Fig. 3. Apoptosis of both motoneurons requires Ubx function at a late developmental stage. NB7-3 neurons in late stage 16 (A,B,G,H) and stage 15 (C) Drosophila embryos and NB2-4 (D-F) neurons in late stage 16 embryos. Dashed circles, NB7-3 and NB2-4; dashed lines mark the midline. (B',C',G',H') Magnified single scans of NB7-3 with GW outlined. (B'',C'',G'',H'') Only the activated is shown; Caspase 3 staining (act Casp3) is shown. (E',F') Magnified single scans of NB2-4t with MNa outlined. (E'',F'') Only the activated Caspase 3 staining (act Casp3) is shown. Anterior is up in all images. (A) Ubx expression levels are high in GWs that undergo apoptosis (arrows) and are barely, if at all, detectable in GWs that survive (arrowheads). (B-B'') Apoptosis of GWs is abolished in a Ubx loss-of-function mutant (arrows). The apoptotic cell in T2 (curved arrow) is not the GW (arrowhead). (C-C'') Ectopic Ubx expression induces apoptosis of GWs in anterior thoracic segments (arrowheads). These neurons now die prematurely in T3 and in abdomen as well (arrows). (D) Ubx is strongly expressed in the anterior, apoptotic NB2-4t motoneuron (arrows). Expression levels are low in the posterior motoneuron. In T2, Ubx is not expressed in these neurons (arrowheads). (E-E'') In a Ubx loss-of-function mutant, apoptosis of the MNa in T3 is abolished (arrows). In A1, these neurons are transformed towards a thoracic phenotype (arrows), and do not undergo apoptosis. T2 is unaffected (arrowheads). (F-F'') Ectopic Ubx expression induces apoptosis of MNa in anterior thoracic segments (arrowheads), and increases MNa apoptosis in T3 (arrow). Asterisks indicate that MNa has already been removed. (G-G'') Early ectopic Ubx expression cannot induce GW apoptosis in anterior thoracic segments (arrowheads). (H-H'') Late ectopic Ubx expression induces GW apoptosis in anterior thoracic segments (arrowheads), and increases it in T3 and abdomen. Genotypes: (A,D) wild type, (B) Ubx¹, (C) en-Gal4/+;UAS-Ubx/+, (E) UAS-CD8::GFP/+;eg-al4,Ubx¹/Ubx¹, (F) poxN-Gal4,UAS-GFP/UAS-Ubx, (G,H) hs-Ubx.