RELATIVE GENETIC-STRUCTURE OF A POPULATION OF RHIZOBIUM-MELILOTI ISOLATED DIRECTLY FROM SOIL AND FROM NODULES OF ALFALFA (MEDICAGO-SATIVA) AND SWEET CLOVER (MELILOTUS-ALBA)

Insertion sequence (IS) hybridization was used to define the structure of a population of Rhizobium meliloti isolated directly from soil and from nodules of Medicago sativa (alfalfa) and Melilotus alba (sweet c lover) grown under controlled conditions and inoculated with a suspens ion of the same soil. The detection of X. meliloti isolated from soil on agar plates was facilitated by use of a highly species specific DNA probe derived from ISRm5. All R. meliloti obtained directly from soil proved to be symbiotic (i.e. nodulated and fixed nitrogen with alfalf a). Analysis of 293 R. meliloti isolates revealed a total of 17 distin ct IS genotypes of which 9, 9 and 15 were from soil, M. alba and M. sa tiva, respectively; 8 genotypes were common to soil and both plant spe cies. The frequency of R. meliloti genotypes from soil differed marked ly from that sampled from nodules of both legume species: 5 genotypes represented about 90% of the isolates from soil whereas a single genot ype predominated among isolates from nodules accounting for more than 55% of the total. The distribution of genotypes differed between M. sa tiva and M. alba indicating species variation in nodulation preference s for indigenous X. meliloti. The data are discussed in the context of competition for nodulation of the host plant and the selection of Rhi zobium strains for use in legume inoculants. This study has ecological implications and suggests that the composition of A. meliloti populat ions sampled by the traditionally used host legume may not be represen tative of that actually present in soil.