Herbicide cycling has diverse effects on evolution of resistance in Chlamydomonas reinhardtii

Abstract

Cycling pesticides has been proposed as a means of retarding the evolution of resistance, but its efficacy has rarely been empirically tested. We evolved populations of Chlamydomonas reinhardtii in the presence of three herbicides: atrazine, glyphosate and carbetamide. Populations were exposed to a weekly, biweekly and triweekly cycling between all three pairwise combinations of herbicides and continuously to each of the three herbicides. We explored the impacts of herbicide cycling on the rate of resistance evolution, the level of resistance selected, the cost of resistance and the degree of generality (cross-resistance) observed. Herbicide cycling resulted in a diversity of outcomes: preventing evolution of resistance for some combinations of herbicides, having no impacts for others and increasing rates of resistance evolution in some instances. Weekly cycling of atrazine and carbetamide resulted in selection of a generalist population. This population had a higher level of resistance, and this generalist resistance was associated with a cost. The level of resistance selected did not vary amongst other regimes. Costs of resistance were generally highest when cycling was more frequent. Our data suggest that the effects of herbicide cycling on the evolution of resistance may be more complex and less favourable than generally assumed.

Expanding the eco‐evolutionary context of herbicide resistance research

“…Brotherton et al established differences in sensitivity to glyphosate between accessions of A. thaliana and tried (but failed) to select for decreased sensitivity. Others have used the single‐celled chlorophyte Chlamydomonas reinhardtii as a model for exploring the evolutionary dynamics of selection for herbicide resistance . The Chlamydomonas system has proved a useful tool for exploring the efficacy of resistance management prescriptions in delaying evolution of resistance, although questions remain about the translation of findings to higher plants.…”

“…The potential benefits of herbicide rotations and mixtures have been demonstrated by a number of empirical and theoretical studies. However, most of these studies have considered the evolution of specialist resistance phenotypes, where selection for discrete resistance traits is independent.…”

Selection history and epistatic interactions impact dynamics of adaptation to novel environmental stresses

“…We have previously shown that resistance to carbetamide is very costly and is not easily acquired by a wild-type population [19]. Epistatic interactions between independent resistance mutations can affect the fitness costs [15], so that presence of resistance mutations to one xenobiotic could reduce the costs associated with resistance to a subsequent herbicide through antagonistic epistasis between resistance mutations [12,25].…”

“…The strain was pre-adapted to laboratory culture conditions and, prior to selection experiments, a selection line was initiated by isolating a single cell colony from an agar plate and subsequent multiplication for 7 days in liquid Bold's medium [19].…”

“…The culture medium was modified Bold's medium [20], and the conditions of growth were as described in Lagator et al [19]. Cultures were propagated every 7 days (see 'selection regimes' below), during which time the ancestral population growing in the absence of herbicides reached the stationary phase (approx.…”