Evidence for heterogeneity in heterochromatin of Drosophila melanogaster

Abstract

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 3^1–4. It appears throughout the cell cycle as darkly staining and highly condensed chromatin^2,3 and replicates later than euchromatin during the synthetic period⁵. 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 effect¹. It has recently been found that the heterochromatin of Drosophila corresponds to the C bands⁶ and contains highly repetitive DNA^7–12. None of these characteristics has, however, so far been of any use in solving the problem of the functional role of the heterochromatin. On the other hand, Drosophila offers unique possibilities for solving this problem, since in this organism it is possible to construct chromosomes containing different quantities of centromeric heterochromatin^4,13 and thus study the functions of this material. Obviously this type of approach requires a preliminary study on the possible heterogeneity of the heterochromatin. An initial result in this type of study has been obtained by showing that the heterochromatin regions of D. melanogaster fluoresce differently after staining both with quinacrine^14–16 and with the compound 33258 Hoechst¹⁷. Here we describe experiments in which the heterochromatin of Drosophila was differentiated by means of treatment of the living gangliar cells with 33258 Hoechst, which is known to decondense the centric heterochromatin of the mouse¹⁸.