Heterochromatin markers: a search for heterochromatin specific middle repetitive sequences in Drosophila.

We sought for cloned sequences of middle repetitive (MR) complexity that mark obligatory heterochromatic regions. Total genome probes were employed in a differential screening procedure to recover X-specific, Y-specific and autosomal specific heterochromatic sequences. X-and Y-linked sequences were recovered in the same experiment. (Y-linked clones will be described elsewhere). All nine independent, non-identical X-specific clones were found to be partially homologous to one another and to type I rDNA insertion. No other X-specific Bam HI or HindIII clones were found. In situ hybridization to normal and inverted chromosomes revealed extensive homology in the heterochromatin spanning the nucleolus organizer (NOR) and the eu-heterochromatin junction. Eleven clones which are underrepresented in polytene chromosomes were selected in another differential screening. None was autosome-specific. Five were of nucleolar origin. Among them a presumptive type II 28SrDNA insertion sequence was clearly localized within the X-chromosome proximal heterochromatin in addition to the known localization of the X and Y nucleolar organizers. We mapped three clones to major sites on the Y chromosome and to secondary autosomal sites. The results are discussed with regard to the complexity of heterochromatin organization.