Separating the effects of competition and cooperation in shared mycorrhizal networks

Background/Question/Methods

Many plants associate with soil fungi in a mutualistic relationship called mycorrhizae. The same fungus may connect multiple plants to form a shared mycorrhizal network. Data fairly clearly show that resources can move between plants along this network. Sometimes the shared network is interpreted as a form of cooperation among the plants, though plants also still compete outside the network. We describe experiments using a non-mycorrhizal mutant of pea competing with the mycorrhizal wildtype, and stable isotopes to quantify trade. The mutant and wildtype differ by just one allele at one locus. We ask how much does inter- and intra-mutant competition shape reproductive outcomes? Next, we explored shared mycorrhizal networks in an experiment where a central focal plant grew inside a 35 μm nylon mesh bag and was the only source of fungal partners for four surrounding competitors. We used mixtures of mutant and wildtype to essentially titrate the number of potential plants in the shared network from 0 (all four neighbours are mutant), 1, 2, 3, or 4 (all neighbours are wildtype). This allowed us to ask how competition and cooperation combined to shape reproductive outcomes when the focal plant was either mutant or wildtype.

Results/Conclusions

In the first experiment, the wildtype was competitively dominant over the mutant. This wasn’t surprising since the mutant was artificially created, and presumably evolution by natural selection favoured the wildtype’s mycorrhizal strategy over non-mycorrhizal ancestors at some point in the evolution of land plants. However, the margins were surprisingly slim, only about 11% more reproduction, and only 3% more nitrogen uptake compared to non-mycorrhizal plants. Preliminary data from the second shared network experiment suggest that the mycorrhizal wildtype performs increasingly better as it is surrounded by more and more wildtype plants that could be connected. Alternatively, the non-mycorrhizal mutant performs worse and worse as the number of wildtype plants that surround it increase. Combined, the results suggest that the cooperative effects between the plant and the mycorrhizal fungus, as well as the apparently cooperative resources that move through the shared network are higher than the negative effects of plant-plant competition. If the shared network is about plant-plant sharing of resources, it remains unclear why the fungus cooperates with the plants to pass resources that it could presumably use for its own fitness. We are using stable isotopes of carbon to track movement of carbon based resources through the network.