The coordination between mechanical and chemical subsystems initiates locomotion of 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. 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). We use both theory and experiments to study how coordination emerges between the different mechanical and chemical subsystems of the fragment to initiate locomotion. The role of many involved factors, such as fragment size, substratum adhesiveness, rheological properties, actin polymerization and traction stresses are investigated, and we find they agree well with our predictive model.