The structure of a local population of phytopathogenic Pseudomonas brassicacearum from agricultural soil indicates development under purifying selection pressure

Among the isolates of a bacterial community from a soil sample taken from a n agricultural plot in northern Germany, a population consisting of 119 str ains was obtained that was identified by 16S rDNA sequencing and genomic fi ngerprinting as belonging to the recently described species Pseudomonas bra ssicacearum. Analysis of the population structure by allozyme electrophores is (11 loci) and random amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR; four primers) showed higher resolution with the latter method. Both methods indicated the presence of three lineages, one of which dominat ed strongly. Stochastic tests derived from the neutral theory of evolution (including Slatkin's exact test, Watterson's homozygosity test and the Taji ma test) indicated that the population had developed under strong purifying selection pressure. The presence of strains clearly divergent from the maj ority of the population can be explained by in situ evolution or by influx of strains as a result of migration or both. Phytopathogenicity of a P. bra ssicacearum strain determined with tomato plants reached the level obtained with the type strain of the known pathogen Pseudomonas corrugata. The resu lts show that a selective sweep was identified in a local population. Previ ously, a local selective sweep had not been seen in several populations of different bacterial species from a variety of environmental habitats.