Two types of root nodule symbioses between higher plants and nitrogen-fixing soil bacteria are known, the legume symbiosis with Gram-negative rhizobia and the actinorhizal symbiosis between a diverse group of ca. 280 species from eight different families of three orders, Fagales, Cucurbitales and Rosales, that interact with nitrogen-fixing soil bacteria of the genus Frankia. All plants able to enter a root nodule symbiosis go back to a common ancestor, i.e., they belong to a single clade. Phylogenomic studies have revealed that non-symbiotic members of this clade have lost symbiosis-related genes. Thus, the common ancestor had evolved a symbiosis which was subsequently lost in the majority of lineages. The microsymbiont of the common ancestor must have been a Frankia strain; eventually, microsymbiont exchange has taken place in legumes. In a project financed by the Tryggers foundation, we are performing family- or order-specific comparisons of the genes induced in nodules compared to roots, and the presence/absence of nodulation-related genes identified in legume symbioses, in different groups of actinorhizal plants. We are also analysing different microsymbiont genomes, which are resistant to genetic manipulation, to identify genes relevant for the symbiosis, (a) by identifying relevant promoter elements, and (b) by comparing different lineages of genome erosion in microsymbionts of Fagales vs. Cucurbitales.