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transposons that McClintock identified and studied were DNA transposons, both gypsy-like and copia-like retrotransposons were soon identified in the maize genome and subsequently in many other plant genomes (Shepherd et al., 1984; Flavell, 1992; Purugganan and Wessler, 1994; White et al., 1994; Suoniemi et al., 1997, 1998). It has also become evident that non-long terminal repeat (LTR) retrotransposons are abundant in maize, as well as other plant genomes (Schwarz-Sommer et al., 1987; Noma et al., 1999). Many additional maize transposon families have been identified through their sequence organization and their presence in or near genes (Spell et al., 1988; Bureau and Wessler, 1992, 1994a, b; Wessler et al., 1995; Bureau et al., 1996). We now know that transposons and retrotransposons comprise half or more of the maize genome (Bennetzen et al., 1998).

WHAT DO TRANSPOSONS DO?

Commencing with McClintock's elegant analyses of transposon-associated chromosomal rearrangements and extending into the literature of today, the range of transposon-associated genetic changes has continued to expand (McClintock, 1951). Insertion of plant transposons, like almost all known transposons, is accompanied by the duplication of a short flanking sequence of a few base pairs (Schwarz-Sommer et al., 1985a). Plant transposons excise imprecisely, generally leaving part of the duplication at the former insertion site (Schwarz-Sommer et al., 1985a). The consequences of insertion and excision of a transposon therefore depend on the location within the coding sequence and excision of an insertion from an exon commonly results in either an altered gene product or a frame-shift mutation. Transposon insertions can alter transcription and transcript processing, and there are cases in which transposons are processed out of transcripts by virtue of the presence of splice donor and acceptor sequences (Kim et al., 1987; Wessler, 1989; Giroux et al., 1994). Transposons also can promote the movement of large segments of DNA either by transposition or by illegitimate recombination (Courage-Tebbe et al., 1983; Schwartz et al., 1998).

THE PARADOX

One might think that given their abundance, transposable elements would rapidly randomize genome order. Yet the results of a decade of comparative plant genome studies has revealed that gene order is surprisingly conserved between species. Close relationships among genomes have been demonstrated in crop plants belonging to the Solanaceae, and the Graminae, between Brassica crops and Arabidopsis, among several legumes, and others (Tanksley et al., 1992; Gale and Devos, 1998; Lager-



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