Abstract
Host-associated bacteria can have both beneficial and detrimental effects on host health, but little is known about the evolution of these distinct outcomes. Using the model plant Arabidopsis, we found that closely related strains within the Pseudomonas fluorescens species complex (Pfl) promote plant growth and occasionally cause disease. To elucidate the genetic basis of the transition between commensalism and pathogenesis, we developed a novel computational pipeline and identified genomic islands that correlate with outcomes for plant health. One island containing lipopeptide biosynthesis and quorum sensing genes is required for pathogenesis and allows distantly related pathogens to cooperate in the environment. We found that genomic loci associated with both pathogenic and commensal lifestyles were convergently gained and lost in multiple lineages through homologous recombination, constituting an early step in the differentiation of pathogenic and beneficial lifestyles and providing insights into the evolution of host-associated bacteria.