Presence/absence polymorphism for alternative pathogenicity islands in Pseudomonas viridiflava, a pathogen of Arabidopsis

Hitoshi Araki, Dacheng Tian, Erica M. Goss, Katrin Jakob, Solveig S. Halldorsdottir, Martin Kreitman, Joy Bergelson

Research output: Contribution to journalArticlepeer-review


The contribution of arms race dynamics to plant-pathogen coevolution has been called into question by the presence of balanced polymorphisms in resistance genes of Arabidopsis thaliana, but less is known about the pathogen side of the interaction. Here we investigate structural polymorphism in pathogenicity islands (PAIs) in Pseudomonas viridiflava, a prevalent bacterial pathogen of A. thaliana. PAIs encode the type III secretion system along with its effectors and are essential for pathogen recognition in plants. P. viridiflava harbors two structurally distinct and highly diverged PAI paralogs (T- and S-PAI) that are integrated in different chromosome locations in the P. viridiflava genome. Both PAIs are segregating as presence/absence polymorphisms such that only one PAI ([T-PAI, ∇S-PAI] and [∇T-PAI, S-PAI]) is present in any individual cell. A worldwide population survey identified no isolate with neither or both PAI. T-PAI and S-PAI genotypes exhibit virulence differences and a host-specificity tradeoff. Orthologs of each PAI can be found in conserved syntenic locations in other Pseudomonas species, indicating vertical phylogenetic transmission in this genus. Molecular evolutionary analysis of PAI sequences also argues against "recent" horizontal transfer. Spikes in nucleotide divergence in flanking regions of PAI and ∇-PAI alleles suggest that the dual PAI polymorphism has been maintained in this species under some form of balancing selection. Virulence differences and host specificities are hypothesized to be responsible for the maintenance of the dual PAI system in this bacterial pathogen.

Original languageEnglish (US)
Pages (from-to)5887-5892
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number15
StatePublished - Apr 11 2006


  • Arms race
  • Bacteria
  • Balancing selection
  • Horizontal gene transfer
  • Plant-pathogen interaction

ASJC Scopus subject areas

  • General


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