Phase switching of oscillatory contraction in relation to the regulation of amoeboid behavior by the plasmodium of Physarum polycephalum.

The plasmodium of the true slime mould Physarum polycephalum is a large aggregate of protoplasm and behaves like an amoeboid cell, exhibiting rhythmic contraction everywhere within the organism. Phase dynamics of these oscillations were studied in relation to the global organization of amoeboid behavior, by analysing the thickness oscillation, isotonic tension and the motive force of the streaming. Usually the plasmodium showed synchrony, the phase of the oscillation being the same everywhere excepting the peripheral part. We found several situations where this in-phase relationship switched to anti-phase. This occurred either at the early stages of the plasmodial coalescence, or when a single plasmodium was nearly separated by partition, or when the streaming of the protoplasm was hindered by applying the hydrostatic pressure. Furthermore, the motive force of the protoplasmic streaming increased once the anti-phase relationship was established. In this way, the weak interactions among plasmodial parts induce the switching of phase relationship from in-phase to anti-phase, and this transition in turn acts to increase the interaction by promoting a rapid mixing of the protoplasm. This global feedback mechanism by phase switching should help maintain a large single plasmodium without separating into parts. The possible mechanism of phase switching is discussed in terms of coupled nonlinear oscillators.