Somatic reversion of P transposable element insertion mutations in thesingedlocus ofDrosophila melanogasterrequiring specific P insertions and atrans- acting factor

Abstract

Destabilization in somatic cells of P-element insertions in the X-linked singed gene of Drosophila melanogaster has been studied. We have shown that some but not all unstable P-element insertions in singed can form mosaics. The cause of this variation is not clear from studies of the restriction maps of the mutations tested. The transposable element movements occur early in development and require, in addition to an appropriate P-element insertion in singed, a /raws-acting maternal effect component. Movements appear to occur preferentially in attached-X stocks. However, the maternal effect component maps to the central region of chromosome 2. IntroductionThe phenomenon of P-M hybrid dysgenesis has been shown to be due to a family of transposable genetic elements called P-elements (Engels, 1983(Engels, , 1986. Pfactors are autonomous P-elements capable of producing a transposase protein which can mobilize both themselves and the smaller, deleted, P-elements. The transposable elements are mobilized at high frequencies in P-M hybrid dysgenesis, generating chromosome breaks, insertion mutations, and reversions of pre-existing P insertions. P-M hybrid dysgenesis occurs in the Fl of a cross between males containing P factors (P-strain males) and females from strains lacking such factors (M-strains). Hybrid dysgenesis does not occur in the reciprocal cross, nor does it occur in crosses between P strains. Thus P-factors are active only in the cytoplasm contributed by M-strain mothers (the M-cytotype) but not in the cytoplasm contributed by P-strain mothers (or P-cytotype). Some mutations revert at particularly high frequencies. The mutation singed wtak (sn w ), for example, reverts, in dysgenic conditions, either to an almost wild-type phenotype or to a much more extreme singed phenotype, at a combined rate of around 50%.With such a high rate of movement, it might be expected that such flies would also be mosaics, with

The molecular basis of instability of thesingedvery weakmutation inDrosophila melanogaster

“…We presume that the 3' ends of both elements are intact in sn vw . Previous restriction mapping of the allele (Brookfield & Lewis, 1989) is consistent with this interpretation. The 3' ends of the two elements are normal in the strongly singed and pseudo-wild type revertants of the allele.…”

“…The element remaining in the singed products is, in each case, the one whose transcription would be in the same direction as singed, while, correspondingly, when the element remaining has transcription in the opposite orientation to singed the pseudo-wild-type product is generated. However, this is probably coincidental since P element insertions in either orientation can generate strong singed mutations (Roiha, Rubin & O'Hare, 1987, Brookfield & Lewis, 1989. Furthermore, while it is easy to imagine how the structure of the strong singed product, in which the transcriptional stop signals at the end of the KP element would be expected to stop singed transcripts, explains the phenotype, it is hard to see how a weaker phenotype is restored by the addition of the second element downstream in sn vw .…”

“…These were derived from the crosses that produced strain 34 (Brookfield & Lewis, 1989) but selected for mosaicism. They would be expected to bear the Mo allele on chromosome 2, although it is not certain that this allele was fixed in the strain.…”

“…A mutation has been described in the Xchromosomal singed gene, singed vervweak (or sn vw ) in which the germline specificity of P element movement is incomplete (Brookfield & Lewis, 1989). The mutation has two inserted sequences, which appear, from their restriction maps, to be deleted P elements of around 1150 and 650 base pairs in length in an inverted tail-to-tail orientation (i.e.…”

Genetic evidence for repression of somatic P element movements in Drosophila melanogaster consistent with a role for the KP element

“…This mutation can show what is, for P element insertion mutations, the very unusual property of being somatically unstable. Sfl' can revert somatically to patches of strong singed tissue in a very weakly singed phenotypic background, but only in the sons of females which have a dominant maternal effect mutation, which we have called Mo (for mosaicism) (BrookfIeld & Lewis, 1989). The two P elements in are apparently inserted next to each other save for the 8 base pair target sequence duplicated between them, at base pair 1580 of the singed sequence (Paterson & O'Hare, 1991;Ortori et a!., 1994).…”