UNUSUAL DROSOPHILA-VIRILIS STRESS-PUFF AT 20CD - CYTOLOGICAL LOCALIZATION OF A HEAT-SENSITIVE LOCUS AND SOME PECULIARITIES OF THE HEAT-SHOCK RESPONSE

Using a series of chromosome aberrations a heat shock (hs) inducible site of the Drosophila virilis heat shock puff 20CD has been cytologically localized in the distal 20C subdivision: the hs puff was not observable in chromosome 2 carrying the Df(2)eb(TG-52) deficiency; the bands in the proximal 20D subdivision could not autonomously be activated by hs in aberrant chromosomes that have this deficiency or the In(2)eb(PC-19) inversion. An electron microscopy (EM) analysis of the successive stages of puff development in the 20CD and 20F regions shows that the 20CD puff is initially formed from the right part of the 20C6-7 band. In the maximally developed puff, the neighboring bands of the 20C and 20D regions are only partially decondensed; they are seen as discrete compact chromatin clumps. Specific differences in the ultrastructural organization and in response to hs have been found between the D. virilis "unusual" 20CD and typical 20F hs puffs.