Discussion
This study highlights how Frankia -host symbiotic communities are shaped through spatial Frankia assembly and filtering forces in nature. We comprehensively investigated genetic diversity and composition of Frankia in different environments, using NGS. The occurrence of host plant individuals promoted the local assemblage of diverse Frankia strains and the greater ASV and OTU richness ofFrankia detected in rhizosphere soils around hosts compared to root nodules and host-absent riparian soil. In addition, our results suggest that the diverse Frankia community assemblies in rhizosphere soils are affected by soil nitrogen. However, theFrankia composition in root nodules consisted of very few members (Fig. 1). Because Frankia richness in root-nodules was significantly lower than in the null model, our findings indicate that members of symbionts within hosts were selected from surrounding pools via filtering forces. The community dissimilarity approach suggests that genetic variation in host plants underlies the symbiosis filtering difference even though host genetics did not affect Frankiacompositions in nodules or the rhizosphere.