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The so-called "DNA-puffs" are a characteristic and exclusive feature of sciarid polytene chromosomes. A number of descriptive and comparative studies on these structures have been carried out over the last two decades both in Rhynchosciara angelae and in Sciara coprophila. They have revealed the fact that concurrent with puffing at these sites an excessive and disproportionate synthesis of DNA occurs.1 2 It has also been suggested that these events are somehow related to development,3 a conclusion which has been recently contested.4 Much of the confusion that exists in the literature with respect to the significance and nature of the DNA-puffs is simply due to a lack of precise data at the morphological and biological level. Recent studies in our laboratory demonstrate that, contrary to a widely held conviction, the DNA-puffs are not just illdefined, chance manifestations of some kind of cellular "senility" preceding histolysis. To the contrary, a number of facts attest to the general significance of the DNA-puffs: (1) They always arise from specific points in the salivary gland chromosome, usually a pair of light-staining bands or the space between the two. (2) Unlike the situation found in RNA-puffs, the DNA unfolds into a characteristic network shaped like a flat saucer. (3) Disproportionate DNA synthesis involves a number of additional rounds of replication. (4) The excessive DNA is not shed from the chromosome but seems to remain integrated. (5) During the process of puffing, DNA synthesis is almost constantly observed in other regions along the chromosome but at a much lower level of magnitude. (6) There are clear-cut and constant differences between the anterior and posterior portions of the salivary gland with respect to the development of specific DNA puffs; and the intermediary portion of the gland does not form any puffs at all. (7) The sequence of DNA-puff formation in metamorphosis is typical and constant. (8) DNA-puffing as a process is intimately linked to a change in salivary gland metabolism which takes place during metamorphosis (pupal molt). The present paper deals with the relationship of DNA-puffing to metamorphosis. By injecting ecdysone into early fourth instar larvae, we were able to demonstrate that DNA-puff formation is under the control of this hormone: within a period of 24 hours the characteristic and complete complement of DNApuffs had been induced in the salivary gland chromosomes. For the youngest larvae injected, this represented an induction that was six to seven days ahead of schedule. A remarkable finding regarding the DNA-puffs of Sciara coprophila was recently published in these PROCEEDINGS,4 namely that metamorphosis without DNA-puff formation could be obtained by cortisone feeding. Furthermore, the authors challenged the phenomenon of chromosome puffing in general as a signifi-

The role of ecdysone in DNA-puff formation and DNA synthesis in the polytene chromosomes of Sciara coprophila.