Genetic diversity of Pseudomonas solanacearum biovars 2 and N2 assessed using restriction endonuclease analysis of total genomic DNA

Abstract

Biovars 2 and N2 of Pseudomonas solanacearum differ in their distribution and metabolic properties, the former generally being recovered from areas of cool climate (highland strains), whereas the latter is widespread in the tropical Amazon basin (lowland strains), and is metabolically more versatile. The relationships between 53 isolates of P. solanacearum conforming to biovar 2 or N2 were assessed using analysis of total genomic DNA digested with Hae III. Separation of these digests on 5% polyacrylamide gels and subsequent silver‐staining was shown to be a rapid and sensitive method of distinguishing between biovar 2 and N2 strains. Five restriction fragment length polymorphism (RFLP) groups could be identified which were further divisible into a total of 11 clonal lines. The majority of isolates examined were biovar 2 and of one clonal type (RFLP 26A), which was found in South America and is also a world‐wide pathogen of potato. All other RFLP groups were found only in South America, where they only occurred east of the Andes, with the exception of RFLP group 27 which was only found west of the Andes. The confinement of these closely related groups to South America supports the hypothesis that this continent was the source of the common potato bacterial wilt pathogen (RFLP 26A). The genomic DNA analysis clearly separated the highland (biovar 2) strains of P. solanacearum into the two subgroups previously recognized on the basis of nutritional and DNA hybridization data (RFLP groups 26 and 27), although these subgroups were shown to be closely related, having a similarity coefficient of 85%. The metabolically more versatile lowland strains (biovar N2) exhibited considerable diversity at the DNA level, which is at odds with reports of their uniform phenotype. To demonstrate the general utility of the total genomic DNA analysis, restriction patterns of a limited number of other biovars (1, 3 and 4) are also presented.