Fig. 6. Model for the establishment of cell fate in the MTs. Wg signalling activates the expression of pntP2 in a subset of tubule cells (brown), and is required within this domain to establish proneural gene expression in the PNC (represented here by ac, dark brown). Moderate levels of Ac induce svp expression in the PNC (grey) but, as Ac strengthens in the tip mother cell, a threshold for the initiation of rho expression is reached (blue). The tip mother cell divides to produce the TC and SC (blue). As ac expression is lost from the SC by stage 12, the maintenance of rho expression is later independent of Ac and may depend on a positive feedback loop. The svp- and pntP2-positive cells, in grey, are descended from the PNC and are competent to respond to EGF signalling. They divide in response to Spitz secreted by the tip and sibling cells.