A model for the prestalk/prespore patterning in the slug of the slime mold Dictyostelium discoideum.

We propose that the prestalk/prespore pattern in Dictyostelium is generated in two steps: In a first process, an intermingled, non-position dependent prestalk/prespore pattern is generated by a cell-restricted autocatalysis and the antagonistic action of a long-ranging substrate which becomes depleted during this autocatalysis. By computer simulations we show that the assumed interaction accounts for several experimentally observed features of the prestalk/prespore pattern: The size-independent ratio of both cell types, the pattern regulation after removal of one cell type, the development towards one or the other pathway before the slug obtains its final shape or even before aggregation is completed. Our hypothetical substrate may be identical with an experimentally found differentiation-inducing factor (DIF). Alternative molecular realizations of the basic mechanism are discussed. A second process leads to the aggregation of the prestalk cells in a particular region of the aggregate, the future tip region. Interactions which enable tip formation and the coupling between the prestalk/prespore and the tip-forming system are discussed. Our model shows that the formation of a single large patch of differentiated cells and its size regulation requires conflicting parameters. By a separation into a mechanism which determines the position and a second one which determines the size of a structure, each mechanism can be optimized individually without requiring compromises for the other. Such a separation also seems to occur in other developmental systems.