Zygotically controlled F-actin establishes cortical compartments to stabilize furrows during Drosophila cellularization

Cortical compartments partition proteins and membrane at the cell surface to define regions of specialized function. Here we ask how cortical compartments arise along the plasma membrane furrows that cellularize the early Drosophila embryo, and investigate the influence that this compartmentalization has on furrow ingression. We find that the zygotic gene product Nullo aids the establishment of discrete cortical compartments, called furrow canals, which form at the tip of incipient furrows. Upon nullo loss-of-function, proteins that are normally restricted to adjacent lateral regions of the furrow, such as Neurotactin and Discs large, spread into the furrow canals. At the same time, cortical components that should concentrate in furrow canals, such as Myosin 2 (Zipper) and Anillin (Scraps), are missing from some furrows. Depletion of these cortical components from the furrow canal compartments precipitates furrow regression. Contrary to previous models, we find that furrow compartmentalization does not require cell-cell junctions that border the furrow canals. Instead, compartmentalization is disrupted by treatments that reduce levels of cortical F-actin. Because the earliest uniform phenotype detected in nullo mutants is reduced levels of F-actin at furrow canals, we propose that Nullo compartmentalizes furrows via its regulation of F-actin, thus gtabilizing furrows and insuring their ingression to complete cellularization.