|II.||Methods for the detection and estimation of herbicide resistance fitness costs||753|
|III.||Fitness costs associated with AHAS target site resistance||756|
|IV.||Fitness costs associated with ACCase target site resistance||757|
|V.||Fitness costs associated with resistance to phenoxy herbicides||758|
|VI.|| Arabidopsis thaliana as a model plant to understand pleiotropic effects of herbicide target-site based resistance alleles||759|
|VII.||Fitness costs associated with herbicide resistance endowed by enhanced rates of herbicide metabolism catalysed by P450 enzymes||760|
|VIII.||Fitness costs associated with glyphosate resistance||760|
Predictions based on evolutionary theory suggest that the adaptive value of evolved herbicide resistance alleles may be compromised by the existence of fitness costs. There have been many studies quantifying the fitness costs associated with novel herbicide resistance alleles, reflecting the importance of fitness costs in determining the evolutionary dynamics of resistance. However, many of these studies have incorrectly defined resistance or used inappropriate plant material and methods to measure fitness. This review has two major objectives. First, to propose a methodological framework that establishes experimental criteria to unequivocally evaluate fitness costs. Second, to present a comprehensive analysis of the literature on fitness costs associated with herbicide resistance alleles. This analysis reveals unquestionable evidence that some herbicide resistance alleles are associated with pleiotropic effects that result in plant fitness costs. Observed costs are evident from herbicide resistance-endowing amino acid substitutions in proteins involved in amino acid, fatty acid, auxin and cellulose biosynthesis, as well as enzymes involved in herbicide metabolism. However, these resistance fitness costs are not universal and their expression depends on particular plant alleles and mutations. The findings of this review are discussed within the context of the plant defence trade-off theory and herbicide resistance evolution.