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

Abstract

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 examine the evolutionary hypotheses motivated by the model, we performed a comprehensive analysis of 11,365 bacterial genomes across the bacterial 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 did not show detectable evolutionary coupling with flagellar motility. Transition-rate analyses further indicated that T6SS gain was strongly biased towards motile bacterial lineages, even though T6SS loss was common overall. These results indicate different macroevolutionary associations of flagellar motility with T6SS and bacteriocins, consistent with the qualitative hypothesis motivated by our model.