The proportion of a wild-type population capable of initiating aggregation centres in starving fields of was investigated using mixed populations of a few wild-type amoebae and a large excess of a mutant incapable of initiating aggregation signals but fully capable of responding to them. For this purpose an aggregation-deficient mutant (designated NP160) was isolated that showed high sensitivity to wild-type strains in synergy tests. This mutant formed normal cell-surface cyclic AMP receptors and phosphodiesterases and was fully capable of chemotaxis to cyclic AMP. Although it had an active adenylate cyclase and formed cyclic AMP, it was deficient in the pulsatile regulation of this enzyme and did not initiate or relay pulsatile cyclic AMP signals and did not form cell adhesive ‘contact sites A’ unless given artificial cyclic AMP pulses. When synergistic mixtures of this mutant and the wild-type were made, the mutant formed fruiting bodies at frequencies indicating that up to 1 in 5 of the wild-type cells could initiate aggregation centres when present at a ratio of between 1 in 500 and 1 in 10000 of the mixed amoebal population.


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