Steady rotations in ZS model, (<i>v</i><sub>0</sub>, m<sub>tot</sub>, α) = (2.5, 0.75p, 0.5) (see also S4 Movie).

<p>(<i>a</i>) Transient distributions of myosin (pseudo-colors) and actin velocities (arrows): <i>t</i> = 2, an initially symmetric cell with centroid at (<i>x</i>, <i>y</i>) = (0,0) self-polarizes and assumes fast unidirectional motility, myosin accumulates in a semi-circular band, pulling the rear inwards to form a ‘dip’; <i>t</i> = 7, the cell slows down and becomes unstable, as myosin is now close enough to cell front to be able to pull it in as well; <i>t</i> = 9, loss of axial symmetry, as the lower part of the cell with steeper myosin gradients is pulled inwards faster than the upper one; <i>t</i> = 23.5: emergence of stable asymmetric myosin distribution and cell shape, as the cell locks in rotations (see <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005862#pcbi.1005862.g001" target="_blank">Fig 1<i>C</i></a>). (<i>b</i>) Cell shape and boundary velocities in steady rotations. Positions of the cell boundary and centroid at <i>t</i> = 23.5, 23.6, and 24 (solid, dashed, and dotted-dashed contours, respectively, and filled circles with larger size corresponding to later time). Faster boundary velocities (arrows) in the high curvature region, consistent with the location of steep myosin gradients (panel (<i>a</i>)), ensure rotational motility with a circular trajectory of the centroid (dotted arc), see also <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005862#pcbi.1005862.g001" target="_blank">Fig 1<i>C</i></a>.</p>