Bridgehead invasion of a monomorphic plant pathogenic bacterium: Xanthomonas citri pv.citri, an emerging citrus pathogen in Mali and Burkina Faso

Molecular epidemiology studies further our understanding of migrations of phytopathogenic bacteria, the major determining factor in their emergence. Asiatic citrus canker, caused by Xanthomonas citri pv.citri, was recently reported in Mali and Burkina Faso, a region remote from other contaminated areas. To identify the origin and pathways of these emergences, we used two sets of markers, minisatellites and microsatellites, for investigating different evolutionary scales. Minisatellite typing suggested the introduction of two groups of strains in Mali (DAPC1 and DAPC2), consistent with microsatellite typing. DAPC2 was restricted to Bamako district, whereas DAPC1 strains were found much more invasive. The latter strains formed a major clonal complex based on microsatellite data with the primary and secondary founders detected in commercial citrus nurseries and orchards. This suggests that human activities played a major role in the spread of DAPC1 strains via the movement of contaminated propagative material, further supported by the frequent lack of differentiation between populations from geographically distant nurseries and orchards. Approximate Bayesian Computation analyses supported the hypothesis that strains from Burkina Faso resulted from a bridgehead invasion from Mali. Multi-locus variable number of tandem repeat analysis and Approximate Bayesian Computation are useful for understanding invasion routes and pathways of monomorphic bacterial pathogens.