A HIERARCHICAL ANALYSIS OF POPULATION GENETIC-STRUCTURE IN RHIZOBIUM-LEGUMINOSARUM BV TRIFOLII

Little is known about the population processes that shape the genetic diversity in natural populations of rhizobia. A sample of 912 Rhizobiu m leguminosarum biovar trifolii isolates were collected from naturaliz ed red clover populations (Trifolium pratense) and analysed for 15 all ozyme loci to determine the levels and distribution of genetic diversi ty. Hierarchical analyses compared different sampling levels, geograph ical separation, and temporal separation. Total genetic diversity acro ss all isolates was H=0.426, with 57.6% of the total diversity found a mong isolates obtained from individual red clover plants. Relatively l ow genetic differentiation among populations and high differentiation among plants within populations was observed; this suggests that gene flow and founder effect act differently at geographical and local. sca les. Significant differences were observed in (i) allele frequencies a mong populations and among plants within populations, and (ii) the fre quency distribution of the most widespread and the most abundant strai ns. When multilocus linkage disequilibrium was calculated, significant levels of disequilibrium were observed in the total sample and in thr ee of the eight populations.