Pangenome analysis indicates evolutionary origins and genetic diversity: emphasis on the role of nodulation in symbiotic Bradyrhizobium

Leonardo Araujo Terra ¹ Milena Serenato Klepa ¹ Marco Antonio Nogueira ² Mariangela Hungria ^2*

• ¹ Brazilian National Council for Scientific and Technological Development (CNPq), Brasília, Distrito Federal, Brazil
• ² Embrapa Soja, Londrina, Paraná, Brazil

The genus Bradyrhizobium is mostly known for encompassing several species capable of forming nodules and establishing a process of biological nitrogen fixation with a wide range of legumes, highly contributing to agriculture and environmental sustainability. Despite its agronomic and environmental importance, questions about the evolution, minimum gene core, and symbiotic genes of Bradyrhizobium are still not well known. In this study, pangenome analyses of symbiotic Bradyrhizobium species from the Northern and Southern Hemispheres were performed to contribute to the genetic knowledge of this important genus. Pangenome analyses of symbiotic Bradyrhizobium species from the Northern and Southern Hemispheres provided valuable information about their diversity, biogeography, origin, and coevolution with legume hosts. The genus carries a relatively small core compared to the expanded accessory genome, a main property to facilitate genetic exchange and the acquisition of new genes, allowing adaptation to a variety of environments. The presence or absence of type III secretion system (T3SS) effector proteins varied significantly according to geographic location, suggesting specific environmental adaptations, as well as a direct relationship with nodulation genes. Comparative analysis revealed that symbiotic Bradyrhizobium species originated in the Northern Hemisphere and present a greater diversity of orthologous groups compared to those from the Southern Hemisphere. These results contribute to our understanding of the evolutionary history of these symbiotic bacteria.