248 The Nature of Biological Diversity 



process which, more rarely, may proceed to the formation of enor- 

 mous rosette-like projections called "Balbiani rings" (Beerman, 1952; 

 Mechelke, 1953; Beerman and Bahr, 1954) . 



Cytochemical studies suggest that the projections from the chromo- 

 somes consist of unraveled DNA (Beerman, 1952, 1959; Gall, 1958). 

 Moreover, the individual "puffs" prove to be the sites of intensive 

 synthetic activity, especially of RNA (Beerman, 1952; Pavan and 

 Breuer, 1952; Ficq and Pavan, 1957; Gall, 1958). It is difficult to 

 avoid the conclusion that the puffing phenomenon is, in fact, a carica- 

 ture of the de-repression of specific gene loci and the coding of 

 messenger RNA. 



In general, one can say that the pattern of puffing is uniform 

 throughout a single tissue, but it differs from that in other tissues of 

 the same individual (Beerman, 1952, 1959; Mechelke, 1953; Breuer 

 and Pavan, 1955 ) . Each tissue is, of course, engaged in synthetic 

 operations peculiar to itself — a fact which is apparently reflected in 

 its pattern of puffing. 



If attention is centered on a single tissue, say, the salivary glands, 

 it is worth inquiring as to whether the chromosomal puffs undergo 

 any systematic change during larval life. Because of the precise syn- 

 chronization within each brood of larvae, Rhynchosciara is obviously 

 the perfect animal for this type of study (Breuer and Pavan^ 1955). 

 However, similar investigations have been carried out with consider- 

 able success on the salivary glands of the Chironomidae (Beerman, 

 1952, 1959; Mechelke, 1953). 



Just prior to pupation, the salivary glands show a pronounced 

 change in their synthetic activity, in that the clear secretion which 

 they previously produced is replaced by a brownish fluid which is now 

 synthesized for the first time. In cytological preparations of salivary 

 glands, the new synthetic acts are signaled by striking changes in the 

 pattern of puffing in the giant chromosomes. Most of the puffs that 

 had been prominent during larval life now collapse, the DNA strands 

 apparently being folded back into the chromosome to re-form com- 

 pact bands (Breuer and Pavan, 1955). Meanwhile, new puffs are 

 formed at a series of other specific loci. 



There is a comprehensive body of evidence to show that the change 

 in the secretory activity of the salivary glands is promoted by the same 

 endocrine agents that control the metamorphosis of the animal as a 

 whole. As we shall later consider, pupation is the response to an 

 increasing titer of "ecdyson" and a decreasing titer of "juvenile hor- 

 mone." Therefore, by injecting ecdyson into immature larvae, one can 



