GENETIC-VARIATION IN LOCAL-POPULATIONS OF SINORHIZOBIUM-MELILOTI

Restriction fragment length polymorphism (RFLP) analysis was used to a ssess genetic variation in samples of two field populations of Sinorhi zobium meliloti (from sites 2 km apart) that had been characterized on the basis of phage sensitivity. Restricted genomic DNAs from 120 isol ates were hybridized to each of four DNA probes from the S. meliloti c hromosome, two from the symbiotic megaplasmid (pnod) and two from the nonsymbiotic megaplasmid (pexo). Data for these RFLPs showed that ther e were totals of 22 chromosomal types, 33 pnod types and 18 pexo types . Analysis of data for chromosomal RFLP's revealed a high proportion o f genotypes common to the two field sites, a very low coefficient of g enetic differentiation and moderate, but similar mean genetic diversit y values for the S. meliloti isolates from each of the sites (H = 0.24 and 0.25, respectively), suggesting that these isolates do not repres ent distinct populations. The population at one field site exhibited a low, but significant level of chromosomal linkage disequilibrium wher eas the population at the other site exhibited a panmictic structure ( linkage equilibrium). Dendrograms (UPGMA) revealed differences in the grouping of isolates based on megaplasmid (pnod and pexo) genotype rel ative to the same isolates defined by chromosomal genotype. The distri bution of pnod megaplasmid types across chromosomal backgrounds was al most random and contrasted with the apparent non-random distribution o f pexo megaplasmid types. Collectively, our data suggest that genetic recombination of chromosomal as well as megaplasmid sequences had occu rred in these populations of S. meliloti. Crown copyright (C) 1998 Pub lished by Elsevier Science Ltd. All rights reserved.