Abstract
Microbial competition drives rapid adaptation, often forcing organisms to specialize in new ecological niches. Adaptations that improve competitive ability can reduce performance in other environments creating trade-offs. Whether such trade-offs persist in nature-or are eroded as lineages adapt through compensatory changes-remains largely unknown. Here we show that a trade-off between competitive ability and host colonization has been stably maintained in natural Pseudomonas populations for centuries. Wild plant-pathogenic Pseudomonas compete using tailocins-phage-derived molecular weapons that bind to specific cell-surface receptors. Genomic surveys and functional assays reveal that the most broadly lethal tailocins remain rare-while the tailocin's production increases competitive killing, it also compromises plant colonization. We determine that the polymorphisms behind this trade-off are not transient - historical genomes spanning two centuries show that the trade-off has been maintained for at least 10(5)-10(6) generations. Our results demonstrate that, in natural populations, a trade-off between competition and pathogenicity is fundamental and not easily overcome.