A simple plant-mycorrhizal fungal resource trade co-evolution model explains mutualism stability, extinction and transitory parasitism via fitness feedback.

The mutualism between mycorrhizal fungi and plants has persisted for over 400 million years, despite the mutualism paradox predicting that mutualisms should be evolutionarily unstable due to the fitness advantages of cheating. It is widely accepted that mutual benefit alone is not sufficient for stable mutualism, and so a search for additional stabilising mechanisms has been the focus of past investigation. In this work, we test the assumption that cheating is an omnipresent threat to mutualism; hence, additional mechanisms for stability are needed. We developed a novel individual-based model of a plant and mycorrhizal fungus, where mechanisms commonly thought necessary for mutualism stability, for example, partner choice, are absent. The organisms take up carbon and phosphorus at different uptake efficiencies, exchange resources, and grow depending on their limiting resource over 10 time steps. We simulated co-evolution of resource trading strategies over 2000 generations, under 231 nutrient uptake efficiency combinations. Evolutionarily stable mutualism evolved in 66% of the nutrient uptake efficiency combinations tested. Parasitism and Darwinian extinction also occurred. Our results suggest that different nutrient uptake efficiencies, Leibig's law of the minimum, and one-to-one resource trade are sufficient to explain stable mutualism, parasitism and Darwinian extinction, via fitness feedback, without additional mechanisms.