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Ds could transpose into and out of the C locus germinally, she inferred that somatic variegation reflects the frequent transposition of Ds during development. Transposition explained both Emerson's and Rhoades' earlier observations. McClintock and Rhoades were good friends, of course, and it is evident from their correspondence that McClintock immediately saw the parallels between the behavior of the c-m1 mutation and Rhoades' a1 mutation (Lee Kass, personal communication).

The Ac and Ds elements are transposition-competent and transposition-defective members of a single transposon family. In the ensuing years, McClintock identified and studied a second transposon family, called Suppressor-mutator (Spm) (McClintock, 1951, 1954). Her studies on these element families were purely genetic, and she was able to make extraordinary progress in understanding the transposition mechanism because she studied the interactions between a single transposition-competent element and one or a small number of genes with insertions of cognate transposition-defective elements (Fedoroff, 1989). Two points about this early history of transposition merit emphasis. First, the active elements were denumerable and manageable as genetic entities, despite their propensity to move. Second, the number of different transposon families and family members uncovered genetically was (and still is) small. Hence the genetic impact of transposable elements was limited. McClintock recognized that the high frequency of new variegating mutations in her cultures was linked to the genetic perturbations associated with the presence of broken chromosomes (McClintock, 1946, 1978). Her inference, extraordinarily prescient, was that transposons are regular inhabitants of the genome, but genetically silent.


With the cloning of the maize transposons, first the Ac element in my laboratory and later the cognate En and Spm elements in Heinz Saedler's and my laboratories, the picture began to change (Fedoroff et al., 1983; Pereira et al., 1985; Masson et al., 1987). To begin with, it became obvious immediately that the maize genome contains more copies of a given transposon than there are genetically identifiable elements. Although most of these sequences are not complete transposons, there are nonetheless more complete transposons than can be perceived genetically (Fedoroff et al., 1984). Importantly, it was clear almost immediately that a genetically active transposon could be distinguished from one that was genetically silent by its methylation pattern (Fedoroff et al., 1984; Banks and Fedoroff, 1989). Both of these observations bear on the genetic visibility of transposons.

As maize genes and genome segments began to be cloned and sequenced, the discovery of new transposons accelerated. Although the

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