Phylogenetic evidence of allopatric speciation of bradyrhizobia nodulating cowpea (Vigna unguiculata L. Walp) in South African and Mozambican soils

Abstract

The legume host and soil environment play a major role in establishing effective symbiosis with diverse rhizobia for plant growth promotion and nodule formation. The aim of this study was to assess the morpho-physiology, distribution and phylogenetic position of rhizobia nodulating cowpea from South Africa and Mozambique. The results showed that the isolates were highly diverse in their appearance on yeast mannitol agar plates. The isolates tested also showed an ability to produce IAA at concentrations ranging from 0.64 to 56.46 µg.ml-1 and to solubilise phosphorus at levels from 0 to 3.55 index. Canonical correspondence analysis showed that soil pH and mineral nutrients significantly influenced bradyrhizobial distribution. Analysis of BOX-PCR placed the isolates in eight major clusters with 0.01 to 1.00 similarity coefficient which resulted in 45 unique BOX-types. Phylogenetic analyses based on 16S rRNA, atpD, glnII, gyrB and recA gene sequences showed distinct novel evolutionary lineages within the genus Bradyrhizobium, with some of them being closely related to Bradyrhizobium kavangense, B. subterraneum and B. pachyrhizi. Furthermore, symbiotic gene phylogenies suggested that the isolates' sym loci probably relates to the isolates' geographical origin. The results indicated that geographical origin did affect the isolates' phylogenetic placement and could be the basis for allopatric speciation.