CYTOGENETIC AND MOLECULAR ASPECTS OF POSITION-EFFECT VARIEGATION IN DROSOPHILA-MELANOGASTER

Position-effect variegation in Drosophila melanogaster is accompanied by compaction of the corresponding chromosomal regions. The compaction can be continuous, so that bands and interbands located distal to the eu-heterochromatic junction fuse into one dense block, or discontinuo us, when two or more zones of compaction are separated by morphologica lly and functionally normal regions. In this work it was found that in both continuous and discontinuous compaction the blocks of dense mate rial contain the immunochemically detectable protein HP1, which has pr eviously been characterized as specific for heterochromatin. The regio ns undergoing compaction do not contain HP1 when they have a normal ba nding pattern. Thus, it may be proposed that HP1 is one of the factors involved in compaction. If two different or two identical rearrangeme nts are combined in the same nucleus, they variegate independently. Th e frequency of compaction of the two rearrangements in the same nucleu s corresponds to the product of the frequencies of the compact state o f the individual elements. The extent of compaction (i.e. the number o f bands involved in heterochromatization) of each rearrangement does n ot depend on the compaction pattern of the other rearranged element.