bottleneck mutant embryos display increased stability and recruitment of myosin-II to the furrows. (A) FRAP experiments in bottleneck mutant embryos (Df(3R)tll^e) expressing Sqh::GFP. In bottleneck mutant embryos the recovery rates of Sqh::GFP are slower than wild-type early embryos and similar to wild-type embryos during late stages. Quantification of three independent FRAP experiments is plotted (Student’s t test, P = 2.98 × 10⁻³⁴ wild-type early compared with Df(3R)tll^e embryos). Errors bars represent mean ± SD. (B) Quantification of Sqh::GFP fluorescence intensity at the furrow in early embryos, late embryos, and bottleneck mutant embryos. In bottleneck mutant embryos the levels of Sqh::GFP are significantly up-regulated (approximately threefold) compared with similar stage wild-type early embryos (Student’s t test, P = 0.0048) again like in PI(4,5)P₂/AM-injected embryos. Each dot represents an embryo. At least three independent embryos were quantified. (C–H) Representative confocal sections of an embryo expressing the PI(4,5)P₂ sensor PLCδ1-PH::GFP, which was fixed and stained with anti-GFP (C) and anti-Bnk (D) antibodies. Cross sections (C–E) and apical view (F–H) are shown, illustrating the colocalization (yellow) between PLCδ1-PH::GFP (green) and Bottleneck (red) during the slow phase of cellularization (E–H). (C–E) Red line indicates the focal plane at which the apical view was imaged. (I–N) Representative confocal sections of an embryo expressing the PI(3,4,5)P₃ sensor GFP::AKT-PH, which was fixed and stained with anti-GFP (I) and anti-Bnk (J) antibodies. Cross sections (I–K) and apical view (L–N) are shown, illustrating the colocalization (yellow) between GFP::AKT-PH (green) and Bottleneck (red) during the slow phase of cellularization (K and N). Red line in I–K indicates the focal plane at which the apical view was imaged. Bars: (all panels) 10 µm.