COMPARATIVE-ANALYSIS OF POSITION-EFFECT VARIEGATION MUTATIONS IN DROSOPHILA-MELANOGASTER DELINEATES THE TARGETS OF MODIFIERS

In Drosophila melanogaster heterochromatin-induced silencing or positi on-effect variegation (PEV) of a reporter gene has provided insights i nto the properties of hetorochromatin. Class I modifiers suppress PEV and class II modifiers enhance PEV when the modifier gene is present i n fewer than two doses. We have examined the effects of both class I a nd class II modifiers on four PEV mutations. These mutations include t he inversions In(I)w(m4) and In(2R)bw(VDe2), which are classical chrom osomal rearrangements that typify PEV mutations. The other mutations a re a derivative of brown(Dominant), in which brown(+) reporters are in activated by a large block of heterochromatin, and a P[white(+)] trans poson insertion associated with second chromosome heterochromatin. In general, we find that class I modifiers affect both classical and nonc lassical PEV mutations, whereas class II modifiers affect only classic al PEV mutations. We suggest that class II modifiers affect chromatin architecture in the vicinity of reporter genes, and only class I modif iers identify proteins that are potentially involved in heterochromati n formation or maintenance. In addition, our observations support a mo del in which there are different constraints on the process of heteroc hromatin-induced silencing in classical m. nonclassical PEV mutations.