For a comprehensive treatment (through 1998) of both structure and function of polytene chromosomes, consult Igor Zhimulev's excellent review occupying three entire volumes of Advances in Genetics (152-154). His monographs cover D. melanogaster and other Drosophila species as well as other insect and noninsect organisms. Daneholt (155) has reviewed the packaging and trafficking of a transcript between nucleus and cytoplasm of a salivary gland cell of the midge Chironomus tentans, and similar mechanisms are likely to be operating in Drosophila. Edgar and Orr-Weaver have reviewed factors controlling the transition from mitotic to endomitotic cycles as well as regulation of endoreplication itself (156).
The salivary gland polytene chromosome complement of D. melanogaster consists of five large arms of similar size and a nubbin (see Chapters 11-15). The large arms correspond to the euchromatic portions of the X chromosome and the left and right arms of chromosomes 2 and 3. The nubbin is polytene chromosome 4, and the heterochromatic Y chromosome is not evident. Both homologs of a chromosome contribute to the polytene arm; consequently, there are half as many chromosome arms in a polytene nucleus as in a 2n (mitotic) nucleus. Each arm has a characteristic and reproducible pattern of alternating dark and light regions, termed bands (or chromomeres) and interbands, respec-tively.1 The banded pattern is a fundamental morphological feature of the chromosome that can be seen in nonstained squashes by phase-contrast microscopy, and in non-fixed, DAPI-stained whole mounts by fluorescence microscopy (see refs. 158,159). The pericentric heterochromatic regions of these chromosomes and the entire Y chromosome, if present, coalesce into a "chromocenter" from which the X and autosomal arms extend. In squash preparations, the single X chromosome of the male appears somewhat thinner and stains less intensely than the two X's of the female. The chromocenter consists of compact a-heterchromatin, which appears as a single dense body in the middle of the chromocenter, surrounded
1Phase dark regions of polytene chromosomes are bands; phase light regions are interbands. This terminology differs from the cytogenetic nomenclature adopted for banded human metaphase chromosomes, where by definition there are no interbands, only dark bands and light bands depending on the staining technique (157).
by diffuse, netlike P-heterochromatin (17,18,158). However, a-heterchromatin of D. melanogaster is normally difficult to see with conventional staining (in constrast to D. virilis). This is remedied by a simple "differential" staining method developed by Belyaeva for phase-contrast microscopy (160).
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