Horizontal Transfer of the Plant Virulence Gene, nec1, and Flanking Sequences among Genetically Distinct Streptomyces Strains in the Diastatochromogenes Cluster

Evidence for the horizontal transfer of a pathogenicity island (PAI) carrying the virulence gene nec1 and flanking sequences among Streptomyces strains in the Diastatochromogenes cluster is presented. Plant-pathogenic, thaxtomin-producing Streptomyces strains, previously classified as S. scabiei based on the conventionally used phenotypic characteristics, were found to be genetically distinct from the type strain of S. scabiei based on DNA relatedness and 16S rDNA sequence analysis. Pairwise DNA-DNA hybridizations between some of these strains and the S. scabiei type strain were as low as 36%, a value much below what is conventionally accepted for species identity (70%). The sequence of the nec1 gene, however, was identical in all the S. scabiei and S. scabiei-like strains tested, irrespective of their DNA relatedness to the type strain of S. scabiei, their geographic origin, or the isolation host. Furthermore, a 26-kb DNA fragment including and flanking nec1 was also conserved among these strains based on restriction and Southern analyses. These data indicate that the etiology of potato scab is more complex than previously recognized; this result has important implications for potato scab management strategies. Previous research has suggested that horizontal transfer of a PAI was the mechanism for evolution of pathogenicity in S. acidiscabies and S. turgidiscabies, species that lie outside of the Diastatochromogenes cluster. Data presented here support this model and indicate that PAI transfer also has occurred frequently in species closely related to S. scabiei