Figure 1.

The Dictyostelium discoideum life cycle includes multicellular stages. (A) During the growth phase of development, amoeboid cells feed on bacteria and replicate by binary fission. The development cycle is initiated upon resource depletion, and aggregation occurs when starving cells secrete cyclic AMP to recruit additional cells (B). The aggregating cells organize to form the mound stage enclosed within an extracellular matrix composed of cellulose and mucopolysaccharide (26) (C) and continue to develop into the standing slug (D). Depending on its environment, the standing slug either falls over to become a migrating slug that moves toward heat and light (E) or proceeds directly to the culmination stages (F) that ultimately produce the fruiting body, which consists of a spore-containing structure, the sorus, held aloft by a stalk of dead cells (G). Spores are released from the sorus and germinate into growing cells (H). Under optimal conditions, the developmental cycle takes around 24 h. If the slug forms underground, it migrates toward the surface to maximize spore dissemination. To protect itself from infection during migration, the slug possesses a rudimentary immune system comprising phagocytic sentinel cells. These cells move throughout the slug, take up bacteria and toxins, and are shed along with extracellular matrix as the slug moves (E). In response to bacteria, sentinel cells release extracellular traps, derived from mitochondrial DNA, via an unknown mechanism involving NADPH oxidase (NOX)-generated reactive oxygen species (ROS) and TirA, a soluble protein containing a toll/interleukin 1 receptor domain (I).