Summary: To determine genetic relationships within and between two pathovars of , strains typical of pv. and selected strains of pv. were characterized by three methods. DNA-DNA hybridization experiments showed that strains of and were, respectively, 86-100% and 37-47% homologous to DNA from a reference strain when tested under stringent conditions. An analysis of electrophoretic variation in enzymes encoded by 26 loci placed 17 strains studied in a group of four electrophoretic types, and these strains had a mean genetic diversity per locus of 0.076. Six strains formed a second group of six electrophoretic types, which had a higher mean genetic diversity per locus of 0.479. The mean genetic distance separating from ( = 0.94) was unexpectedly large for strains of a single species. An analysis of restriction fragment length polymorphisms (RFLPs) with three cloned hybridization probes demonstrated that each of the and strains was unique. A method was developed to quantify the RFLP difference between pairs of strains, and cluster analysis revealed relationships among , but not among , that were similar to those based on enzyme polymorphisms. Implications of these findings for bacterial systematics and epidemiology are discussed.


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