Experimental design
In both whole plant greenhouse and in-vitro root organ culture experiments, we employed a basic three-compartment setup (Olsson et al.2014). One compartment contained the focal plant or root, which was then consistently inoculated with the model strain Rhizophagus irregularis strain A5 (Sanders Lab, hereafter A5). The second compartment contained a second root inoculated with one of three fungal treatments, one selfing: R. irregularis A5, and two non-selfing fungi: R. irregularis strain B12 (Sanders Lab, hereafter B12) or R. aggregatum (hereafter Agg), listed in order of decreasing relatedness to the focal strain (Roger et al. 2013). These strains were chosen because they allowed us to test three levels of relatedness in a genetically well-characterized genus (Roger et al. 2013). In both the whole plant and in-vitro setup, the roots compartments were physically separated by a ‘fungus-only’ compartment in which the fungi from the two hosts could directly interact (Fig. 1a&b). To study the physical structure of fungal networks in in-vitroroot organ cultures, we covered the fungus-only compartment with a cellophane sheet to restrict network growth to 2D top layer (Crawford et al.1993; Hitchcock et al. 1996; Ritz et al. 1996) (Fig. 1c). To determine the nutrient transport from the fungal network into the host roots, we added quantum-dot tagged apatite to the partner compartment of the in-vitro root organ cultures, and determined how much was transferred to the focal root (Fig. 1d).