THE DROSOPHILA SALIVARY-GLAND CHROMOCENTER CONTAINS HIGHLY POLYTENIZED SUBDOMAINS OF MITOTIC HETEROCHROMATIN

Peri-centromeric regions of Drosophila melanogaster chromosomes appear heterochromatic in mitotic cells and become greatly underrepresented in giant polytene chromosomes, where they aggregate into a central mas s called the chromocenter. We used P elements inserted at sites disper sed throughout much of the mitotic heterochromatin to analyze the fate of 31 individual sites during polytenization. Analysis of DNA sequenc es flanking many of these elements revealed that middle repetitive or unique sequence DNAs frequently are interspersed with satellite DNAs i n mitotic heterochromatin. All nine Y chromosome sites tested were und errepresented > 20-fold on Southern blots of polytene DNA and were rar ely or never detected by in situ hybridization to salivary gland chrom osomes. In contrast, nine tested insertions in autosomal centromeric h eterochromatin were represented fully in salivary gland DNA, despite t he fact that at least six were located proximal to known blocks of sat ellite DNA. The inserted sequences formed diverse, site-specific morph ologies in the chromocenter of salivary gland chromosomes, suggesting that domains dispersed at multiple sites in the centromeric heterochro matin of mitotic chromosomes contribute to polytene beta-heterochromat in. We suggest that regions containing heterochromatic genes are organ ized into dispersed chromatin configurations that are important for th eir function in vivo.