Root nodule symbiosis evolved ca. 100 Mya between a nitrogen-fixing bacterium and the common ancestor to the Fabales, Fagales, Rosales, and Cucurbitales plant orders. Over time the majority of the lineages derived from this ancestor lost their symbiotic capability. While extant symbiotic members found in the Fabales order (legumes) all engage in symbiosis with rhizobia, extant symbiotic members of the latter three plant orders are referred to as actinorhizal plants. These engage in symbiosis with Frankia.

Frankia is a genus of soil actinobacteria, which can be split into four phylogenetically distinct clades. The earliest divergent symbiotic clade, Frankia cluster-2, encompasses strains that have a broad host range and could not be cultured in vitro thus far with two exceptions. Based on Frankia enriched meta-genomes from whole nodules collected at different locations across the globe, it is clear there is very little diversity of Frankia cluster-2 in continental Eurasia, spanning from France to Japan. These strains are also closely related to strains found in North America. However, very little is known about strains occurring in the islands in the Pacific Ocean and the southern hemisphere.

In short, this thesis aimed to investigate the biodiversity of the earliest divergent symbiotic Frankia clade and to understand how Frankia spread across the globe (Study 1 and Study 2). From nodules collected in study 1, a novel Streptomyces species was identified and declared (Study 3). The thesis also aimed to study genetic changes within Frankia cluster-2 which might be associated with their endosymbiotic lifestyle and low saprotrophic potential (Study 4 and Study 5).