Arp2/3 is required for displacing actin bundles away from each other and toward the cap circumference. (A–D) Moe-ABD–GFP in single cycle 11 caps. Time = 0 is arbitrary. Genotypes begin at the same stage. (A) In control RNAi during early cap growth, strongest bundle-like signal is observed just below apical cap surface at the base of cap folds (top row; purple asterisks at either side of a bundle in side and surface views). Bottom two rows show bundle displacements away from each other (double-headed arrows) and to cap circumference (green dots). At final time point, apical cap is flattened and largely devoid of bundles. Observed in 7/7 embryos. (B) With dia RNAi, Moe-ABD–GFP only labeled puncta across the cap. Bundles not detected. Observed in 7/10 embryos (3/10 had fragmented bundles). (C) With Arp3 RNAi, bundles detected but not associated with folds (side views) and collapsed toward the cap center over time (purple dots). 7/10 embryos lacked folds. Bundles collapsed in 5/10 embryos. No bundle movement in 3/10 embryos. Centrifugal bundle displacement in 2/10 embryos. (D) With zip RNAi, bundles associated with folds (side views) displaced from each other (double-headed arrows) and toward cap circumference (green dots). Note abnormal overall cap shape. Observed in 5/5 embryos. (E) Model of bundle induction mechanism and how bundles might function as part of a composite material for metaphase compartment formation (see Discussion). Time given in minutes and seconds.