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The discovery of transposable elements has been a major step in the evolution of our concepts about heredity. Mobile genetic elements, by their ability to intercalate at different positions in the genome, and to create mutations and genetic rearrangements, appeared then as a challenge to the strict mendelians laws accepted as the basis of eucaryotic genetics. Transposable elements have been now characterized in the widest range of organisms, but the major credit for their discovery must be given to plant geneticists, and especially to Barbara McClintock, for her analysis of maize somatic instabilities in the early 40s. Molecular analysis of plant transposable elements is however quite recent, and the cloning of the first element characterized by McClintock, the maize Ac element, was reported in 1983 (Fedoroff et al). Great progress has been made in this field in the last seven years, mostly sustained by the hopes of considerable applications for genetic engineering, especially for the cloning of genes in higher plants. Gene-tagging techniques allow the cloning of any plant gene, provided that screening for mutants of the corresponding phenotype is possible, and have been successfully used in maize and snapdragon, in which more than a dozen of genes have been cloned this way (Fedoroff et al, 1984; Martin et al, 1985; O’Reilly et al, 1985; Cone et al, 1986; Paz-Ares et al 1986; Schmidt et al, 1987; Motto et al, 1988; Martienssen et al, 1989). Most of these genes, however, were genes coding for a limited range of functions visible at the plant level, such as flower or seed coloration, or seed starch content, whose cloning required the screening of large population of plants. |
