The X-linked singed locus is concerned with the bristle phenotype and female sterility, and is known as a hot spot of P element insertion. A moderate allele of singed, singed-weak (snw) (Engels, 1979; 1984) is inserted with P elements. It is used as an index of P element activity, since it mutates at a high frequency to either a more extreme allele, singed-extreme (sne), or to a phenotype that is equivalent to the wild type (SK+) when an autonomous P element exists. We show here that mw is inserted with two defective P elements in reverse orientation, and the two alternate mutational events (sn+ and sne) are caused by the excision of one or the other of the P elements present in the singed gene. It is interesting that sn+ and sne are inserted with a single P element in the same position, but show very different phenotypes. The insertional sites of P elements in the singed locus possibly contain an unidentified repetitive sequence, which is repeated dozens of times per haploid genome of the wild-type strain Canton-S.
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