A Dictyostelium anterior-like cell mutant reveals sequential steps in the prespore prestalk differentiation pathway.

We have isolated Dictyostelium cell type proportioning mutants in a screen for cAMP signal transduction mutants. All mutants showed reduced numbers of prespore cells as analyzed with prespore-specific antibodies and prespore- and prestalk-specific reporter gene constructs. Formation of prestalk and prespore cells in the mutant HP1 is initially almost normal; however, many prespore cells redifferentiate as anterior-like cells leading to altered cell type proportions. Synergy experiments of HP1 transformant cells with wild-type Ax2 slugs showed the existence of stable subpopulations of cells in the prestalk and prespore regions of the slug. HP1 cells sort exclusively to the back of the prestalk and prespore zone in slugs. These cells will form the upper and lower cup regions of the culminate. Ax2 transformant cells sort to the tip of the anterior prestalk and prespore zone in HP1 slugs and will form the stalk and spores in the culminate. These experiments show that upper and lower cup derive from cells in different differentiation pathways. The lower cup cell derive from prespore cells, which redifferentiate as anterior-like cells in the back of the prespore zone. The upper cup cells derive from cells from the back of the prestalk zone. Synergy experiments showed that the mutation in HP1 results in an altered sensitivity to proportioning signals. The mutation in HP1 may result in a reduced sensitivity to cAMP, which facilitates the redifferentiation of prespore cells via anterior-like to lower cup cells and inhibits the transition from upper cup to stalk cells.