Genetic diversity of weeds growing in continuous wheat

Genome fingerprinting by simple sequence repeat (SSR)-anchored polymerase c hain reaction (PCR) was used to analyse diversity in four weed species (Alo pecurus myosuroides Huds., Medicago lupulina L., Stellaria media (L.) Vill. and Aphanes arvensis L.) growing in the Broadbalk long-term wheat experime nt. The experiment is made up of individual plots, to which different ferti lizer regimes have been applied annually for more than 150 years. Sixteen p olymorphic loci were analysed in the obligate outcrosser A. myosuroides (24 plants sampled on each of four plots); the proportion of total diversity c aused by differences between populations (G(st)) was low (0.02). Twelve loc i were analysed in the self-pollinating species M. lupulina (24 plants samp led on each of four plots, G(st)=0.12) and S. media (24 plants sampled on e ach of three plots, G(st)=0.11); and 11 loci in the apomict A. arvensis (24 plants sampled on each of four plots, G(st)=0.19). There was no relationsh ip between the geographic distances separating plots and the genetic distan ces between samples from the weed populations growing on each plot. Soil fe rtility 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.