NODULATING STRAINS OF RHIZOBIUM-LOTI ARISE THROUGH CHROMOSOMAL SYMBIOTIC GENE-TRANSFER IN THE ENVIRONMENT

Rhizobia were isolated from nodules off a stand of Lotus corniculatus established with a single inoculant strain, ICMP3153, 7 years earlier in an area devoid of naturalized Rhizobium loti. The isolates showed d iversity in growth rate, Spe I fingerprint of genomic DNA, and hybirdi zation pattern to genomic DNA probes, The 19% of isolates that grew at the same rate as strain ICMP3153 were the only isolates that had the same fingerprint as strain ICMP3153. Sequencing of part of the 168 rRN A gene of several diverse isolates confirmed that they were not derive d from the inoculant strain. Nevertheless, all non-ICMP3153 strains ga ve EcoRI and Spe I hybridization patterns identical to ICMP3153 when h ybridized to nodulation gene cosmids. Hybridization of digests generat ed by the very rare cutting enzyme Swa I revealed that the symbiotic D NA region (at least 105 kb) was chromosomally integrated in the strain s. The results suggest that the diverse strains arose by transfer of c hromosomal symbiotic genes from ICMP3153 to nonsymbiotic rhizobia in t he environment.