Crop-to-wild gene flow has received close attention over the past ten years in connection with the development and cultivation of transgenic crops. In this paper, we review key examples of crop/wild sympatry and overlapping flowering phenology, pollen and seed dispersal, the barriers to hybridisation and introgression, the evolution and fate of interspecific hybrids, their fitness, and the potential cost of transgenes. We pay particular attention to ways in which the evolution and divergence

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... ween crops and their wild relatives may interfere with these successive steps. Our review suggests that crop-to-weed gene flow is highly idiosyncratic and that crop gene dispersion will certainly be very difficult to preclude totally. Future directions for research should thus focus on the long-term establishment and effects of transgenes on natural communities. Arias DM, Rieseberg LH (1994) Gene flow between cultivated and wild sunflowers. Theor. Appl. Genet. 89: 655-660 Arnold ML (1997) Natural hybridization and evolution. Oxford University Press, Oxford Arriola PE, Ellstrand NC (1996) Crop-to-weed gene flow in the genus Sorghum (Poaceae): spontaneous interspecific hybridization between johnsongrass, Sorghum halepense, and crop sorghum, S. bicolor. Am. J. Bot. 83: 1153-1160 Arriola PE, Ellstrand NC (1997) Fitness of interspecific hybrids in the genus Sorghum: persistence of crop genes in wild populations. Ecol. 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