Upon starvation, the cellular slime mould Dictyostelium discoideum initiates a 24-h programme of differentiation1. Within 6 h, cells move towards aggregation centres in response to pulsatile synthesis and secretion of cyclic AMP2-4. At about 12 h, aggregates of 10 5 cells are formed, held together by newly made surface adhesion molecules5-8. The cells then differentiate into the two principal types found in the terminal stage of development, spores and stalks. Here we show that the chemotaxis and aggregation stages of this developmental programme can be described as a series of sequential events in which these extracellular signals - starvation, cyclic AMP and cell-cell contact - induce specific, sequential changes in the pattern of gene expression.
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