Localized origins of herbicide resistance in Alopecurus myosuroides

The process of evolution of resistance to acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicides was investigated in four distinct patches of Alopecurus myosuroides Huds. (black-grass) that occur within adjacent fields on a cereal farm in Nottinghamshire, UK. In one field, there was a `main' patch containing 96% resistant plants and two `satellite' patches containing ≈2.9% and 4.4% resistant plants, and in an adjacent field another patch contained 25% resistant plants. Genome fingerprinting by simple sequence repeat (SSR)-anchored polymerase chain reaction (PCR) was used to analyse variation at 30 genetic loci in at least 20 resistant and 20 sensitive individual plants from each patch, from additional resistant populations from Essex and Lincolnshire, and from a sensitive reference population. Banding patterns were found to be highly repeatable. Each patch contained a high level of genetic diversity, regardless of its resistance status, and there was evidence for genetic differences between the patches (Gst = 0.14, Nei's distances up to 0.26). There was no evidence that resistance had spread from the `main' patch to the others, as resistant and sensitive plants in the same patch were more closely related on average than were resistant plants from neighbouring patches. The most likely explanations of this distribution, and their implications, are discussed.