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Fig. 3. Embryos overexpressing Reaper and Ricin exhibit distinct cell shape and cytoskeletal changes. (A-C) Frames from time-lapse movies taken under Nomarski optics showing a wild-type embryo (A), an embryo overexpressing Reaper (B, see supplementary material Movie 4), and an embryo overexpressing Ricin (C, see supplementary material Movie 5). Times are indicated from the end of cellularisation. In reaper^OVER embryos, the cells round up shortly after the end of cellularisation, precluding any morphogenetic movements (C+10 minutes). When all cells have rounded up, the cells are found in multiple layers at the surface of the embryo (C+40 minutes). In ricin^OVER embryos, the embryo development is also arrested shortly after the end of cellularisation, before the start of morphogenetic movements. But in contrast to reaper^OVER embryos, cells in ricin^OVER embryos still appear attached basally to the yolk cell and do not round up. (D-G) Close-up of fixed embryos taken under Nomarski optics. In wild-type embryos, the epithelium is made up of a single layer of columnar cells arranged on top of the yolk cell (y). In reaper^OVER embryos, cells round up abruptly at the end of cellularisation (arrowhead), losing both lateral and basal contacts (E) and rearrange to form multiple layers at the surface of the embryo (F). In ricin^OVER embryos, development also arrests at the end of cellularisation, but the cells stay columnar in shape and in a single layer (G). (H-J) Wild-type, reaper^OVER (supplementary material Movie 6) and ricin^OVER (supplementary material Movie 7) embryos labelled with DEcadGFP to visualise the adherens junctions. In wild-type, DEcadGFP is enriched in an apical and basal junction towards the end of cellularisation (arrowheads in C–10-minute frame). Later on, the basal staining disappears and most of the signal is concentrated in the apical junction (C+10 minutes). As germ-band extension proceeds, the cells become less columnar but DEcadGFP is still clearly localised in an apical junction. In reaper^OVER embryos, DEcadGFP localisation is normal up to the beginning of germ-band extension (frames C–10 minutes and C+10 minutes). When the embryo starts contracting, DEcadGFP is still localised at the cell apices (C+20 minutes) and then becomes delocalised in dots once cells have rounded up (C+35 minutes). By contrast, DEcadGFP is not delocalised in arrested ricin^OVER embryos and remains associated with the apical cortex (Frame C+35 minutes in J). Apical (K-M) and sagital (N-P) views of wild-type, reaper^OVER and ricin^OVER embryos labelled with phalloidin. Actin is dramatically delocalised in reaper^OVER embryos (L,O), and concentrates in a dense spot on one side of the cell. By contrast, actin is still cortical in ricin^OVER embryos (M,P). Moreover, the basal actin at the cellularisation front is still present (P). Note that in the ricin^OVER embryo shown in P, the actin rings at the basal side of the cell have closed, whereas actin rings in WT embryos are still open at late cellularisation (N). This suggests that in arrested ricin^OVER embryos, actin ring closure proceeds in the absence of translation.

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