Limited fitness costs of herbicide-resistance traits in Amaranthus tuberculatus facilitate resistance evolution

Authors: WU, CHENXI - University Of Illinois; Davis, Adam; TRANEL, PATRICK - University Of Illinois

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/11/2017
Publication Date: 1/5/2018
Citation: Wu, C., Davis, A.S., Tranel, P.J. 2017. Limited fitness costs of herbicide-resistance traits in Amaranthus tuberculatus facilitate resistance evolution. New Phytologist. 74(2):293-301. doi.org/10.1002/ps.4706.
DOI: https://doi.org/10.1002/ps.4706

Interpretive Summary: A common extension recommendation to farmers for delaying the onset of herbicide resistance on their farms is to switch among different herbicide families over time. This recommendation is based on the idea that plant defenses are costly (i.e., have a ‘fitness cost’), and when a defense trait isn’t needed, it should become less prevalent in the population. To evaluate this resistance prevention strategy, we conducted a study of the fitness costs of five herbicide resistance traits in an economically dominant weed of corn and soybean production in the upper Midwest U.S., common waterhemp (Amaranthus tuberculatus). We subjected a common waterhemp population containing five types of herbicide resistance to competitive growth conditions in the absence of herbicide selection for six generations. Fitness costs were quantified by using a combination of phenotyping and genotyping to monitor herbicide resistance frequency changes over generations. In the absence of herbicide selection, a significant fitness cost was observed for only one out of five herbicides. Our results indicate that herbicide-resistance mitigation strategies (e.g., herbicide rotation) that rely on the existence of fitness costs in the absence of herbicide selection likely will be largely ineffective in many cases.

Technical Abstract: The fitness cost of herbicide resistance (HR) in the absence of herbicide selection plays a key role in HR evolution. Quantifying the fitness cost of resistance, however, is challenging, and there exists a knowledge gap in this area. A synthetic Amaranthus tuberculatus population segregating for five types of HR was subjected to competitive growth conditions in the absence of herbicide selection for six generations. Fitness costs were quantified by using a combination of phenotyping and genotyping to monitor HR frequency changes over generations. In the absence of herbicide selection, a significant fitness cost was observed for resistance to acetolactate-synthase-inhibiting-herbicides, but not for resistances to atrazine (nontarget-site resistance mechanism), protoporphyrinogen oxidase inhibitors, 4-hydroxyphenylpryuvate-dioxygenase inhibitors, or glyphosate. Glyphosate resistance was conferred by multiple mechanisms in the synthetic population, and further analysis of this resistance revealed that one mechanism, amplification of the 5-enolypyruvyl-shikimate-3-phosphate synthase gene, did have a significant fitness cost. Our results indicate that herbicide-resistance mitigation strategies (e.g., herbicide rotation) that rely on the existence of fitness costs in the absence of herbicide selection likely will be largely ineffective in many cases.