Question

Bacteria in the genus Rhizobium and in the genus Bartonella are believed to have come from the same common ancestor based on 16S rRNA and they are grouped in the same order (Rhizobiales). However, bacteria in the rhizobium group but not Bartonella can convert nitrogen from the air to ammonia even though both of them came from the same ancestor. How is this possible and what could have happened to bacteria in the group rhizobium to enable it to convert nitrogen from the air to ammonia?

Answer 1

Answer

Though Bartonella and Rhizoboium are descendant of same ancestor but they are diverged from each other because of

1. Nitrogen fixation(nif) genes were spread by lateral gene transfer relatively late in evolution.

2. Phylogenetic reconstructions for Fix, Nif, Nod, Vir, and Trb showed possible horizontal gene transfer events, grouping species of different phenotypes.

3. functional integration of the nodulation and nitrogen fixation genes over time. The ability to fix nitrogen has a more promiscuous nature, as observed in phylogenetic reconstructions of structural genes, such as the 16S rRNA, and nif and fix genes, while nodulation has a very specialized character which evolved in function of the host plant.

4. Finally, although nitrogen fixation and nodulation genes originated in divergent times, it is believed that through the mechanisms of gene transfer the genes related to both processes were grouped in operons and probably co-evolved in symbiotic bacteria. Despite being widely distributed in the Archae and especially in the Bacteria domains, the process of biological nitrogen fixation is not monophyletic, with its origin and distribution being modified in function of selective pressures and processes as gene duplication, loss, and gene transfer

5. The reconstruction with NodD (codified by nodD orthologous, preceded by nodABC genes) presented the most divergent topology among all trees obtained . All groups are highly distinct from those observed in the model phylogeny, and then it was not possible to evidence the two main groups - one composed of photosynthetic, methylotrophic, and bioremediation bacteria, and another composed of symbiotic and pathogenic bacteria