Repeated DNA Sequences in the Heteroch) of Adult Drosophila melanogaster (DNA-DNA hybridization/thermal dissociation of hybrid

DNA isolated from Drosophila melano- gaster adults (XX, XY, XXY, XYY) was used in DNA- DNA hybridization experiments on nitrocellulose filters. Filter-bound DNA of "high-heterochromatin" flies (those with one or more Y chromosomes) is more effective in forming hybrid duplexes than is XX DNA. The quantita- tive difference in hybridization efficiency is due primarily to molecules with a relatively high thermal stability (du- plexes that dissociate in the temperature range 80-90?). Hybridization experiments with DNA samples that have been fractionated into reiterated and unique portions show that the majority of the hybrid duplexes formed involve reiterated DNA. A small, but highly specific, inter- action of unique DNA sequences has also been detected in our experiments. These data indicate that a class of repeated DNA sequences is associated with the constitu- tive heterochromatin, specifically with the Y chromosome of D. melanogaster. Evidence is presented that this same class of molecules, or very similar ones, is toa be found in other portions of the genome, presumably in X-chromo- somal or centromeric heterochromatin.