Evidence for heterogeneity in heterochromatin of Drosophila melanogaster

In Drosophila melanogaster the heterochromatin comprises the whole of the Y chromosome, about the proximal third of the X chromosome and the centromeric areas of chromosomes 2 and 3. It appears throughout the cell cycle as darkly staining and highly condensed chromatin and replicates later than euchromatin during the synthetic period. The heterochromatin of Drosophila is considered genetically inert because it contains very few mappable genes, although it has marked genetic effects in determining the well known position effect. It has recently been found that the heterochromatin of Drosophila corresponds to the C bands and contains highly repetitive DNA. None of these characteristics has, however, so far been of any use in solving the problem of the functional role of the heterochromatin. The heterochromatin regions of D. melanogaster fluoresce differently after staining both with quinacrine and with the compound 33258 Hoechst. Here are described experiments in which the heterochromatin of Drosophila was differentiated by means of treatment of the living ganglia cells with 33258 Hoechst, which is known to decondense the centric heterochromatin of the mouse.