Fitness effects of the triazine resistance mutation in Amaranthus hybridus: relative fitness in maize and soyabean crops

Fitness costs of the triazine resistance mutation were estimated in experimental populations of Amaranthus hybridus L. from Maryland and Virginia (USA). Estimated costs were attributed directly to effects of the resistance mutation, through comparisons of lines bearing similar nuclear genomes, but either resistant or susceptible cytoplasm. Experimental weed plants were established from seed in the field at a wide range of A. hybridus and crop (maize or soyabean) densities, and experiments were repeated over 3 years. Resistant cytoplasm caused large reductions in several components of fitness, but substantial differences were observed among years in these fitness costs of resistance. For the fitness component of total biomass above-ground biomass production, relative fitnesses of resistant genotypes (resistant/susceptible ratios) ranged between 0.42 and 0.70. Costs of resistance were similar in weeds growing with either maize or soyabean crops. Certain costs of resistance differed between populations: impairment of seedling establishment and early growth by the resistance mutation was evident in the Virginia population, but not in the Maryland population. This functional difference between populations was associated with greater costs of resistance in the Virginia population with respect to mid-season leaf number and total above-ground biomass production; however, only the former population difference in fitness cost was statistically significant.