Fig. 2. Genetic evidence for cell death in the vmd1a lineage. (A-C) Ventral Cut-positive md neurons and es organs in segments A1-A7 of wild-type (A), Df(1)H99 (B) and arm-Gal4 UAS-p35 (C) embryos stained for Cut (red), Elav (blue) and Pros (green). (D) Diagram of the ventral-most Cut-positive md neurons and es organs in segments A1-A7 of Df(1)H99 or arm-Gal4 UAS-p35 embryos. The Cut-positive md neurons (diamond-shaped cells) are either highly Cut-positive (pink) or weakly Cut-positive (light purple). Es neurons, sheath cells and socket/shaft cell pairs are as in Fig. 1A. One ectopic es organ (bold outline in D) is observed near the vmd1a neuron in Df(1)H99 (B) and arm-Gal4 UAS-p35 (C) embryos. This es organ is composed of one Elav-positive es neuron (arrow), one Pros-positive sheath cell (arrow) and two Cut-positive cells identified as the socket and shaft cells (arrowheads; one of these two cells accumulates Suppressor of Hairless [Su(H)], a socket cell marker; data not shown (Gho et al., 1996)). This phenotype is observed in 40% of the segments of arm-Gal4 UAS-p35 embryos (n=52). In other segments, we observed three Cut-positive cells (12%), two Cut-positive cells (12%) or no additional Cut-positive cell (36%). Therefore, in these cases, the level of p35 accumulation may not be high enough to fully inhibit apoptosis in the vmd1a lineage. (E) Model proposed for the vmd1a lineage. The pIIa and pIIIb cells die.

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