IMPACTS OF DOMESTICATION ON THE LEGUME-RHIZOBIA MUTUALISM

Niall Millar

doi:10.7273/000006526

Domestication is essential for improving plants for human use, but it can come with unintended side effects. The rhizobial mutualism, which provides legumes with nitrogen, is an essential part of the nitrogen cycle and makes legumes highly efficient crops. In this dissertation, I investigate how the mutualism between legumes and rhizobial bacteria has evolved over the course of domestication, asking whether the mutualism has been improved, degraded, or unaffected by artificial selection. In my first chapter, I compare the benefit that wild and domesticated legumes receive from rhizobia, and find that, in most cases, the benefits of the mutualism are maintained in domesticated legumes. There are some exceptions; domesticated soybeans benefit less from rhizobia than wild soybeans, and domesticated chickpeas cease the mutualism at a lower level of nitrogen fertilisation than wild chickpeas. In my second chapter, I investigate the genetic correlation between yield traits and mutualism traits in pea crops. I find that the highest-yielding pea cultivars tend to get a greater proportion of their nitrogen from rhizobia, which means that selection for yield should also maintain, or even improve, the rhizobial mutualism in peas. In my third chapter, I compare the ability of wild and domesticated peas to exert control over the mutualism by sanctioning against rhizobia that fail to provide them with nitrogen. I find that wild and domesticated peas have largely similar abilities to control the mutualism. I also find that the control the of the mutualism is strongest when plants can compare strains of varying quality, and that the overall benefit a plant receives from its partners does not determine its ability to exert control over them. In summation, my dissertation paints a positive picture of the rhizobial mutualism in legume crops. The genetic variation in the outcomes of the mutualism show that there is potential for it to be improved through selection, particularly in the few cases where domestication has reduced the benefits received by crops. Despite these few cases, the rhizobial mutualism remains essential for food security without total reliance on artificial fertilisers.

IMPACTS OF DOMESTICATION ON THE LEGUME-RHIZOBIA MUTUALISM

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