Dictyostelium discoideum: Cell-type proportioning, cell-differentiation preference, cell fate, and the behavior of anterior-like cells in Hs1/Hs2 and G+/G− mixtures

Abstract

We determined the proportions of prestalk, prespore, and anterior-like cells in slugs of a ‘short-prestalk’ mutant, Hs2, and its parent strain Hs1 using a prespore-specific monoclonal antibody. Hs2 has fewer prestalk cells and more prespore cells than its parent; the fraction of anterior-like cells is unchanged. In experiments in which vegetative Hs1 and Hs2 cells were mixed and XRITC was used to stain one of the strains, we found that Hs2 amebae preferentially form prespore cells, while Hs1 amebae preferentially become prestalk cells. Using the data obtained from these experiments and additional data concerning the overall cell-type proportions of the chimeric slugs, we calculated the cell-type proportions in the individual strains in the mixtures. In both strains the fraction of prestalk cells increased, and the fraction of prespore cells decreased as the proportion of Hs2 cells in the mixture increased. This shows that the cell-differentiation decision involves some form of supracellular communication. Both the cell-type proportions of pure Hs2 cells and their effect on cell behavior in mixtures can be explained using a simple negative-feedback model in which Hs2 has increased sensitivity to a ‘prestalk inhibitor’ — a hypothetical substance, produced by prestalk cells, that inhibits prestalk differentiation. We also performed mixing experiments using axenic cells grown with or without glucose (G+ and G− cells). The results were similiar to those obtained for Hs1/Hs2 mixtures, except for a striking difference in the behavior of anterior-like cells. In Hs1/Hs2 mixtures, the Hs1/Hs2 ratio among anterior-like cells was similar to the ratio among prespore cells. In C+/G- mixtures, in contrast, the anterior-like cells ‘behaved’ like prestalk cells. This suggests that there are, in fact, two mechanisms controlling cell differentiation, one controlling the ratio of prestalk cells to anterior-like cells, and the other controlling the ratio of anterior-like cells to prespore cells.