Defence against the herbicide RoundUp® predates its widespread use

Background: The herbicide RoundUp® became widely used in agriculture in the mid-1990s. Current populations of the crop weed Ipomoea purpurea exhibit both resistance and tolerance to RoundUp, which are two types of defence that have previously been shown to have a genetic basis in this weed species. Questions: Did the genetic basis of RoundUp defence antedate the introduction of RoundUp? If so, how does defence today compare with defence then? Organism: A preserved collection of Ipomoea purpurea seeds (the common morning glory) was germinated from 10 accessions sampled from the early 1980s. A contemporary population of I. purpurea derived from a crossing scheme was used for comparison. Methods: Replicate seeds from both the historical and contemporary populations were planted into a field experiment located at the University of Georgia’s Plant Sciences Farm. Individuals were sprayed with the field rate of glyphosate, the active ingredient in the herbicide RoundUp. Tolerance, or the fitness response following herbicide application, and resistance, a measure of the amount of damage a plant exhibits after herbicide application, were determined for each line of both the historical and contemporary populations. Results: Genetic variation for both types of defence was already present in this weed species prior to the herbicide’s introduction in the mid-1990s. Furthermore, the level of tolerance to glyphosate has not increased over time. In contrast, our results suggest that resistance is higher in the contemporary population than in the historical accessions.