Impacts of domestication on the legume-rhizobium mutualism

Abstract Aims Domestication has improved plant traits to facilitate human use, but can have unintended consequences for traits that are not the target of selection. Disruption in the mycorrhizal mutualism has already been observed in domesticated plants, but little is known about whether the legume-rhizobia mutualism can be similarly disrupted, despite the ecological importance of symbiotic nitrogen fixation. We aim to identify differences in the outcome of this mutualism between wild and domesticated legumes, in terms of reaction norms across varying nitrogen levels. Methods In a series of greenhouse experiments, we characterise rhizobial symbiosis traits in chickpeas, lentils, peas, peanuts, and soybeans across six levels of nitrogen fertilisation to characterise whether and how symbiosis traits differ across continuous variation in soil conditions for wild and domesticated lineages. Results Our reaction norm characterisations reveal that at lower levels of nitrogen fertilisation, wild soybeans receive a greater benefit from rhizobia than do domesticated soybeans. Additionally, domesticated chickpeas abandon symbiosis at a lower level of nitrogen fertilizer than do wild chickpeas. However, other crops we tested express symbiosis trait reaction norms that are indistinguishable from those of their wild relatives. Conclusions Differences are evident between the mutualism reaction norms of domesticated and wild chickpea and soybean, but not lentil, pea, or peanut. This indicates that mutualism disruption is a possible, but not necessary, consequence of domestication.