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Abstract. Weed species are known to evolve rapidly with their associated crops. A better understanding of the mechanisms and rates of weed evolution could aid in limiting or at least anticipating this process. Spontaneous hybridization between crops and related weed species can transfer crop genes coding for fitness-enhancing traits to wild populations, but little is known about how easily this takes place in various weed-crop complexes. We studied interspecific hybrids between wild and cultivated radishes (Raphanus raphanistrum ϫ R. sativus), which often co-occur and share pollinators. To determine whether the F 1 generation represents a strong barrier to subsequent introgression, we compared the fitness of wild plants and wild-crop hybrids. Two experiments were carried out in Michigan, USA, one with potted plants and the other involving four artificially established populations. In the artificial populations, we used white flower color, a dominant, crop-specific allele, to document the persistence of crop genes over time. Wild plants had yellow flowers, which is a recessive trait. F 1 hybrids had lower fitness than wild plants due to lower pollen fertility, fewer seeds per plant, and delayed flowering. Despite these disadvantages, hybrids contributed substantially to each population's gene pool. After 3 yr, frequencies of whiteflowered plants in the artificial populations ranged from 8% to 22%, demonstrating that crop genes persisted. Other studies of flower color variation in wild populations of R. raphanistrum provide circumstantial evidence for frequent crop-to-wild gene flow. We predict that, if cultivated radish is engineered to possess transgenes coding for traits such as resistance to insect herbivores, disease, herbicides, or environmental stress, these fitnessrelated crop genes will easily spread to R. raphanistrum.

Few studies of genetic variation have focused on species that reproduce through both showy, chasmogamous (CH)¯owers and self-pollinated, cleistogamous (CL)¯owers. Using two dierent techniques, genetic variation was measured in six populations of Viola pubescens Aiton, a yellowowered violet found in the temperate forests of eastern North America. Results from eight allozyme loci showed that there was considerable genetic variation in the species, and population structuring was indicated by the presence of unique alleles and a h (F ST ) value of 0.29. High genetic variation was also found using ISSR (inter-simple sequence repeat) markers, and population structuring was again evident with unique bands. Viola pubescens appears to have a true mixed-mating system in which sel®ng through CL and CH¯owers contributes to population dierentiation, and outcrossing through CH¯owers increases genetic variation and gene¯ow among populations. Overall, allozyme and ISSR techniques yielded similar results, indicating that ISSR markers show potential for use in population genetic studies.