3 PHYLOGENETIC GROUPS OF NODA AND NIFH GENES IN SINORHIZOBIUM AND MESORHIZOBIUM ISOLATES FROM LEGUMINOUS TREES GROWING IN AFRICA AND LATIN-AMERICA

The diversity and phylogeny of nodA and nifH genes were studied by usi ng 52 rhizobial isolates from Acacia senegal, Prosopis chilensis, and related leguminous trees growing in Africa and Latin America. All of t he strains had similar host ranges and belonged to the genera Sinorhiz obium and Mesorhizobium, as previously determined by 16S rRNA gene seq uence analysis. The restriction patterns and a sequence analysis of th e nodA and nifH genes divided the strains into the following three dis tinct groups: sinorhizobia from Africa, sinorhizobia from Latin Americ a, and mesorhizobia from both regions. In a phylogenetic tree also con taining previously published sequences, the nodA genes of our rhizobia formed a branch of their own, but within the branch no correlation be tween symbiotic genes and host trees was apparent. Within the large gr oup of African sinorhizobia, similar symbiotic gene types were found i n different chromosomal backgrounds, suggesting that transfer of symbi otic genes has occurred across species boundaries. Most strains had pl asmids, and the presence of plasmid-borne nifH was demonstrated by hyb ridization for some examples. The nodA and nifH genes of Sinorhizobium teranga ORS1009(T) grouped with the nodA and nifH genes of the other African sinorhizobia, but Sinorhizobium saheli ORS609(T) had a totally different nodA sequence, although it was closely related based on the 16S rRNA gene and nifH data. This might be because this S. saheli str ain was originally isolated from Sesbania sp., which belongs to a diff erent cross-nodulation group than Acacia and Prosopis spp. The factors that appear to have influenced the evolution of rhizobial symbiotic g enes vary in importance at different taxonomic levels.