Understanding how genetic variation within a foundation species determines the structure of associated communities and ecosystem processes has been an emerging frontier in ecology. Previous studies in common gardens identified close links between intraspecific variation and multispecies community structure, and these findings are now being evaluated directly in the complex natural ecosystem. In this study, we examined to what extent genomic variation in a foundation tree species explains the structure of associated arthropod communities in the field, comparing with spatial, temporal and environmental factors. In a continuous mixed forest, arthropods were surveyed on 85 mature alders (Alnus hirsuta) in 2 years. Moreover, we estimated Nei's genetic distance among the alders based on 1,077 single nucleotide polymorphisms obtained from restricted-site-associated DNA sequencing of the alders' genome. In both years, we detected significant correlations between genetic distance and dissimilarity of arthropod communities. A generalized dissimilarity modelling indicated that the genetic distance of alder populations was the most important predictor to explain the variance of arthropod communities. Among arthropod functional groups, carnivores were consistently correlated with genetic distance of the foundation species in both years. Furthermore, the extent of year-to-year changes in arthropod communities was more similar between more genetically closed alder populations. This study demonstrates that the genetic similarity rule would be primarily prominent in community assembly of plant-associated arthropods under temporally and spatially variable environments in the field.

Revealing genetic diversity in a root nodulation symbiosis under field conditions is critical to understand the formation of ecological communities of organisms associated with hosts and the nitrogen cycle in natural ecosystems. However, our knowledge of genetic diversity of bacterial mutualists on a local scale is still poor because of the assumption that the genetic diversity of mutualistic bacteria is constrained by their hosts. MethodsWe thoroughly investigated genetic diversity of Frankia in a local forest stand. We collected root nodules from 213 Alnus hirsuta seedlings covering the spatial range of the continuous population, which means that Alnus individuals occurred in a relatively homogeneous distribution in a continuous forest. Then, a phylogenetic analysis was performed for the nifD-K IGS region, including global Frankia sequences from Alnus hosts. ResultsThe genetic diversity of Frankia detected even on a local scale measured as high as that shown by previous studies conducted on a regional scale. Moreover, a genetic structure analysis revealed a spatially mosaic-like distribution of genetic variation in Frankia despite the small spatial scale. Conclusions 3The genetic diversity and composition of bacterial mutualists are heterogeneous on a local scale. Our findings demonstrate that genetically different bacterial symbionts simultaneously interact with a single host population and interaction partnerships spatially vary. The standing variation could produce dynamic ecological and evolutionary outcomes in a heterogeneous forest ecosystem.

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Community assembly of diverse mutualistic symbionts in host plants have recently received much attention. On the other hand, for decades, researchers have also focused on a role of genetic variation for shaping ecological communities. However, it remains unclear how local interaction networks between a host and symbionts are shaped from the pool of genetically diverse microorganisms in the natural rhizosphere. In this study, we comprehensively analyzed local and regional genetic communities of Frankia in host individuals and in surrounding soils using metabarcoding to unravel community assembly in shaping interaction networks between plants and root nodule symbionts, both of which are genetically diverse, in natural ecosystems. We found that Frankia communities in rhizosphere soil were consisted of tremendously diverse strains. The diverse Frankia communities in rhizosphere soils were heterogeneously structured by the presence of host individuals and soil nutritional status. Furthermore, quantification of the filtering forces by community analysis demonstrated that the filtering forces that shape symbiotic networks would depend on genetic variation in the host plant. This study suggests that filtering mechanisms and host genetics would have a profound role in shaping plant-bacterium symbiotic networks in natural environments.