Fitness cost associated with enhancedEPSPSgene copy number and glyphosate resistance in anAmaranthus tuberculatuspopulation

The evolution of resistance to pesticides in agricultural systems provides an opportunity to study the fitness costs and benefits of novel adaptive traits. Here, we studied a population of Amaranthus tuberculatus (common waterhemp), which has evolved resistance to glyphosate. Following the production of seed families with contrasting levels of glyphosate resistance, we assessed the growth and fitness of seed families in the absence of glyphosate and determined their ability to compete for resources under intra- and interspecific competition. Further investigation revealed a positive correlation between the level of resistance and gene copy number for the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) glyphosate target, thus indicating gene amplification as the mechanism of resistance within the population. Resistant common waterhemp plants were found to have a lower competitive response when compared to the susceptible phenotypes. A substitution rate of 2.76 glyphosate resistant plants was required to have an equal competitive effect as a single susceptible plant. A growth trade-off was associated with the gene amplification mechanism under intra-phenotypic competition where 20 extra gene copies were associated with a 26.5 % reduction in dry biomass. Interestingly, this growth trade-off was mitigated when assessed under interspecific competition from maize.