A molecular analysis of three unstable alleles in drosophila : (transposable elements, mutable alleles, white locus)
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We have determined the structure of several unstable mutant alleles of the white locus in Drosophila melanogaster. The white ivory (w[superscript i]) allele is a moderately unstable allele, which gave rise to the highly unstable white-crimson (w[superscript C]) allele. We have determined that the w[superscript i] mutation is due to the duplication of 2.9 kilobases (kb) of DNA within the white locus, and that reversion of w[superscript i] to wild type usually occurs by simple loss of one copy of the duplication. We have also analyzed two highly unstable alleles of the white locus, we and white dominant zeste-like (w[superscript DZL]) and have shown that both are insertion mutations. The w[superscript C] mutation results from the insertion of 10 kb of DNA into the w[superscript i] duplication, and the w[superscript DZL] mutation results from the insertion of 13 kb of DNA at or near the right end of the white locus. The w[superscript C] and w[superscript DZL] insertions are structurally related, but not identical, and are related to a previously characterized family of transposable elements, the fold back(FB) elements. The we insertion consists of a single FB elementwith a low eopy number sequence between the moderately repetitive terminal inverted repeats. The wDZL insertion contains two FB elements which flank a single copy sequence in the middle of the insertion. Reversion of w[superscript C] to w[superscript i] is mediated by an apparently precise excision event, while reversion of w[superscript DZL] to wild type occurs by an imprecise excision of the insertion. We suggest that structural differences in the two insertions may account for these different modes of reversion.