Auto-oscillatory Processes and Feedback Mechanisms in Physarum Plasmodium Motility

Abstract

The multinucleate plasmodium of the acellular slime mold Physarum polycephalum is one of the most suitable organisms for the study of protoplasmic streaming and amoeboid motion. During migration on the surface of agar or any moist surface the plasmodium differentiates into a leading frontal zone and a network of interconnected strands at the posterior region (Fig. 1). The strands, which can be up to 2 mm in diameter, contain the gel-like ectoplasm forming a cortical tube and the sol-like liquid endoplasm representing the core of the tube. The endoplasm exhibits a regular back-and-forth streaming activity throughout the plasmodium with a velocity up to 1 mm/s and a characteristic periodicity for changing the streaming direction of about 2 min (Wohlfarth-Bottermann 1979). This so-called shuttle streaming of the endoplasm is caused by non-stationary gradients of the intracellular pressure, which are generated by periodic contractions of the ectoplasm (Kamiya 1981). Any excised fragments of the plasmodium or even droplets of the endoplasm extruded by puncturing plasmodial strands are capable of rhythmical contractions (Achenbach et al. 1979) indicating that both the force-generating ability and the driving oscillator are distributed throughout the plasmodium.