Analytical studies on migrating movement of the pseudo-plasmodium ofDictyostelium discoideum

Summary Migrating movement of a pseudoplasmodium (slug) of the cellular slime mouldDictyostelium discoideum was analyzed using a time-lapse video tape recorder. Since slugs usually migrated with repeated interruptions of advance, migrating velocities were measured only within a period of forward movement. On the basis of some known facts and assumptions, a dynamical model for slug movement was formulated, which consists of motive force generated by slug cells against their intrinsic resistance and resistance of slime sheath at the tip. The migrating velocity of a slug depended neither on its width nor its volume, but solely on its length. Under any experimental conditions tested, a linear relationship always held between reciprocals of the two variables. The results were in good agreement with predictions of the model. Quantitative analyses of experimental results by the use of the model lead to the conclusions that a decrease in velocity at a low temperature is due to an increase in resistance of slime sheath at the tip, but that a decrease in velocity during prolonged migration is due to a decrease in motive force of constituent cells. An anterior isolate dissected from a slug migrated at a velocity greater than that of an intact slug of the same length. This was interpreted by the model to be due to the fact that the anterior cells have greater motive forces and intrinsic resistances than the posterior cells. The heterogeneous distributions of the two variables in the cell mass is discussed in reference to the mechanism of sorting out of cells.