A MODEL FOR PATTERN-FORMATION IN DICTYOSTELIUM-DISCOIDEUM

A model for regulation of cell-type proportions in the mound stage of the cellular slime mold Dictyostelium discoideum has been developed. T he model is based on current information about morphogenetic signaling that controls the differentiation of prestalk and prespore cells from differentiation-competent cells. The morphogens that can control the cell-type transitions are: (i) cAMP, which is required for both presta lk and prespore differentiation, (ii) differentiation-inducing factor (DIF), which induces prestalk and inhibits prespore differentiation, a nd (iii) NH3, which antagonizes all DIF-induced responses. cAMP is ass umed to be a background species at the mound stage produced by oscilla tory cAMP signaling. In addition to its effects on differentiation, cA MP also induces DIF synthesis. DIF levels are controlled by DIF-dechlo rinase, which is produced by the prestalk population in response to DI F stimulation. Induction of DIF-dechlorinase is inhibited by cAMP and NH,. NH, is produced by prestalk cells, which show relatively high cat abolism. As far as possible, we have incorporated quantitative informa tion on transition rates between cell types and the dependence of thos e rates on the morphogen concentrations. Computations using these data produce correct proportions of prestalk cells, but yield a ratio of p respore to undifferentiated cells that is too low. This can be remedie d by assuming that maintenance of prespore differentiation requires a 10-fold lower cAMP concentration than induction. In the absence of NH, and DIF-dechlorinase production, all cells rapidly differentiate into prestalk cells. DIF-dechlorinase is of major importance in setting pr estalk and prespore proportions, but inclusion of NH3 in the presence of DIF-dechlorinase is not strictly necessary in the spatially uniform system studied here. The regulatory capacity of the system is very ro bust, and proportion regulation is not very sensitive to changes in mo rphogen concentrations, beyond a certain level required for differenti ation induction.