Housekeeping Gene Sequencing and Multilocus Variable-Number Tandem-Repeat Analysis To Identify Subpopulations within Pseudomonas syringae pv. maculicola and Pseudomonas syringae pv. tomato That Correlate with Host Specificity

Pseudomonas syringae pv. maculicola causes bacterial spot onBrassicaceae worldwide, and for the last 10 years severe outbreaks have been reported in the Loire Valley, France.P. syringae pv. maculicola resemblesP. syringae pv. tomato in that it is also pathogenic for tomato and causes the same types of symptoms. We used a collection of 106 strains ofP. syringae to characterize the relationships betweenP. syringae pv. maculicola and related pathovars, paying special attention toP. syringae pv. tomato. Phylogenetic analysis ofgyrB andrpoD gene sequences showed thatP. syringae pv. maculicola, which causes diseases inBrassicaceae , forms six genetic lineages within genomospecies 3 ofP. syringae strains as defined by L. Gardan et al. (Int. J. Syst. Bacteriol. 49[Pt 2]:469–478, 1999), whereasP. syringae pv. tomato forms two distinct genetic lineages. A multilocus variable-number tandem-repeat (VNTR) analysis (MLVA) conducted with eight minisatellite loci confirmed the genetic structure obtained withrpoD andgyrB sequence analyses. These results provide promising tools for fine-scale epidemiological studies on diseases caused byP. syringae pv. maculicola andP. syringae pv. tomato. The two pathovars had distinct host ranges; onlyP. syringae pv. maculicola strains were pathogenic forBrassicaceae . A subpopulation ofP. syringae pv. maculicola strains that are pathogenic for Pto-expressing tomato plants were shown to lackavrPto1 andavrPtoB or to contain a disruptedavrPtoB homolog. Taking phylogenetic and pathological features into account, our data suggest that the DC3000 strain belongs toP. syringae pv. maculicola. This study shows thatP. syringae pv. maculicola andP. syringae pv. tomato appear multiclonal, as they did not diverge from a single common ancestral group within the ancestralP. syringae genomospecies 3, and suggests that pathovar specificity withinP. syringae may be due to independent genetic events.