Evolutionary stability of bacterial motility to spatially dependent selection*

Bacterial chemotaxis is one of the well-studied systems. Attention has been focused mostly on the migrating cells. Much less is known about the adaptive value of chemotaxis to the population as a whole. Here we introduce a simple evolution protocol to study the effect of chemotaxis on the growth and colonization of E. coli behind the migrating front. The migration speeds of the selected populations depended distinctly on the selection distance: Cells selected at large distances migrated faster while those selected at small distances migrated slower. This surprising result, which relates the migration speed of a population to its fitness behind the front, is elucidated by performing quantitative spatial competition assays for co-migrating strains with different motilities: We reveal an intriguing parasitic interaction by which a slow strain can surf in the wake of a fast strain and preferentially propagate its own progeny, thereby dominating the population behind the front for an extended distance. Mathematical models are developed to relate the outcome of this competition process quantitatively to the evolution dynamics, predicting what migration speed is stable to selection at which distance. These predictions are quantitatively validated by repeating the evolution in conditions supporting different migration speeds of the ancestor. The precise relation between the migration speed of a strain and the position it is evolutionarily stable, despite the ease by which the migration speed can evolve, suggests that E. coli fine tune their motility to stably occupy ecological niches of defined spatial extent.