Fitness cost due to herbicide resistance may trigger genetic background evolution.

This article investigates the possible existence of mechanisms counterbalancing the negative pleiotropic effects on development and reproduction that are conferred by alleles responsible for herbicide resistance in the weed Alopecurus myosuroides. We considered three herbicide-resistant, mutant acetyl-coenzyme A carboxylase (ACCase) alleles, Leu1781, Asn2041, and Gly2078, found in eight resistant populations. Of these, Gly2078 is the only allele with a known fitness cost. We compared plants homozygous for wild-type ACCase alleles that were siblings of plants carrying a given mutant resistant ACCase allele with plants from three populations where resistance did not evolve. In each of two series of experiments, we measured germination dynamics, seedling vigor, plant height, vegetative biomass, and seed production. The wild-type siblings of plants carrying Gly2078 performed better in the field, on average, than wild-type plants that were sibling of plants carrying other mutant ACCase alleles, and particularly those carrying Leu1781. We propose that rapid evolution of the genetic background of plants from the populations where the Gly2078 allele originally arose could partially counterbalance Gly2078 fitness cost, enhancing the spread of the resistant genotypes.