COMPARATIVE-ANALYSIS OF THE GENETIC-STRUCTURE OF A RHIZOBIUM-MELILOTIFIELD POPULATION BEFORE AND AFTER ENVIRONMENTAL RELEASE OF THE HIGHLYCOMPETITIVE RHIZOBIUM-MELILOTI STRAIN GR4

Rhizobium meliloti strain GR4, which exhibits a highly competitive abi lity for alfalfa root nodule occupancy, was used in a field release ex periment in Granada, Spain. In order to analyze the ecological impact of the GR4 release, we characterized the R. meliloti indigenous popula tion of the field site by ERIC-(enterobacterial repetitive intergenic consensus) PCR and IS (insertion sequence) fingerprinting. Both finger printing methods resulted in the same grouping of the isolates. Data o btained were compared with a previous analysis by plasmid based sequen ce-specific PCR. Isolates belonging to the major infective group, as d efined by dominant plasmid types, were shown to have identical or near ly identical ERIC and IS fingerprint patterns. Hence, we conclude that all three typing methods are suited to characterize the genetic struc ture of the field population. The possible impact of the introduction of strain GR4 was examined two years after its release in its original environment. No effect on the genetic structure of the indigenous R. meliloti field population was observed.