We focus here on the intercalary heterochromatin (IH) of Drosophila melanogaster and, in particular, its molecular properties. In the polytene chromosomes of Drosophila, IH is represented by a reproducible set of dense bands scattered along the euchromatic arms. IH contains mainly unique DNA sequences, and shares certain features with other heterochromatin types such as pericentric, telomeric, and PEV‐induced heterochromatin, the inactive mammalian X‐chromosome and the heterochromatized male chromosome set in coccids. These features are transcriptional silencing, chromatin compactness, late DNA replication, (...) underrreplication or elimination in somatic cells, and formation of the heterochromatin state in early embryogenesis. Post‐translational modification of histones and the specific nonhistone protein complexes are shown to participate in the establishment and maintenance of silencing for all heterochromatin types. Many IH regions contain binding sites for HP1 and/or Pc‐G proteins and all the regions are sites of heterochromatin‐associated SuUR protein. Some IH regions are known to contain homeotic genes. Summarizing these data, we suggest that IH regions comprise stable inactivated genes, whose silencing is developmentally programmed. BioEssays 25:1040–1051, 2003. © 2003 Wiley Periodicals, Inc. (shrink)

The most enigmatic feature of polytene chromosomes is their banding pattern, the genetic organization of which has been a very attractive puzzle for many years. Recent genome‐wide protein mapping efforts have produced a wealth of data for the chromosome proteins of Drosophila cells. Based on their specific protein composition, the chromosomes comprise two types of bands, as well as interbands. These differ in terms of time of replication and specific types of proteins. The interbands are characterized by their association with (...) “active” chromatin proteins, nucleosome remodeling, and origin recognition complexes, and so they have three functions: acting as binding sites for RNA pol II, initiation of replication and nucleosome remodeling of short fragments of DNA. The borders and organization of the same band and interband regions are largely identical, irrespective of the cell type studied. This demonstrates that the banding pattern is a universal principle of the organization of interphase polytene and non‐polytene chromosomes.Editor's suggested further reading in BioEssaysCaught in the act: Rapid, symbiont‐driven evolution AbstractFunction and evolution of sex determination mechanisms, genes and pathways in insects Abstract. (shrink)