Abstract
Microdissection and microcloning of the euchromatin-heterochromatin transition region of the Drosophila melanogaster polytene X chromosome and part of the euchromatin of chromosome 4 reveals that they share certain features characteristic of beta-heterochromatin, which is morphologically defined as the loosely textured material at the bases of some polytene chromosome arms. Both are mosaics of many different middle-repetitive DNA sequences interspersed with single-copy DNA sequences. Sixty percent of cloned inserts derived from division 20 and about 40 percent from subdivisions 19EF of the X chromosome harbor at least one repetitive DNA sequence in an average insert of 4.5 kilobases. No repeats have significant cross-hybridization to any of the eleven satellite DNAs, or to the clustered-scrambled sequences present in pDm1. The repetitive elements are, in general, confined to the beta-heterochromatic regions of polytene chromosomes, but some are adjacent to nomadic elements. Chromosome 4, however, has some repeats spread throughout its entire euchromatin. These data have implications for the structure of transition zones between euchromatin and heterochromatin of mitotic chromosomes and also provide a molecular basis for reexamining some of the unusual classical properties of chromosome 4.
