Fig. 6. Histoblast proliferation and LEC death are triggered by Ecdysone signaling. (A) Blocking Ecdysone signaling autonomously in histoblasts abolished their proliferation. The overexpression of an Ecdysone receptor EcR-RNAi construct (Esg-Gal4) results in a prolonged delay in cell division. The left image shows a dorsal anterior nest at the prepupal stage expressing EcR-RNAi. The number of histoblasts in this nest is similar to equivalent nests in wild-type pupae (see Fig. 1B). The middle image shows the same nest 300 minutes later; histoblasts have not divided. In this period, wild-type animals undergo three rounds of division (right image). (B) Clonal autonomous inhibition of Ecdysone signalling in LECs (dominant-negative form of the Ecdysone receptor EcR-DN) blocks their death (see Materials and methods). The left image shows a control animal in which the abdominal epithelial fusion proceeds normally. As a result of the overexpression of EcR-DN (middle image) the expansion of histoblast nests does not succeed, resulting in a dorsal scar phenotype (arrowheads). Simultaneous expression of a GFP marker reveals that LECs (asterisk) have not been eliminated from the abdomen of pharate adult mutant flies (green: right image).