GENETIC-STRUCTURE OF RHIZOBIUM-LEGUMINOSARUM BIOVAR TRIFOLII AND VICIAE POPULATIONS FOUND IN 2 OREGON SOILS UNDER DIFFERENT PLANT-COMMUNITIES

An investigation was carried out to determine the genetic structure in soil populations of Rhizobium leguminosarum bv. trifolii and viciae a t each of two Oregon sites (A and C) that were 1 km apart. Although th e soils were similar, the plant communities were quite different becau se grazing by domestic animals had been allowed (site A) or prevented (site C). Analysis of allelic variation at 13 enzyme encoding loci by multilocus enzyme electrophoresis delineated 202 chromosomal types (ET s) among a total of 456 isolates representing two populations of R. le guminosarum bv. trifolii (AT and CT) and two populations of R. legumin osarum bv. viciae (AV and CV). Regardless of their site of origin or b iovar affiliation, isolates of the same ET were confirmed to be more c losely related to each other than to isolates of other ETs by repetiti ve extragenic palindromic and enterobacterial repetitive intergeneric consensus sequences and the PCR technique. Despite the wide range in d ensities of the Rhizobium populations (<10(2) to >10(5)/g of soil), th eir overall genetic diversities were similar (mean genetic diversity, 0.45 to 0.51), indicating that low-density populations of soil-borne b acterial species are not necessarily of little genetic diversity. Link age disequilibrium analysis revealed significant multilocus structure (nonrandom associations of alleles) within each of the four population s. From a combination of cluster and linkage disequilibrium analyses, a total of eight distinct groups of ETs were defined in the four popul ations. Two groups (I and III) contributed significant numbers of ETs and isolates to each population. The two populations of R. leguminosar um bv. viciae (AV and CV) exhibited similar genetic structures despite existing at different densities, in different plant communities, and in the presence (CV) or absence (AV) of their local Vicia hosts. In co ntrast, total linkage disequilibrium was partitioned differently in tw o biovar populations occupying the same soil (AV and AT), with disequi librium in the latter being due entirely to the presence of group V.