Starvation is a crisis that the cells confront with immediate action. Polysomes are degraded; transcripts that are required for growth disappear; and the cell cycle halts. All of development is accomplished without the addition of new metabolic reserves, and so there is selective advantage in using the reserves that have been accumulated during the trophic phase as parsimoniously as possible, without wasting energy on constituents necessary only for growth. Slowly, after the onset of starvation, new protein synthesis begins – only after a few hours do many of the transcripts and proteins that will mediate aggregation appear. This earliest period of development leads to the induction of genes necessary for aggregation, and it is constructive to think of it as a separate and essential series of events. Chemotaxis and aggregation are not the earliest developmental events.

Cells can detect imminent starvation

In the laboratory, when development is induced by abruptly washing away nutrients, the onset of starvation is sudden. In the soil, depletion is more gradual and the cells have mechanisms to sense when hard times are approaching. There are two density-sensing mechanisms that function during the early stages of development. One mechanism is mediated by a molecule called prestarvation factor (PSF) and controls induction of certain very early genes (Rathi and Clarke, 1992). The other mechanism, mediated by a molecule called conditioned medium factor (CMF) (Gomer et al., 1991), helps the cells to assess density at a slightly later period – during aggregation (see below).