Article

Gene flow is understood here as the movement of genes among sympatric or neighbouring populations of crops and related taxa or the introduction of genes from related taxa into the recipient genepool of a crop, ie. it describes the processes also known as introgression. We must differ between artificial or induced and spontaneously occuring gene flow. Atificial or human-caused gene flow has a rather long history in plant breeding and applied genetics and is documented by the efforts to use wild or weedy relatives of crops as donors of economically important characters. The successful breeding e.g. of disease resistant cultivars in many crop species by the transfer of resistance genes from wild taxa arc well-known examples. The spontaneous occurence of gene flow between crops and relatives however is a poorly understood phenomenon and has been described only anecdotically in most cases. There were gene flow processes between wild taxa and crops in the past which led to obvious evolutionary changes, ¢.g. to the formation of new species (hexaploid wheats). Actual gene flow between crops and relatives can be detected only by careful field studics. In general such gene flow is a rare event and can happen only in crops if their primary genepools includes related taxa and if crop and relatives are more or less sympatric. In Europe the possibility of gene flow between crops and relatives is greatly restricted, because European crops are mostly exotic plants from other regions of the world which lack relatives in the European flora. Some examples of gene flow occuring on different taxonomic levels had been described in some detail for carrot, beet, alfalfa, rice, foxtail millet and Chenopodium and the importance of ethnobotanical observations for the understanding of gene flow processes is underlined.