Flow-dependent myosin recruitment during Drosophila cellularization requires zygotic dunk activity

Abstract

Actomyosin contractility underlies force generation in morphogenesis ranging from cytokinesis to epithelial extension or invagination. In Drosophila, the cleavage of the syncytial blastoderm is initiated by an actomyosin network at the base of membrane furrows that invaginate from the surface of the embryo. It remains unclear how this network forms and how it affects tissue mechanics. Here, we show that during Drosophila cleavage, myosin recruitment to the cleavage furrows proceeds in temporally distinct phases of tension-driven cortical flow and direct recruitment, regulated by different zygotic genes. We identify the gene dunk, which we show is transiently transcribed when cellularization starts and functions to maintain cortical myosin during the flow phase. The subsequent direct myosin recruitment, however, is Dunk-independent but requires Slam. The Slam-dependent direct recruitment of myosin is sufficient to drive cleavage in the dunk mutant, and the subsequent development of the mutant is normal. In the dunk mutant, cortical myosin loss triggers misdirected flow and disrupts the hexagonal packing of the ingressing furrows. Computer simulation coupled with laser ablation suggests that Dunk-dependent maintenance of cortical myosin enables mechanical tension build-up, thereby providing a mechanism to guide myosin flow and define the hexagonal symmetry of the furrows.

Spatiotemporal recruitment of RhoGTPase protein GRAF inhibits actomyosin ring constriction in Drosophila cellularization

“…Since Sqh-mCherry marked the furrow tip, it was also used to estimate furrow lengths during cellularization. The dynamics of furrow length increase in the slow and fast phase in GRAF-OE were similar to controls during cellularization (Figure 6-figure supplement 1A) (Figard et al, 2013;He et al, 2016;Lecuit and Wieschaus, 2000;Merrill et al, 1988;Royou et al, 2004;Warn and Magrath, 1983). In contrast, the slow phase in Graf CR57 was extended and the fast phase started at a later time point (Figure 6-figure supplement 1A).…”

“…Drosophila cellularization occurs by plasma membrane furrow extension between adjacent nuclei from 3 μm to 40 μm in approximately 45 min at 25 o C (He et al, 2016).…”

“…Control and mutant embryos were stained with fluorescently coupled phalloidin which marks cortical F-actin (Figure 1C,D). During early stages of cellularization (furrow length 3-6 μm), the F-actin network at the basal part of furrow was organised as a polygonal array (white arrowheads, Figure 1C) (He et al, 2016;Xue and Sokac, 2016). The polygonal architecture transitioned to a circular shape on constriction at the furrow tip in mid cellularization (furrow length 6-16 μm) and constricted further in late cellularization (furrow length 16-40 μm) (Xue and Sokac, 2016) (Figure 1B).…”

Measurement of cortical elasticity in Drosophila melanogaster embryos using ferrofluids

“…For example, in early Drosophila development, nuclear division and mitosis during syncytial division take <2 min. The redistribution of actomyosin to the site of incipient cellularization furrow invagination occurs over ∼14 min (18). During gastrulation, mesoderm apical constriction (which precedes ventral furrow invagination) completes in about 10 min (15).…”

Transcytosis in the development and morphogenesis of epithelial tissues

“…Subsequent electron microscopy showed that the apical membrane is initially rich with microvilli that flatten as cellularization progresses. Both endocytosis and mechanical unfolding of these microvilli have been proposed to drive this change in apical membrane topology (Fabrowski et al, 2013;Figard et al, 2013;Figard et al, 2016;He et al, 2016). Other evidence suggests that Rab8 facilitates directed exocytosis to lateral membrane faces, allowing endosomal membrane to be added to these growing membrane faces (Mavor et al, 2016).…”