Multilocus sequence analysis of bradyrhizobia isolated from Aeschynomene species in Senegal

This study reports the multilocus sequence analysis (MLSA) of nine house-keeping gene fragments (atpD, dnaK, glnA, glnB, gltA, gyrB, recA, rpoB and thrC) on a collection of 38 Bradyrhizobium isolated from Aeschynomene species in Senegal, which had previously been characterised by several phenotypic and genotypic techniques, allowing a comparative analysis of MLSA resolution power for species delineation in this genus. The nifH locus was also studied to compare house-keeping and symbiotic gene phylogenies and obtain insights into the unusual symbiotic properties of these Aeschynomene symbionts. Phylogenetic analyses (maximum likelihood, Bayesian) of concatenated nine loci produced a well-resolved phylogeny of the strain collection separating photosynthetic bradyrhizobial strains (PB) from non-photosynthetic bradyrhizobial (NPB) ones. The PB clade was interpreted as the remains an expanding ancient species that presently shows high diversification, giving rise to potential new species. B. denitrificans LMG8443 and BTAi1 strains formed a sub-clade that was identified as recently emerging new species. Congruence analyses (by Shimodaira-Hasegawa (S-H) tests) identified three gene-fragments (dnaK, glnB and recA) that should be preferred for MLSA analyses in Bradyrhizobium genus. The nine loci or nifH phylogenies were not correlated with the unusual symbiotic properties of PB (nod-dependent/nod-independent). Advantages and drawbacks of MLSA for species delineation in Bradyrhizobium are discussed.