Some strains that have converged to infect Prunus spp. trees are members of distinct Pseudomonas syringae genomospecies and ecotypes as revealed by in silico genomic comparison

Simone Marcelletti, Marco Scortichini

A complementary taxonomic and population genetic study was performed to delineate genetically and ecologically distinct species within the Pseudomonas syringae complex by assessing 16 strains including pathovar strains that have converged to infect Prunus spp. trees, and two outgroups. Both average nucleotide identity and genome-to-genome distance comparison methods revealed the occurrence of distinct genomospecies, namely 1, 2, 3 and 8 (sensu Gardan et al.), with the latter two being closely related. Strains classified as P. s. pv. morsprunorum clustered into two distinct genomospecies, namely 2 and 8. Both the AdaptML and hierarchical Bayesian analysis of population structure methods highlighted the presence of three ecotypes, and the taxonomically related genomospecies 3 and 8 strains were members of the same ecotype. The distribution of pathogenic and virulence-associated genetic traits among Pseudomonas strains did not reveal any distinct type III secretion system effector or phytotoxin distribution pattern that characterized single genomospecies and strains that infect Prunus spp. The complete WHOP (Woody HOst and Pseudomonas spp.) genomic region and the entire β-ketoadipate gene cluster, including the catBCA operon, were found only in the members of genomospecies 2 and in the two P. s. pv. morsprunorum strains of genomospecies 8. A reduced gene flow between the three ecotypes suggested that point mutations played a larger role during the evolution of the strains than recombination. Our data support the idea that Prunus trees can be infected by different strains of distinct Pseudomonas genomospecies/ecotypes through diverse mechanisms of host colonization and infection. Such strains may represent particular lineages that emerged from environments other than that of the infected plant upon acquiring genetic traits that gave them the ability to cause plant diseases. The complementary assessment of bacterial strains using both taxonomic approaches and methods that reveal ecologically homogeneous populations has proven useful in confirming the cohesion of bacterial clusters.