The Phylogenetic Tree Of The ‘Nod D’ Host-Specific Factor Is Incongruent With The Rhizobial Species Tree And Shows a Possible Hgt Event Between Highly Unrelated Rhizobial Bacteria

Abstract

Rhizobium is a symbiotic biological nitrogen-fixing (BNF) bacteria. and has high diversity in different geographical regions worldwide. The BNF associated with the legume trees in tropical environments improves the efficiency of nitrogen and increases the soil organic matter, and soil fertility. Rhizobium consists of Mesorhizobium, Sinorhizobium, and Bradyrhizobium, the species belonging to each genera infecting a specific host. This host specificity is dictated by several factors among which Nod D was secreted by the bacterium and plays an important role. We investigated how Nod D gene sequences may have evolved to confer host specificity. We constructed phylogenetic trees of 16srRNA and Nod D trees and compared the trees with a manually built host tree. We find that the Nod D tree is incongruent with the 16srRNA tree and the topology does not show any specific pattern correlative of the host tree. We also uncover a possible HGT of the Nod D gene between highly unrelated bacterial species. This finding may have implications for the development of Rhizobium strains specific to a particular host for Agricultural purposes.