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Escape, Resist, or Tolerate? Evolution of Defence Strategies in Response to Glyphosate Herbicide in an Agricultural Weed (Amaranthus palmeri)

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Title: Escape, Resist, or Tolerate? Evolution of Defence Strategies in Response to Glyphosate Herbicide in an Agricultural Weed (Amaranthus palmeri)
Author: Teitel, Zachary
Department: Department of Integrative Biology
Program: Integrative Biology
Advisor: Caruso, Christina
Abstract: Weeds can respond to herbicide stress with three defence strategies: escape, resistance, and tolerance. Escape occurs when a weed changes the timing of its life cycle to avoid a herbicide; resistance occurs when a weed reduces the extent of damage it receives from a herbicide; and tolerance occurs when a weed withstands damage from a herbicide, but compensates for the loss of fitness resulting from that damage. Whether a weed population evolves to escape, resist, or tolerate a herbicide depends on tradeoffs among defence strategies, limits to the evolution of defence strategies, and selection on defence strategies in competitive agricultural environments. I examined how each of these factors shape the evolution of defence strategies in the noxious agricultural weed, Amaranthus palmeri S. Wats., in response to glyphosate herbicide. First, I examined whether escape and tolerance are correlated both within and among populations, which would indicate whether defence strategies are trading off. Second, I tested whether genotype by environment interactions (GEI) and estimated fitness costs have the potential to limit the evolution of escape and tolerance. Third, I measured selection acting on glyphosate escape and resistance within two environmental contexts: a high competition corn (Zea mays L.) crop environment and a low competition no crop environment. I found that both within and among populations there were no tradeoffs between escape and tolerance, suggesting that populations can potentially evolve high magnitudes of both escape and tolerance. Further indicating this, I did not find fitness costs for escape or tolerance in the absence of glyphosate, though one potential limit I discovered was from a GEI, showing that variable environments could limit the evolution of escape and tolerance. Finally, though I found no selection acting on glyphosate escape, there was selection for increased resistance in the low competition environment, and selection against high magnitudes of resistance in the high competition environment, indicating that competition with crops could slow the evolution of glyphosate resistance. Overall, I found that glyphosate escape and tolerance are less likely to evolve than resistance, though the evolution of resistance could be managed by manipulating the competitive crop growing environment.
Date: 2021-09
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