Feedback between intracellular flow, signaling and active stresses in Physarum plasmodial fragments

Physarum polycephalum is a multinucleated slime mold whose endoplasm flows periodically driven by the contraction of its ectoplasm, a dense shell of F-actin cross-linked by myosin molecular motors and attached to the cell membrane. Ectoplasm contractions are regulated by calcium ions whose propagation is in turn governed by the flow. We study experimentally how this feedback leads to auto-oscillation by simultaneously measuring endoplasmic flow speed and rheological properties, the traction stresses between the ectoplasm and its substratum and the distribution of endoplasmic free calcium ions. We find that physarum fragments smaller than 100 microns remain round and stay in place. However, larger fragments break symmetry leading to sustained forward locomotion, in process that is reminiscent of an interfacial instability that seems to settle around two different limit cycles (traveling waves and standing waves). By using different adhesive coatings in the substratum we investigate the role of substratum friction in the emergence of coherent endoplasmic flow patterns and overall physarum fragment locomotion.