The transposon A(R)4-24P[white, rosy] in Drosophila melanogaster is subject to position-effect variegation at a non-centromeric insertion site

The white gene within the transposon A(R)4-24P [white, rosy] inserted at cy tological location 24D1-2 in the euchromatic portion of the Drosophila mela nogaster genome exhibits a mosaic pattern of expression which is modified b y temperature and Y-chromosome number, as in cases of classical position-ef fect variegation (PEV). The eye colour of the flies in this variegated stoc k remains mosaic in the presence of the PEV modifier Su(var)3-6, slightly l ess so with Su(var)3-9 and Su(var)2-5, and full suppression of variegation occurs in the presence of Su(var)3-7. We have induced further transposition of A(R)4-24 and isolated two mosaic stocks with this transgene at new cyto logical locations. In these stocks, the A(R)4-24 transposon was flanked by the same genomic DNA fragments as in the original location. Spontaneous los s of these fragments leads to reversion of the variegated eye colour to wil d-type. We suggest that the flanking DNA fragments from 24D1-2 are capable of inducing position-effect variegation without any association with centro meric heterochromatin. In situ hybridisation and Southern analysis demonstr ate that the 5' flanking genomic fragment contains repeated sequences which are abundantly present in heterochromatin.