A locus controlling nodulation specificity in Amphicarpaea bracteata (Leguminosae)

Abstract

Plants of the annual legume Amphicarpaea bracteata sampled from 29 sites over a 1000 km region differed strongly in their degree of symbiotic specialization with two divergent genotypes of associated root-nodule bacteria (Bradyrhizobium sp.). Plants from every site were uniformly compatible with one bacterial isolate, while only 12 plant populations were capable of forming an effective nitrogen-fixing symbiosis with the other isolate. Crosses between populations indicated that variation in nodule formation ability with the second bacterial isolate was controlled by alleles at a single locus, with the absence of nodulation a dominant trait. Based on isozyme data, the 12 populations that interacted successfully with both bacterial types constituted a genetically homogeneous group (termed lineage la). These 12 populations grouped together on one distal branch of the tree defining isozyme relationships among plant populations, so their broader symbiotic compatibility may be a derived trait, with more specialized symbiotic behavior possibly representing the ancestral condition for this group.