The persistence of mutualisms is paradoxical, as there are fitness incentives for exploitation.  This is particularly true for plant-microbe mutualisms like arbuscular mycorrhizae (AM), which are promiscuously horizontally-transmitted.  Preferential allocation by hosts to the best mutualist can stabilize horizontal mutualisms, however, preferential allocation is imperfect, with its fidelity likely depending upon the spatial structure of symbionts in plant roots.  In this study, we tested AM mutualisms’ dependence on two dimensions of spatial structure: the initial spatial association of fungi and the ease of fungal dispersal, through three complementary experiments. We found that fitness of the beneficial AM fungus increased when fungi were initially separate, while initial spatial mixing benefited the fitness of the non-beneficial fungus. These effects were strongest when dispersal was limited, and hosts could discriminate.  Additionally, we found that changes in AM fungal proportional abundance induced by spatial structure in roots of a preferentially allocating host produced positive feedbacks on plant growth, showing that interactions between spatial structure and host choice can determine the direction of plant-soil feedbacks.  Our results suggest that symbiont spatial structure within plant roots may act as an important modifier of plant preferential allocation and the dynamics of mycorrhizal mutualisms, with potentially cascading effects on plant-plant interactions.

An updated version of this dataset has been published. Please visit this link: https://doi.org/10.5061/dryad.s1rn8pk8z