NONRANDOM GENOTYPIC ASSOCIATIONS IN A LEGUME - BRADYRHIZOBIUM MUTUALISM

Genetically divergent lineages often coexist within populations of the annual legume Amphicarpaea bracteata. At one site dominated by two su ch lineages (termed biotypes ''C'' and ''S''), isolates of root-nodule bacteria (Bradyrhizobium sp.) were sampled from both hosts and analyz ed by enzyme electrophoresis. Symbiont populations on the two plant bi otypes were highly distinct. Out of 15 bacterial multilocus genotypes detected (among 51 isolates analyzed),only one was shared in common by the two plant biotypes. Cluster analysis revealed three bacterial lin eages (designated I, II, and III), with lineage I found exclusively on biotype C plants, and the two other lineages almost completely restri cted to biotype S hosts. Laboratory inoculation tests indicated that l ineage I bacteria were strictly specialized on biotype C hosts, formin g few or no nodules on plants of the other host biotype. Bacterial lin eages II and III were capable of forming nodules on both kinds of plan ts, but nodule numbers were often significantly higher on biotype S ho sts. The nonrandom association between plant and bacterial lineages at this site implies that genetic diversity of hosts is an important fac tor in the maintenance of polymorphism within the symbiont population.