Genetic diversity and dynamics of Sinorhizobium meliloti populations nodulating different alfalfa cultivars in Italian soils

We analyzed the genetic diversity of 531 Sinorhizobium meliloti strains iso lated from nodules of Medicago sativa cultivars in two different Italian so ils during 4 years of plant growth. The isolates were analyzed for DNA poly morphism with the random amplified polymorphic DNA method. The populations showed a high level of genetic polymorphism distributed throughout all the isolates, with 440 different haplotypes. Analysis of molecular variance all owed us to relate the genetic structure of the symbiotic population to vari ous factors, including soil type, alfalfa cultivar, individual plants withi n a cultivar, and time. Some of these factors significantly affected the ge netic structure of the population, and their relative influence changed wit h time. At the beginning of the experiment, the soil of origin and, even mo re, the cultivar significantly influenced the distribution of genetic varia bility of S. meliloti, After 3 years, the rhizobium population was altered; it showed a genetic structure based mainly on differences among plants, wh ile the effects of soil and cultivar were not significant.