Alterations in Life-History Associated With Non-target-site Herbicide Resistance in Alopecurus myosuroides

Abstract

The evolution of resistance to herbicides is a classic example of rapid contemporary adaptation in the face of a novel environmental stress. Evolutionary theory predicts that selection for resistance will be accompanied by fitness trade-offs in environments where the stress is absent. Alopecurus myosuroides , an autumn-germinating grass weed of cereal crops in North-West Europe, has evolved resistance to seven herbicide modes-of-action, making this an ideal species to examine the presence and magnitudes of such fitness costs. Here, we use two contrasting A. myosuroides phenotypes derived from a common genetic background, one with enhanced metabolism resistance to a commercial formulation of the sulfonylurea (ALS) actives mesosulfuron and iodosulfuron, and the other with susceptibility to these actives (S). Comparisons of plant establishment, growth, and reproductive potential were made under conditions of intraspecific competition, interspecific competition with wheat, and over a gradient of nitrogen deprivation. Herbicide dose response assays confirmed that the two lines had contrasting resistance phenotypes, with a 20-fold difference in resistance between them. Pleiotropic effects of resistance were observed during plant development, with R plants having a greater intraspecific competitive effect and longer tiller lengths than S plants during vegetative growth, but with S plants allocating proportionally more biomass to reproductive tissues during flowering. Direct evidence of a reproductive cost of resistance was evident in the nitrogen deprivation experiment with R plants producing 27% fewer seed heads per plant, and a corresponding 23% reduction in total seed head length. However, these direct effects of resistance on fecundity were not consistent across experiments. Our results demonstrate that a resistance phenotype based on enhanced herbicide metabolism has pleiotropic impacts on plant growth, development and resource partitioning but does not support the hypothesis that resistance is associated with a consistent reproductive fitness cost in this species. Given the continued difficulties associated with unequivocally detecting costs of herbicide resistance, we advocate future studies that adopt classical evolutionary quantitative genetics approaches to determine genetic correlations between resistance and fitness-related plant life history traits.

The role of population and quantitative genetics and modern sequencing technologies to understand evolved herbicide resistance and weed fitness

“…Although resistant individuals produced 27% less seed than susceptible individuals under low fertility conditions, the former competed and produced the same seed amount as the latter when growing in competition with wheat (Triticum aestivum). 37 This illustrates how genetic changes in other parts of the genome can impact how well HR populations will be able to compete within the crop. In other words, the effect on fitness of the HR allele on individual plants provides only a limited view of how the HR allele will affect the fitness of the population.…”

“…As discussed above, HR and especially NTSR and multiple HR, are among the most immediate concerns that should be addressed using population and quantitative genetics. 37 It can be argued that, at this point, HR management could be considered a moot point for several species because they have evolved resistance to multiple (almost all) sites of action registered for specific crops. However, the recent reliance on tank mixtures combining herbicides with different sites of action and herbicide tolerance crops with stacked traits might favor selection of NTSR to multiple sites of action.…”

The costs of human-induced evolution in an agricultural system

“…We assume that the resistant BG phenotype has the same impact on yield as the susceptible wildtype. There is good evidence illustrating how limited the effects of both non-target-site resistance (NTSR) and some predominant target-site resistance (TSR) mutations are on relative performance of R and S BG biotypes [52][53][54] , and thus any influence on competition with the crop is likely to be negligible. Comparisons of NTSR and susceptible BG found no consistent fitness costs, either when grown alone or in competition with winter wheat 52,54 .…”

“…There is good evidence illustrating how limited the effects of both non-target-site resistance (NTSR) and some predominant target-site resistance (TSR) mutations are on relative performance of R and S BG biotypes [52][53][54] , and thus any influence on competition with the crop is likely to be negligible. Comparisons of NTSR and susceptible BG found no consistent fitness costs, either when grown alone or in competition with winter wheat 52,54 . In a study of three ACCase TSR mutations in BG 53 , one mutant allele (Gly-2078) did result in a small reduction in biomass and seed production; however, this mutation is rare, with a frequency of only 0.34% based on previous genotyping of 82 56 haplotypes from UK BG55.…”

Fitness of Herbicide-Resistant Weeds: Current Knowledge and Implications for Management

“…In other cases, herbicide resistance mutations have been shown to alter morphological, developmental, or phenological traits in weeds without necessarily a direct impact on plant fitness per se [58,59,60,61,62,63,64,65,66]. The changes in these traits are best thought of as changes in life history characters due to either subtle pleiotropic effects of resistance mutations or their coevolution with non-resistance life history traits in response to the wide range of selective factors operating in agroecosystems [9,67].…”