Mosaic analysis of ligand-induced neurogenesis A-C. Mosaic thoraces from y hsFLP; FRT42 109-68 [y+]/FRT42; UAS-SESI flies. Yellow bristles (arrows) derive from recombinant cells lacking 109-68-dependent Ser expression. A maximum of one yellow bristle is seen in each proneural region. D. Mosaic thorax derived from f36a hsFLP; [abx>f+>Gal4]; UAS-DESI fly. Forked bristles derive from recombinant cells expressing the Dl transgene. Normal (not forked) bristles lack transgenic Dl expression (arrows). A maximum of one normal bristle is seen in each proneural region. These mosaics show that Ser and Dl overexpression are cell autonomously neurogenic and do not recruit neighboring cells to neural fates. They further suggest that each proneural region contains at least one cell not sensitive to high levels of ligand expression nearby.