Figure 2.

Evolution of developmental signalling from a stress response. Dictyostelia are members of the mostly unicellular kingdom of Amoebozoa. They can be subdivided into two branches each containing two major groups [6]. Comparative analysis of cAMP and DIF-1 signalling across the phylogeny suggests a scenario for the evolution of developmental signalling. cAMP was first used as intermediate for stress-induced encystation in unicellular amoebozoa, with stress acting on sensor histidine phosphatases to inhibit RegA, causing cAMP produced by AcrA, to increase and activate PKA and thereby encystation [34•, 35]. During Dictyostelid evolution sensor histidine kinases and phosphatase acquired novel roles in sensing developmental signals that control timely spore and stalk maturation. Early aggregating prototypes use secreted cAMP accumulating in aggregates as a signal for spore formation [39]. In early Dictyostelia, an emerging network of CarA, AcaA and PdsA produces cAMP pulses to coordinate fruiting body morphogenesis [38, 43]. Finally, group 4 acquires DIF-1 as a signal for basal disc formation, while addition of distal ‘early’ promoters to carA and acaA genes, and increased affinity of PdsA enables the use of cAMP as chemoattractant for aggregation in group 4 [38, 43, 48].