Excision of one of two defective P elements as the cause of alternate mutational events (sn⁺ and sn^e) of the singed bristle allele sn^w in Drosophila melanogaster

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 (sn^w) (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 (sn^e), 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 sn^e) 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 sn^e 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.