NBs delaminate at different but reproducible ti me points during neurogenesis (S1-S5) and it has been shown for early delam inating NBs (S1/S2) that their identities depend on positional information conferred by segment polarity genes and dorsoventral patterning genes. We h ave studied mechanisms leading to the fate specification of a set of late d elaminating neuroblasts, NB 6-4 and NB 7-3, both of which arise from the en grailed (en) expression domain, with NB 6-4 delaminating first. In contrast to former reports, we did not find any evidence for a direct role of hedge hog in the process of NB 7-3 specification. Instead, we present evidence to show that the interplay of the segmentation genes naked cuticle (nkd) and gooseberry (gsb), both of which are targets of wingless (wg) activity, lead s to differential commitment to NB 6-4 and NB 7-3 cell fate. In the absence of either nkd or gsb, one NB fate is replaced by the other. However, the t emporal sequence of delamination is maintained, suggesting that formation a nd specification of these two NBs are under independent control.