Glyphosate resistance reduces kochia fitness: Comparison of segregating resistant and susceptible F2 populations

Glyphosate is considered the world's most important herbicide, but widespread and continual use has resulted in the evolution of resistance. Kochia scoparia (kochia) has evolved resistance via tandem gene amplification of glyphosate's target, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) and resistant populations have been reported from the Canadian Prairies and the Northern Great Plains. Here, we evaluated the fitness costs of EPSPS amplification in kochia by comparing susceptible and resistant full siblings from segregating F2 populations generated from within six populations. Kochia was expected to be highly diverse because of strong gene flow; however, six of the seven field-collected parents with higher EPSPS copy number were homozygous. Under competitive greenhouse conditions, the EPSPS type of the line's maternal parent showed persistent effects: delayed emergence, delayed flowering, and reductions in viable seed count and weight overall. High EPSPS copy number individuals had reduced seed count and weight, reduced competitive ability, and reduced final height in mixed stands, but better germination of the F3. However, all characteristics were highly variable and fitness costs were not constant across genetic backgrounds. In the absence of selection from glyphosate, kochia with increased EPSPS copy number will be at a competitive disadvantage in some genetic backgrounds.