Mj. Hagen et Jl. Hamrick, A HIERARCHICAL ANALYSIS OF POPULATION GENETIC-STRUCTURE IN RHIZOBIUM-LEGUMINOSARUM BV TRIFOLII, Molecular ecology, 5(2), 1996, pp. 177-186
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.