| Antero-posterior polarity in Caenorhabditis elegans is established by a large-scale flow of the actomyosin cortex. Flows appear to arise when active cortical myosin motors become unevenly distributed. We have shown that polarising cortical flow in the C. elegans zygote is established by two essential factors: a gradient in contractility to drive flow and a sufficiently large viscosity of the cortex to allow cortical flow to be long-ranged. We arrive at this conclusion by reporting on the subcellular distribution of cortical tension in the living C. elegans zygote, which we determine in a position- and direction-sensitive manner using a novel laser ablation assay. In contrast to previous proposals, we find that cortical flow is not associated with tension gradients, but instead induces anisotropies in cortical tension. Anisotropies are the signature of a highly viscous cortex that resists compression, which we show by a theoretical description of active cortical mechanics. Our results reveal the underlying physical mechanisms that allow for large-scale cortical flow, to allow for efficient polarity establishment in C. elegans. |
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