Emergence and host range expansion of an epidemic lineage of Ralstonia solanacearum

Members of the Ralstonia solanacearum species complex are soil-borne pathogens of important crop plants. Strain-level host specialisation is considered typical, yet outbreak monitoring in Martinique revealed a new lineage with dramatically expanded host range had emerged. Our analysis shows the outbreak in Martinique was caused by the expansion of lineages from two different Ralstonia species. One lineage is of Asian origin and retains a signature of host specialisation, while the emergent broad host range lineage originated in the Americas. The presence of similar integrative and conjugative elements (ICEs) in both lineages suggests they are exchanging genetic material. The emergent lineage acquired novel prophage along with genes encoding novel catalytic and metabolic functions, but shows little variation among the virulence proteins considered host specificity determinants. We postulate that host range expansion is not caused by the evolution of a virulence strategy targeting host immunity, but is rather due to the acquisition of novel pathways for substrate metabolism enhancing pathogen fitness in soil or plant tissues.