Evolutionary Coupling of Flagellar Motility and Type VI Secretion Systems Across Bacteria

Bacteria use both motility and antagonism to compete in spatially structured environments, but whether these traits evolve together across broad bacterial diversity remains unclear. We developed a spatial kin-competition model predicting that motility should couple robustly with contact-dependent weapons by increasing encounter rates with competitors, whereas coupling with diffusible weapons should be weaker and context-dependent. To test these predictions, we performed a comprehensive analysis of 11,365 bacterial genomes across the Tree of Life (ToL). By utilising large-scale homology-based searches, we annotated flagellar, T6SS, and bacteriocin components and then applied phylogenetic comparative models to examine evolutionary associations. T6SS presence was strongly associated with flagellar motility: T6SS-positive lineages were predominantly flagellated, and BayesTraits supported a dependent model of FliC and T6SS evolution. In contrast, bacteriocins showed no detectable evolutionary coupling with flagellar motility. Transition-rate analyses further indicated that T6SS gain was strongly biased towards motile lineages, even though T6SS loss was common overall. These results support an asymmetric macroevolutionary relationship between bacterial motility and antagonism, in which flagellar motility is robustly coupled to contact-dependent competition but not to diffusible antagonistic systems.