Many aspects of metazoan morphogenesis find parallels in the communal behavior of microorganisms. The cellular slime mold D. discoideum has long provided a metaphor for multicellular embryogenesis. However, the spatial patterns in D.d. colonies are generated by an intercellular communication system based on diffusible morphogens, whereas the interactions between embryonic cells are more often mediated by direct cell contact. For this reason, the myxobacteria have emerged as a contending system in which to study spatial pattern formation, for their colony strutures rival those of D.d. in complexity, yet communication between cells in a colony is carried out by direct cell contacts.

Myxococcus xanthus are Gram-negative bacteria that glide on solid surfaces, periodically reversing their direction of movement. When starved, M. xanthus cells organize their movements into waves of cell density that sweep over the colony surface. These waves are unique: although they appear to interpenetrate, they actually reflect off one another when they collide, so that each wave crest oscillates back and forth with no net displacement. Since the waves reflect the coordinated back and forth oscillations of the individual bacteria, we call them accordion waves. The spatial oscillations of individuals are a manifestation of an internal biochemical oscillator, probably involving the Frz chemosensory system. These internal ‘clocks’—each of which is quite variable—are synchronized by collisions between individual cells utilizing a contact mediated signal transduction system. The result of collision signaling is that the collective spatial behavior is much less variable than the individual oscillators. In this work we present new experimental observations in which individual cells marked with GFP can be followed in groups of unlabelled cells in monolayer cultures. This data, together with a new agent-based computational model demonstrates that the only properties required to explain the ripple patterns are an asymmetric biochemical limit cycle that controls direction reversals, and asymmetric contact induced signaling between cells: head-to-head signaling is stronger than head-to-tail signaling. Together, the experimental and computational data provide new insights into how populations of interacting oscillators can synchronize and organize spatially to produce morphogenetic patterns that may have parallels in higher organisms.