An exploration of the dynamics of selection for resistance to herbicides

Herbicide resistance in weeds is a major threat to agricultural productivity, but the incidence of herbicide resistance in agricultural systems continues to increase. It is increasingly recognised that sustainable weed management can only be achieved by considering the ecological and evolutionary drivers of herbicide resistance evolution. Within this context, it has been suggested that selection within pre-existing variation in herbicide susceptibility, underpinned by additive genetic variation, may result in rapid evolution of herbicide resistance. In this thesis, these principles were tested in the major agricultural weed Alopecurus myosuroides (black-grass). Dose-response experiments demonstrated pre-existing phenotypic variation in response to two commercial herbicides with alternative modes of action (the ACCase inhibitor herbicide fenoxaprop-P-ethyl and the ALS inhibitors mesosulfuron-methyl and iodosulfuron-methyl-sodium) in A. myosuroides with no history of exposure to herbicide. Selection within this pre-existing variation was then tested using low doses of the herbicide fenoxaprop-P-ethyl, demonstrating rapid increases in resistance following a single generation of selection. Recurrent selection showed further, but non-significant, increases in resistance. Competition experiments were then performed to compare fitness in the absence of herbicide between the original susceptible population, and a population following two generations of selection for fenoxaprop-P-ethyl resistance. Results suggested that both resistant and susceptible A. myosuroides populations were of similar fitness and competitiveness: seed output was highly variable, and both resistant and susceptible populations were highly competitive against wheat. Finally, the possibility of exploiting dose-dependent selection to limit resistance evolution was tested in a long-term dose rotation experiment using the unicellular alga Chlamydomonas reinhardtii. The results showed that rotating between a very high herbicide dose and no herbicide application could limit resistance evolution, but overall population control was poor. There were indications that rotation between intermediate higher and lower herbicide doses could successfully control population sizes and limit resistance evolution, but these effects were only present in the earlier stages of the experiment