MODIFICATION OF THE DROSOPHILA HETEROCHROMATIC MUTATION BROWN(DOMINANT) BY LINKAGE ALTERATIONS

The variegating mutation brown(Dominant) (bw(D)) of Drosophila melanog aster is associated with an insertion of heterochromatin into chromoso me arm 2R at 59E, the site of the bw gene. Mutagenesis produced 150 do minant suppressors of bw(D) variegation. These fall into two classes: unlinked suppressors, which also suppress other variegating mutations; and linked chromosome rearrangements, which suppress only bw(D). Some rearrangements are broken at 59E, and so might directly interfere wit h variegation caused by the heterochromatic insertion at that site. Ho wever, most rearrangements are translocations broken proximal to bw wi thin the 52D-57D region of 2R. Translocation breakpoints on the X chro mosome are scattered throughout the X euchromatin, while those on chro mosome 3 are confined to the tips. This suggests that a special proper ty of the X chromosome suppresses bw(D) variegation, as does a distal autosomal location. Conversely, two enhancers of bw(D) are caused by t ranslocations from the same part of 2R to proximal heterochromatin, br inging the bw(D) heterochromatic insertion close to the chromocenter w ith which it strongly associates. These results support the notion tha t heterochromatin formation at a genetic locus depends on its location within the nucleus.