Loss of Notch pathway activity reveals the existence of a proneural band in the notum.

The red and yellow arrows in various panels point to regions (PNP/ASA and DC/SC) where the cells adopted the SOP fate latest. (A) Expression pattern of sca-lacZ in the notum of a late third instar wing disc. The arrows point to the region with low expression between the PNCs. (B–D) The comparison of sca-lacZ with the expression of the DC-E confirmed that sca is expressed also between PNCs (arrows). (E) Expression of sca-lacZ is elevated between PNCs in mib1 mutant disc causing some PNCs to fuse (arrows, compare with (A)). (F) Pattern of the SOPs in the disc shown in (E). The arrowhead highlights an ectopic SOP. (G, H) Expression of the early SOP marker E(spl)m8SM in wt (G) and mib1 mutant discs (H). (I–K) In the absence of Psn function, the regions between the PNC up-regulate the expression of sca-lacZ and eventually form of a band-like region at 2 h apf. The vast majority express Hnt and sca-lacZ, indicating that they became SOPs. (L-N) The same is observed in nic mutant wing discs. The duplicated white and yellow arrows in (L) highlight the duplication of the proneural band caused by a wing to notum transformation often observed in these mutants. White scale bar 50µm; cyan scale bar: 10µm.