Cell surface cAMP receptors (cARs) mark the use of cAMP as an extracellular signal. D. discoideum has four homologous cARs. cAR1 is expressed shortly after starvation, while cAR3, cAR2 and cAR4 are expressed at progressively later stages. (45, 46) Recent studies show that the cAR1 gene is deeply conserved in social amoeba evolution and is present in all four taxon groups. (25) The gene duplications that gave rise to cAR2, cAR3 and cAR4 only occurred in group 4 (Y. Kawabe and P. Schaap, unpublished results). The basal cAR1s are functionally identical to D. discoideum cAR1, but there is a marked difference in their developmental regulation. The cAR1s from the basal groups are expressed as a single mRNA after aggregation while, in group 4 species, a second cAR1 mRNA is expressed before and during aggregation. (25) Transcription of this early mRNA is driven by a second promoter that is more distal from the cAR1 coding sequence than the promoter that drives expression after aggregation. (47) Also in the gene encoding the extracellular cAMP phosphodiesterase, the promoter that drives late expression is proximal to the coding sequence and the promoters that drive expression before and during aggregation are more distal. (48) This arrangement suggests that the use of cAMP as chemoattractant by the evolutionary younger group 4 species was achieved by addition of distal promoters to existing cAMP signalling genes.