Myosin comes and goes during fly embryogenesis.

Early in Drosophila development, the embryo is a syncytium, and the cloud of dividing nuclei expands along the long axis of the embryo. This nuclear axial expansion requires tight coordination between mitosis and the actin–myosin cytoskeleton; but how do these two systems communicate? On page 127, Royou et al. show that a spatially and temporally regulated cycle of myosin recruitment to and dispersion from the embryo cortex in this system is indirectly controlled by the cell cycle regulator Cdc2. A similar mechanism may be at work in other types of mitotic cells.

Using time-lapse confocal microscopy of embryos expressing fluorescently tagged myosin, the authors found that cytoplasmic myosin is repeatedly recruited and dispersed with the same rhythm as the mitotic cycle. Myosin is recruited only to the cortical regions above the cloud of nuclei, and each cycle of recruitment is accompanied by a cortical contraction, coinciding with interphase. The myosin then disperses, and the cortex relaxes during metaphase. The cycles of contraction and relaxation may cause a cytoplasmic flux that drives the nuclei poleward.

Cdc2 activity regulates this process indirectly, acting through Rho kinase. Since ordinary mitotic cells must coordinate actin–myosin cytoskeletal movements with the cell cycle to form the contractile ring, this pathway may be a general feature of eukaryotic mitosis. ▪