DIVERSITY AND GENETIC-STRUCTURE OF A NATURAL-POPULATION OF RHIZOBIUM-LEGUMINOSARUM BV TRIFOLII ISOLATED FROM TRIFOLIUM-SUBTERRANEUM L

A collection of 121 isolates of Rhizobium leguminosarum biovar (by.) t rifolii was obtained from root nodules of Trifolium subterraneum L. (s ubclover) plants growing in an established pasture. The collection con sisted of a single isolate from each of 18 plants sampled from seven m icroplots. The following year, a further 28 and 27 isolates were colle cted from the first and seventh sampling points, respectively. Analysi s of restriction fragment length polymorphisms (RFLPs) of both chromos omal and Sym (symbiotic) plasmid DNA and multilocus enzyme electrophor esis (MLEE) were used to assess the diversity, genetic relationships a nd structure of this population. Symbiotic effectiveness tests were us ed to examine the symbiotic phenotype of each isolate collected in the first year. Analysis of RFLPs of the first year isolates revealed 13 chromosomal types and 25 Sym plasmid types. Similar Sym plasmid types were grouped into 14 families containing 1-6 members. No new chromosom al types and six new Sym plasmid types were detected in the second yea r. The symbiotic effectiveness of the first year isolates of the same Sym plasmid type was similar. Significant differences in symbiotic eff ectiveness were detected between different Sym plasmid types in the sa me plasmid family. Representative isolates of each chromosomal type x Sym plasmid type identified in the first year were analysed using mult ilocus enzyme electrophoresis. Mean genetic diversity per locus was hi gh (0.559). Enzyme electrophoresis revealed 17 electrophoretic types ( ETs). Cluster analysis of the enzyme data revealed large genetic diver sity amongst the ETs. Strong linkage disequilibrium was observed for t he population as a whole, i.e. clonal population structure, but signif icantly less disequilibrium was observed among a cluster of ETs sugges ting that recombination occurred between ETs within the cluster. Our r esults revealed that a population of naturally occurring isolates of R hizobium leguminosarum by. trifolii can be genetically diverse and sup port the possibility that recombination plays a role in generating new genotypes.