Microbial cross-feeding interactions reshape evolutionary trajectories of consumers by preserving motility.

Cross-feeding interactions, in which a producer species release by-products that serve as resources for a consumer species, play an important role in shaping microbial community diversity. Producers create opportunities for consumers by supplying high-energy resources that are often scarce in the environment. However, they also exert strong top-down effects by releasing metabolites in pulses and generating spatial gradients of resource availability. How these spatiotemporal constraints shape consumer evolution remains poorly understood. To address this question, we used a two-species cross-feeding system in which Significance Statement: Cross-feeding interactions are pervasive in microbial communities, often arising from metabolite leakage into the environment. Our study focused on understanding how cross-feeding interactions shape the evolution of populations that cross-feed on these resources. Using a highly trackable cross-feeding system and linking genotypic to phenotypic changes, we showed that cross-feeding interactions reshape the evolutionary trajectories of consumer species. Notably, selection acts on flagellar genes, but their effects on function depend on the origin of resources. When resources are externally supplied, selection favors loss of flagellar motility, whereas when resources are generated through cross-feeding, selection not only maintains functional motility but also enhance it. These findings highlight that navigating the environment is essential for exploiting high-energy resources generated through cross-feeding.