Evolutionary crossroads in developmental biology: Dictyostelium discoideum

Abstract

Dictyostelium discoideum belongs to a group of multicellular life forms that can also exist for long periods as single cells. This ability to shift between uni- and multicellularity makes the group ideal for studying the genetic changes that occurred at the crossroads between uni- and multicellular life. In this Primer, I discuss the mechanisms that control multicellular development in Dictyostelium discoideum and reconstruct how some of these mechanisms evolved from a stress response in the unicellular ancestor.

Fig. 1.

The life cycle of Dictyostelium discoideum. (A) Starving D. discoideum amoebas secrete pulses of cyclic adenosine monophosphate (cAMP, indicated by grey rings) that (B) cause the chemotactic aggregation of cells into mounds. Cells in the mound move upwards in response to continued cAMP emission from its top, (C) causing the formation of a tipped mound. The differentiation of cells into two cell types, prespore and prestalk A, begins in these mounds. (D) A mound then forms a slug, in which further differentiation of cells into prestalk O, B and AB cells occurs. (E) The slug then falls over and starts to migrate. (F) Finally, it undergoes fruiting body formation, during which the different cell types migrate to specified locations in the fruiting body and (G) ultimately differentiate into spores, stalk cells and the structures that support the stalk and spore head. (H) After their dispersal to nutrient-rich habitats, spores germinate and (I) resume proliferation as individual amoebae. Modified with permission from Schaap, 2007 (Schaap, 2007).

Fig. 2.

Phylogeny of Dictyostelia. All known species of social amoebas can be subdivided into four major groups based on phylogenetic analysis of their small subunit ribosomal RNA sequences (Schaap et al., 2006). Group 1 (red) is most closely related to the solitary amoebozoan ancestors. Groups 1-3 have retained the ancestral survival strategy of encystation of individual amoebas. This strategy is lost in Group 4 (purple), the most recently diverged group, which contains the model organism D. discoideum. Group 4 species also show other features, such as the use of cyclic adenosine monophosphate (cAMP) as a chemoattractant and cellular supports for the fruiting body, that are not (or seldomly) displayed by species in Groups 1-3. Coloured arrows point to species with completely sequenced genomes. Sequencing of the D. lacteum genome (group 3, indicated by a dashed arrow) is still in progress. Scale bar: 0.1 substitutions per site. Line thicknesses of branches correlate to the probability that nodes that connect immediate descendants are correct, as determined by Bayesian inference (Ronquist and Huelsenbeck, 2003). Modified with permission from Schaap et al. (Schaap et al., 2006).

Fig. 3.

Regulation of the cAMP phosphodiesterase RegA. RegA controls intracellular cAMP levels in D. discoideum cells and, thus, the level of PKA activity. The schematic shows the extracellular and intracellular environment of a D. discoideum cell and how RegA (green) is activated by phosphorylation (P) of an aspartate (D) in its response regulator (RR) domain. This phosphorylation is regulated by a sensor histidine kinase, DhkC. (A) The binding of ammonia (NH₃) to the sensor domain of DhkC (outlined in black) leads to the activation of its histidine kinase activity and to the autophosphorylation of a DhkC histidine (H) residue. The phosphoryl group is then transferred to an aspartate in the same protein, and from there to a histidine in the phospho-transfer intermediate RdeA (B), and finally to RegA (C). By contrast, the binding of SDF-2, which is present extracellularly, to the sensor domain of DhkA (outlined in black), causes histidine-mediated dephosphorylation of an attached aspartate residue (D) and reverse phosphotransfer via RdeA (E). This leaves RegA without a phosphoryl group, thereby inactivating cAMP hydrolysis. ADP, adenosine diphosphate; ATP, adenosine triphosphate; cAMP, cyclic adenosine monophosphate; DhkA, histidine phosphatase A; DhkC, histidine kinase C; PKA, cAMP-dependent protein kinase A; RdeA, phosphotransfer intermediate Rapid Development A; RegA, intracellular cAMP phosphodiesterase with response regulator; SDF-2, spore differentiation factor 2.

Fig. 4.

Coordination of D. discoideum stalk and spore maturation by cAMP. (A) In prestalk cells at the slug tip, cAMP is synthesized by ACR and can either be hydrolysed by RegA or bind to PKA to trigger stalk encapsulation, which normally occurs when the tip projects upward at the onset of fruiting body formation and loses ammonia (NH₃) by gaseous diffusion. NH₃ inhibits PKA by indirectly activating cAMP hydrolysis via DhkC-RdeA-mediated phosphorylation of RegA. (B) In prespore cells, PKA activation occurs as a consequence of a signalling cascade that is activated by SDF-3, which triggers the synthesis of GABA by prespore cells. Secreted GABA then acts on prestalk cells to induce the cell-surface exposure of the serine protease domain of the ABC transporter TagC. GABA also acts on prespore cells to induce secretion of AcbA, which in turn is cleaved by the TagC protease to produce the peptide SDF-2. SDF-2 next activates the histidine phosphatase activity of DhkA, causing inactivation of cAMP hydrolysis by RegA. This allows cAMP to accumulate and activate PKA, which in turn induces spore encapsulation. ABC, ATP-binding cassette; AcbA, acyl-coenzyme A binding protein; ACR, adenylate cyclase R; ATP, adenosine triphosphate; DhkA, histidine phosphatase A; cAMP, cyclic adenosine monophosphate; DhkC, histidine kinase C; GABA, γ-aminobutyric acid; GadA, glutamate decarboxylase A; GrlA, G-protein coupled receptor-like protein A; GrlE, G-protein coupled receptor-like protein E; PKA, cAMP-dependent protein kinase A, with R and C denoting the regulatory and catalytic subunits, respectively, and a star to denote that the catalytic subunit is active in the presence of cAMP; RdeA, phosphotransfer intermediate Rapid Development A; RegA, intracellular cAMP phosphodiesterase with response regulator; SDF-2, spore differentiation factor 2; SDF-3, spore differentiation factor 3; TagC: tight aggregate C, an ABC transporter with intrinsic serine protease domain.

Fig. 5.

Evolution of cAMP signalling in Dictyostelia. A tentative hypothesis for the evolution of developmental cAMP signalling in Dictyostelia from a cAMP-mediated stress response in the solitary ancestor. The four groups of Dictyostelia (see Fig. 2) are roughly estimated from accumulated changes in their small subunit ribosomal RNA and α-tubulin sequences to have diverged from their last common amoebozoan ancestor about 1 billion years ago (Schaap et al., 2006). (A) The amoebozoan ancestor is likely to have used cAMP as an intracellular second messenger for stress-induced encystation. During the long era of Dictyostelia evolution, the mechanisms associated with synthesis, production and degradation of cAMP increased in complexity and acquired novel roles in cell differentiation and control of cell movement. (B) Accumulation of secreted cAMP was probably first used as a signal in aggregates to prompt cells to form spores. (C) Next, pulsatile cAMP secretion evolved to coordinate cell movement during fruiting body formation. (D) Finally this process was brought forward in development to coordinate the aggregation process. ACA, adenylate cyclase A; ACG, adenylate cyclase G; ACR, adenylate cyclase R; cAMP, cyclic adenosine monophosphate; cAR, cAMP receptor; PKA, cAMP dependent protein kinase A.