How the Dictyostelium discoideum grex crawls

We propose a new model for propulsion of the Dictyostelium discoideum grex (pseudoplasmodium). We concentrate upon the mechanics of the problem: how does each participating amoeba contribute motive force, and how do the myriad force contributions produce a coordinated collective effort? Experiments we report here show that when a Dictyostelium discoideum grex’s migration is stalled by mechanically arresting the motion of its boundary, the amoebae in it actively circulate in a reverse fountain flow extending the length of the grex. The velocity of individual cells relative to the grex boundary is commensurate with the migration speed of a grex: approximately one grex length per hour. We argue that cell circulation constitutes the propulsive engine of migrating grexes. More precisely, we believe each participating amoeba orients its attempted motion by the same cAMP chemotaxis used during aggregation. The cAMP concentration field within the grex consists of pulses, emitted periodically at the tip, propagating rearward by the same cAMP relay behaviour seen during aggregation. Existing literature documents chemotactic migration within grexes and generally reinforces the preceding description. The principal new contribution of this paper is to resolve the following conceptual difficulty: in a close-packed three-dimensional mass of cells, each amoeba trying to crawl can exert traction only upon its neighbours which, in turn, exert traction on it. In the interior of the grex, with no rigid agar substratum to crawl upon, equal efforts by a cohort of amoebae to crawl in the same direction, each upon similarly crawling neighbours, cancel and produce no net mechanical result.