Genetic diversity of weeds growing in continuous wheat

Genome fingerprinting by simple sequence repeat (SSR)‐anchored polymerase chain reaction (PCR) was used to analyse diversity in four weed species (Alopecurus myosuroides Huds., Medicago lupulina L., Stellaria media (L.) Vill. and Aphanes arvensis L.) growing in the Broadbalk long‐term wheat experiment. The experiment is made up of individual plots, to which different fertilizer regimes have been applied annually for more than 150 years. Sixteen polymorphic loci were analysed in the obligate outcrosser A. myosuroides (24 plants sampled on each of four plots); the proportion of total diversity caused by differences between populations (Gst) was low (0.02). Twelve loci were analysed in the self‐pollinating species M. lupulina (24 plants sampled on each of four plots, Gst=0.12) and S. media (24 plants sampled on each of three plots, Gst=0.11); and 11 loci in the apomict A. arvensis (24 plants sampled on each of four plots, Gst=0.19). There was no relationship between the geographic distances separating plots and the genetic distances between samples from the weed populations growing on each plot. Soil fertility had an effect on molecular diversity in S. media: plants growing on low‐nitrogen showed divergence from those sampled on high‐nitrogen plots, which contained slightly more diversity.