BACKGROUND: Host specialization is a hallmark of numerous plant pathogens including
bacteria, fungi, oomycetes and viruses. Yet, the molecular and evolutionary bases of host
specificity are poorly understood. In some cases, pathological convergence is observed for
individuals belonging to distant phylogenetic clades. This is the case for
Xanthomonas strains responsible for common bacterial blight of bean, spread across
four genetic lineages. All the strains from these four lineages converged for
pathogenicity on common bean, implying possible gene convergences and/or sharing of a
common arsenal of genes conferring the ability to infect common bean.
RESULTS: To search for genes involved in common bean specificity, we used a
combination of whole-genome analyses without a priori, including a genome scan based on
k-mer search. Analysis of 72 genomes from a collection of Xanthomonas
pathovars unveiled 115 genes bearing DNA sequences specific to strains responsible for
common bacterial blight, including 20 genes located on a plasmid. Of these 115 genes,
88 were involved in successive events of horizontal gene transfers among the four genetic
lineages, and 44 contained nonsynonymous polymorphisms unique to the causal agents of
common bacterial blight.
CONCLUSIONS: Our study revealed that host specificity of common bacterial
blight agents is associated with a combination of horizontal transfers of genes, and
highlights the role of plasmids in these horizontal transfers.
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