Effects of the triazine-resistance mutation on fitness in Amaranthus hybridus (smooth pigweed)

1. Fitness costs of maternally inherited triazine resistance were estimated in populations of Amaranthus hybridus (smooth pigweed) from Maryland and Virginia (USA), through fitness comparisons of lines that bear comparable nuclear genomes but had either resistant or susceptible cytoplasm. Under glasshouse conditions only modest costs were evident. However, larger fitness costs were often evident when seedlings were transplanted, over 3 years of field studies, into a diverse community of annual weeds in the field; the relative fitness of resistant lines ranged from 0·24 to 1·12 (mean 0·62). 2. Fitness costs of resistance appeared to be larger in the population from Virginia. In the glasshouse, resistant plants of this population suffered a definite cost in terms of early seedling growth and biomass production (mean relative fitness of resistant lines = 0-82), while very little cost was evident in the Maryland population. In the field, the relative fitness of resistant lines from the Virginia population averaged 0·44 over 3 years, while that of resistant lines frm Maryland averaged 0·82. However, at high levels of neighbour interference in the field, the fitness costs were similar in the two populations and very large (relative fitness of resistant lines = 0·26). 3. A path analysis was used to estimate fitness differences between resistant and susceptible lines at different stages of growth in the field. The analysis apportioned total differences in seed production between resistant and susceptible lines into a series of independent components associated with three successive periods of growth. Large (e.g. 33%) fitness differences were evident during some growth periods in some years. However, costs were absent or statistically insignificant in other instances. These results do not support the hypothesis that a fitness cost of triazine resistance was consistently operative over the whole growing season. Rather, the mechanism(s) causing the fitness costs appeared to function sporadically.