Fitness costs of herbicide resistance traits in common waterhemp (Amaranthus tuberculatus)

A three-year multigenerational greenhouse study was conducted to determine the fitness costs of five herbicide resistances (HR) in waterhemp. In the study, a synthetic waterhemp population segregating for five types of HR was subjected to competitive growth conditions in the absence of herbicide selection for six generations. The resistance frequencies of each generation were determined from both whole-plant herbicide treatments (glyphosate, atrazine, 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors) and molecular markers (acetolactate synthase (ALS) and protoporphyrinogen oxidase (PPO) inhibitors). Our result is the first report of the fitness costs for NTR to atrazine and HPPD inhibitors, as well as for the different fitness costs of two EPSPS alterations. Our results indicate that the resistance traits have no fitness costs, with the exception that ALS inhibitor resistance conferred by T574L target-site substitution had a minor fitness cost. Specifically, the relative fitnesses determined from the overall resistance frequency changes were 0.92, 1.02, 1.09, 0.97, and 1.07 for resistances to ALS and PPO inhibitors, atrazine, HPPD inhibitors, and glyphosate, respectively. It was also determined that glyphosate resistance (GR) in the study population was endowed by at least two resistance mechanisms (P106S target-site mutation and amplification of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene). A phenotype-genotype association analysis was conducted to determine if the two GR mechanisms differ in their fitness costs and significance in conferring resistance. No fitness penalty was observed for the P106S substitution, while EPSPS amplification had a significant fitness cost (relative fitness of 1.12 and 0.72, respectively). EPSPS amplification and the P106S mutation did not fully account for glyphosate resistance in the population. The evolution of GR likely is a result of the interplay of different resistance mechanisms, with relative fitness of the mechanisms— both in the presence and absence of glyphosate—playing roles. The results from this novel study add to a growing body of evidence indicating that herbicide rotation is not an effective resistance management strategy because most herbicide resistances lack significant fitness costs