Asymmetric cortical extension leads to asymmetric cell division in Drosophila neuroblasts

Drosophila neuroblasts divide asymmetrically to produce two daughter cells of different sizes: a larger neuroblast and a smaller ganglion mother cell (GMC). Live imaging of neuroblasts demonstrates that they divide asymmetrically due to the localization of myosin to the basal pole of the neuroblast (the side that will become the smaller GMC) during anaphase. We hypothesized that this asymmetric myosin localization would lead to reduced extension at the basal pole while extension at the apical pole would not be restricted. We measured the position of the apical and the basal cortex in neuroblasts and observed preferential displacement of the apical cortex that becomes the larger daughter cell during anaphase, effectively shifting the cleavage furrow toward the smaller daughter cell. In neuroblasts the apical pole extends ~3 times more than the basal pole producing an apical daughter cell (neuroblast) that is ~2 times larger than the basal daughter cell (GMC). In mutants that have lost the basal myosin domain, Pins (Partner of Inscuteable) and Gβ, there is equal cortical extension at both poles and the division produces daughter cells of equal size. This asymmetry is not due to unequal addition of membrane at the apical pole as the amount of surface area created during mitosis (determined by 3D reconstruction of the cell) is insignificant compared to the difference in surface area between the daughter cells. We propose a model in which contraction‐driven asymmetric polar extension of the neuroblast cortex during anaphase contributes to asymmetric furrow position and daughter cell size. National Institutes of Health Grant 068032.