GENETIC DIVERSITY OF AN ITALIAN RHIZOBIUM-MELILOTI POPULATION FROM DIFFERENT MEDICAGO-SATIVA VARIETIES

We investigated the genetic diversity of 96 Rhizobium meliloti strains isolated from nodules of four Medicago sativa varieties from distinct geographic areas and planted in two different northern Italian soils. The 96 isolates, which were phenotypically indistinguishable, were an alyzed for DNA polymorphism with the following three methods: (i) a ra ndomly amplified polymorphic DNA (RAPD) method, (ii) a restriction fra gment length polymorphism (RFLP) analysis of the 16S-23S ribosomal ope ron spacer region, and (iii) an RFLP analysis of a 25-kb region of the pSym plasmid containing nod genes. Although the bacteria which were s tudied constituted a unique genetic population, a considerable level o f genetic diversity was found. The new analysis of molecular variance (AMOVA) method was used to estimate the variance among the RAPD patter ns. The results indicated that there was significant genetic diversity among strains nodulating different varieties. The AMOVA method was co nfirmed to be a useful tool for investigating the genetic variation in an intraspecific population. Moreover, the data obtained with the two RFLP methods were consistent with the RAPD results. The genetic diver sity of the population was found to reside on the whole bacterial geno me, as suggested by the RAPD analysis results, and seemed to be distri buted on both the chromosome and plasmid pSym.