The evolution of tolerance in the common morning glory Ipomoea purpurea
Baucom, Regina S.
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The evolution of a trait in nature is dependent on the amount of genetic variation underlying the trait, the presence of selection on the trait, and the inheritance pattern of the trait. I investigated each of these components using a trait that has important implications for agriculture: the ability of Ipomoea purpurea, a noxious crop weed, to tolerate glyphosate, or the active ingredient in the herbicide RoundUp®. In every investigation, I found the presence of genetic variation underlying tolerance, suggesting that adequate fuel for the evolutionary process exists in nature. In addition, I found the presence of positive, directional selection on tolerance, providing evidence that glyphosate is a potent force of selection on tolerance in populations of I. purpurea. I also found that, in the absence of glyphosate, a significant fitness cost exists such that the continued evolution of tolerance might be mitigated by removing use of glyphosate in cropping systems. Finally, I uncovered an inheritance pattern for tolerance, in that it is primarily under additive nuclear genetic control, and is not under the direct influence of a maternal or paternal effect. The work presented here also shows the presence of genetic variation for tolerance in pre-glyphosate lines, suggesting that tolerance is a pre-adaptation, and that the evolutionary dynamics controlling its increase or decrease within populations is not as simple as previously thought. However, a geographical survey of tolerance in the Southeastern US indicates that variation for tolerance exists at multiple scales of study, from within-populations to among-regions. This broad-scale investigation of glyphosate tolerance in I. purpurea, from its genetics to its geographical partitioning, represents the first in-depth evolutionary consideration of tolerance to an herbicide.