The monodominant species Spirotropis longifolia is mainly nodulated by strains of the genus Bradyrhizobium outside the B. japonicum and B. elkanii superclades

Spirotropis longifolia (DC.) Baill. is organized in monodominant forest patches in French Guiana. S. longifolia root nodules and forest soil samples were collected in three monodominant populations of S. longifolia and in four zones, from the S. longifolia core dominance to the adjacent mixed forest where this species was absent. S. longifolia roots presented arbuscular mycorrhizal fungus (AMF) structures, but no ectomycorrhizae. The presence of myconodule-like structures was only noted once. Isolates of S. longifolia and of diverse French Guianese legume trees were 95% identified as Bradyrhizobium spp. and rarely as Rhizobium spp. On a partial 16S rDNA phylogenetic tree, S. longifolia-associated bradyrhizobia were positioned in a separate cluster including the Bradyrhizobium sp. Tv2a-2 strain isolated from Tachigali versicolor in Panama. Bradyrhizobia of other forest legume trees were positioned identically, or differently, in various clusters. A partial 16S-23S rDNA internal transcribed spacer (ITS) phylogenetic tree confirmed that the main cluster accounting for 82% of the direct or trapped bradyrhizobia associated with S. longifolia was situated outside the B. japonicum and B. elkanii superclades, in the Tv2a-2 superclade/Kakadu supergroup. In this cluster, other bradyrhizobia appeared that were associated with legumes from the tribes Ormosieae, Brongniartieae and Dalbergieae, suggesting the possibility of a shared pool of the most ancestral symbionts that are bradyrhizobia with the Caesalpinoid legumes and the early-branching Papilionoid legumes. Curiously, the seven monodominant Fabaceae of the Amazonian forests were exclusively part of these subfamilies. The link between ancestral symbiosis and monodominance still remains to be studied.