Drosophila nonmuscle myosin II, Zipper, shows dynamic localization during heart morphogenesis and is required for lumen formation. (A) Before dorsal closure, Zipper (red) is present at high levels at the leading edge of epidermal cells (arrows). Zipper foci are also present in variable numbers at the leading and trailing edge of CBs (inset, arrowheads; CB nuclei are green). (A′) After dorsal closure, high levels of Zipper are at the heart lumen (a, arrowheads) and within (b, arrow) the heart. (B and C) Still frames from a time-lapse movie showing Zipper movement during early and late steps of heart morphogenesis. (B) Before dorsal closure, Zipper accumulates at the leading edge of the cell, disperses through the cytoplasm, and then reaccumulates to initiate another cycle. The broken line indicates the dorsal midline. (C and C′) After dorsal closure, Zipper is found dynamically at the heart lumen (arrowheads). (D) Apical constriction of CBs (arrows) coincides with apical concentration of cardiac-specific Zipper^GFP. Neighboring CBs do not show accumulation of Zip at that time point (arrowheads). (E) Lumen formation requires Zipper. An early stage 17 embryo heterozygous for the zip² null allele has a regular aortic lumen, which is delineated by Slit (arrowheads in center panel; cross-section at the broken line of the side panels). (F) In contrast, the complete zip^2/IIX62 null mutant fails to form a lumen, although Slit still accumulates at the contralateral CB interface. Note that there is a cardiac closure defect that is secondary to an epidermal dorsal closure defect (asterisk). Broken lines indicate the position of the cross section.