Unraveling cytoplasmic streaming using a coarse-grained model of microtubule hydrodynamics

During the development of the fruit fly oocyte, flows with short-ranged correlations transition to a dramatic cell-spanning vortex, accompanied by coherent deformations in the microtubule cytoskeleton. Using a coarse-grained model for the hydrodynamics of ordered fibers, we show that sufficiently dense microtubule arrays, forced only by molecular motors transporting cargo, undergo a ``swirling transition'' that is fundamentally different than the buckling transition which leads to the flapping motion of isolated filaments. Our model produces streaming velocities consistent with \emph{in vivo} measurements, and allows us to place bounds on the number density of kinesin-1 motors transporting cargo within the microtubule array.