Native salivary chromosomes of Drosophila melanogaster: retrospect and prospect.

A method has been developed which, for the first time, allows the preparation of mappable cytological spreads of salivary chromosomes from D. melanogaster without exposure to acid fixatives. These isolated native chromosomes show the best preservation of ultrastructure observed to date--ribonucleoprotein particles may be seen to be organized in linear arrays in transcriptionally active puffs and the repeating nucleosome module is present. Native salivary chromosomes are proving useful for the localization of nuclear proteins both at the light microscope and ultrastructural levels. They display only background-level binding of antibodies specific for the Z-DNA conformation. However, Z-DNA immunoreactivity is activated by exposure to acid fixative, first in interbands and then in bands. The Z-conformation in the chromosomes is held in place by elastic torsional strain which appears in the DNA following acid fixation. Native D. melanogaster salivary chromosomes offer promise for enabling the probing of the chromatin of known genetic loci for properties dependent on the preservation of macromolecular integrity.