The cellular basis of movement of the migrating grex of the slime mould Dictyostelium discoideum: chemotactic and reaggregation behaviour of grex cells

The chemotactic behaviour of cells of the migrating grex of D. discoideum has been studied by observing the responses of small groups of cells placed near to aggregation streams, agar blocks containing cyclic AMP or migrating grexes. Grex cells placed near aggregation streams were not attracted to the streams initially; after about 1 h, however, some cells were attracted to the streams. Similarly, grex cells placed near agar blocks containing cyclic AMP were not initially attracted towards the blocks, but some attraction was observed after about 1 h; the most effective cyclic AMP concentration was 10–4m. Grex cells which were placed in heaps against the sides of aggregation streams, instead of a short distance away, were often incorporated into the streams within ½ h. Grex cells were not attracted towards migrating grexes. Because of these results it is suggested that (1) migrating grex cells are not chemotactically sensitive to cyclic AMP, but they acquire chemotactic sensitivity after separation from the grex for about 1 h; (2) chemotaxis is probably not involved in the movement of the migrating grex; (3) grex cells retain the adhesive property found in aggregating cells, which is involved in the formation of end-to-end contacts necessary for the behaviour known as contact following. Reaggregation of dispersed grex cells was studied by time-lapse photography. Reaggregation was found to be a two-stage process: first, an adhesive phase dependent on the development of intercellular contacts at the expense of adhesions with the substratum; secondly, a chemotactic phase which begins about 1 h after dispersal of the grex cells. It is suggested that the first phase may be accounted for by assuming that the cells are more cohesive than adhesive.