Fitness costs associated with three mutant acetyl-coenzyme A carboxylase alleles endowing herbicide resistance in black-grass Alopecurus myosuroides

1. Pleiotropic effects associated with genes endowing resistance to herbicides are generally predicted to reduce plant fitness. Quantifying these effects is necessary to develop management strategies against herbicide-resistant weeds. We assessed the pleiotropic effects associated with three mutant alleles of the herbicide target enzyme acetyl-coenzyme A carboxylase (ACCase) on plant growth and seed production in black-grass Alopecurus myosuroides. 2. In each of two field experiments, black-grass populations segregating for Leu-1781 (five populations), Asn-2041 (three populations) or Gly-2078 (two populations) ACCase were produced to obtain several distinct, homogenized genetic backgrounds and to permit reliable comparisons among wild-type, heterozygous and homozygous mutant ACCase plants grown in competition with a wheat crop. 3. No significant differences from wild-type plants in vegetative biomass, height and seed production were observed in Leu-1781 or Asn-2041 ACCase plants. 4. Over both experiments, homozygous Gly-2078 ACCase plants displayed a significant reduction in biomass (42%), height (6%) and seed production (36%). Reduction varied with the segregating population and between field experiments. 5. Synthesis and applications. Our work illustrates the variation in fitness cost depending on the resistance gene, the plant genetic background and the environment. This underlines the necessity to identify the resistance gene(s) present in a weed population before designing resistance-management strategies. Competitive crops should be effective against Gly-2078 ACCase plants. The effect of resistant ACCase alleles on seed survival and germination needs to be studied in order to develop cultural practices creating or maximizing fitness costs in resistant plants. However, the variation of fitness cost with the environment and the weed population, as well as the likely absence of fitness cost associated with resistance genes such as Leu-1781 ACCase, renders the success of specific cultural control practices uncertain. A solution could be a weed control programme maximizing the diversity of cultural practices and including anti-resistance cultural practices to avoid or reduce selection for resistant ACCase alleles.