Breaking symmetry in myxobacteria

Multicellular animals and plants develop from genetically equivalent cells, yet neighboring cells often adopt different fates. Some well-studied examples are the formation of sensory bristles from a sheet of uniform neuroectodermal cells in Drosophila, and the formation of hair or feathers in the uniform skin ectoderm of vertebrates. This breaking of symmetry within groups of equivalent cells depends on cell–cell interactions that stabilize themselves. Common regulatory systems that break symmetry in flies, worms, and vertebrates use cell-surface signals such as Notch, Hedgehog and Wnt. Although the mechanisms of cell–cell interaction during eukaryotic organogenesis have received the most attention, genetic programs capable of breaking symmetry are also found in bacteria. The view that bacteria are asocial cells with no need for intercellular communication was demolished by the discovery of homoserine lactones which carry extracellular signals and by the phenomenon of quorum sensing. In this Primer, we shall discuss the amazing patterns that arise in colonies of myxobacteria as a consequence of contact-mediated cell interactions. Myxobacteria are common inhabitants of the soil where they enjoy a rich social life. In behavior and development, they resemble the cellular slime molds and, in some aspects of development, animals and plants as well. Myxobacteria prey on other bacteria: feeding cooperatively, they secrete enzymes that digest their prey. They compete with other soil micro-predators and, when their prey are exhausted, they stop hunting, build multicellular fruiting bodies and sporulate for survival (Figure 1C,D). This developmental program uses two cell–cell signals: first, the diffusible, quorum sensing A-signal that initiates fruiting body construction; and second, the cell surface bound Csignal that coordinates the motion of individual cells by cell-contact. We shall see some striking differences between the contact signaling system of myxobacteria and the slime molds that communicate by diffusible morphogens to pattern their fruiting bodies.