Fig. 1: The traits of the myxobacterial outer membrane exchange and visual description of the handshake-dependent rock-paper-scissors-like model.

a Two myxobacteria with compatible TraA receptors can merge their outer membrane (OME). Cao and Wall⁵ referred to OME as the molecular handshake. b The horizontal half-section of the myxobacteria performing OME. Outer and inner membranes are illustrated with green and purple membranes, respectively. c TraA receptors regulate the specificity of OME. The cohort protein TraB is necessary for OME, but it is not involved in the specificity. In this model, all myxobacteria were assumed to have functional TraA/B proteins. During OME, proteins embedded in the outer membrane and the constituent lipids are bidirectionally exchanged including SitA toxins. d The evolutionary dynamics of green-beard interaction of the sitAI alleles is similar to a rock-paper-scissors (RPS) game. As sitAI interactions require OME, I propose that OME is an epistatic green-beard effect while toxin transfer is hypostatic to OME. Such hierarchical social interaction can be labeled as a handshake-dependent RPS-like model. However, the actual hypostatic green-beard interactions are different from classical RPS games because there could be multiple types of the scissors and competition between scissors could result in mutual damage rather than a tie.