Check out Elena Colombi's latest paper in Microbial Genomics. She discovered whole other world of Mesorhizobium spp. in Australian soils. They were all non-symbiotic species, but fell into the same clades as symbiotic species isolated worldwide. Furthermore, through gene transfer she demonstrated most were able to become nitrogen-fixing symbionts following gene transfer of symbiosis genes.
https://doi.org/10.1099/mgen.0.000918
