Bradyrhizobium spp. (TGx) isolates nodulating the new soybean cultivars in Africa are diverse and distinct from bradyrhizobia that nodulate North American soybeans.

The newly developed cultivars of soybean in Africa, known as Tropical Glycine cross (TGx), are nodulated by bradyrhizobia indigenous to African soils, here designated Bradyrhizobium spp. (TGx). Isolates of Bradyrhizobium spp. (TGx) obtained from nodules of TGx soybeans that were inoculated with soils from 65 locations in six African countries were characterized and grouped into 11 phylogenetic clusters on the basis of RFLP of the 16S rRNA gene. Five restriction enzymes (RsaI, HinfI, MspI, CfoI and HaeIII) established RFLP groups within these Bradyrhizobium spp. (TGx) isolates, which were used to construct a phylogenetic tree showing their genetic relationship with other Bradyrhizobium species. RFLP analysis indicated that Bradyrhizobium spp. (TGx) is a heterogeneous group with some isolates related to Bradyrhizobium japonicum and Bradyrhizobium elkanii strains and some to Bradyrhizobium spp. (misc.) reference strains isolated from a variety of tropical legumes. The heterogeneity within the large phylogenetic clusters was further examined through analysis of randomly amplified polymorphic DNA (RAPD) using GC-rich PCR primers. The RAPD analysis showed additional heterogeneity in the Bradyrhizobium spp. (TGx) phylogenetic clusters, which was not revealed by separations based on RFLP analysis. The Bradyrhizobium spp. (TGx) isolates were classified into effective and ineffective types based on their symbiotic performance on TGx soybean. The isolates were randomly distributed throughout the phylogenetic clusters regardless of their symbiotic effectiveness on TGx soybean.