Regulation of cell-type-specific differentiation in Dictyostelium.

During development of Dictyostelium, there are at least a dozen discrete stages of differentiation that can be distinguished by the expression of specific genes. The early stages are triggered by amino acid starvation and are dependent on a small heat-stable effector secreted by the cells to indicate a critical cell density. After development has proceeded for 12 hours, late genes are expressed that are dependent on the conditions found in multicellular aggregates. Cells monitor these conditions and appear to respond by raising their internal cAMP levels to act as a second messenger. Multicellularity can be bypassed as an essential condition if high levels of cAMP are added to the environment. The EDTA-resistant cell-adhesion mechanism that develops by 12 hours is not a required aspect of multicellularity. A final set genes can be induced in 18-hour-developed cells by lowering the pNH3. The spore coat proteins are well-characterized markers for prespore differentiation; their genes are first expressed at 12 hours. Prestalk cells do not express these genes. A small number of prestalk cells become redistributed in the posterior during slug migration and appear to undergo respecification when their position is changed. Prestalk genes become repressed in these "anterior-like" cells and prespore genes are activated. These results clearly indicate that a fieldwide system of positional information regulates cell-type differentiation in Dictyostelium.