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Fig. 1. Abdominal metamorphosis: cell proliferation dynamics of histoblasts. (A) During embryonic stages, four nests of abdominal histoblasts (adult epidermal cells precursors) can be distinguished in each hemisegment: anterior dorsal (green mask color), posterior dorsal (red), ventral (blue) and spiracular (yellow). During metamorphosis, histoblasts form the different structures that compose the abdominal adult epidermis, tergites (green), intersegmental membranes (red), pleurites and sternites (blue) and spiracle (yellow) (Roseland and Schneiderman, 1979). (B) In vivo time-lapse observation of histoblast proliferation in prepupal stages (1-6 hours APF) shows a synchronic cadence of three cell cycles leading to cell doubling every 2 hours. The number of cells calculated at the shown time points steadily increase (18, 36, 72 and 128). Cell sizes decrease after each mitosis (856, 605, 396 and 271 arbitrary 2-dimensional units; see Materials and methods). Histoblasts (anterior dorsal nest) expressed UAS-GFP under the control of Esg-Gal4 (see Movie 1 in the supplementary material). (C) Early histoblast cell divisions show planar orientation. Histoblasts (anterior dorsal nest) expressed UAS-Tau-GFP under the control of Esg-Gal4. In the first cell division, spindles orient predominantly along the dorsoventral axis. (D) Doubling times of histoblasts during pupal stages (from 15 hours APF onwards) increase up to 9 hours. Proliferation is stochastic and coupled to cell growth. Cellular outlines (anterior dorsal nest) were highlighted by ubiquitously expressing a DE-Cadherin-GFP fusion. Mitoses of individual cells were followed by in vivo time-lapse (see Movie 2 in the supplementary material). (E) Schematic of histoblast cell cycle dynamics. During prepupal stages (left), histoblasts do not grow between cycles (68% cell size decrease in the first three divisions). By contrast, during the pupal stages, histoblasts undergo intermitotic growth and their sizes remain constant (right). (F) FACS analysis showing cell cycle profiles of dissociated histoblasts from prepupal (black) and pupal stages (red). During prepupal stages, histoblasts lack or have a very reduced G1 phase. In pupal stages, the length of G1 phase increases by 70%.

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