Section 6 — Cytology 



development of gamete nuclei and plays a 

 fundamental role as a factor constituting the 

 high metabolic properties of the cell. 



vacuolae and their chromosomes do not show 

 any evidence of puff formation in the region. 



6.21. Natural and Experimental Modification of 

 Chromocenters in Interphase Nuclei of Hyme- 

 nolepis diminuta. L. T. Douglas (Baltimore, 

 U.S.A.). 



The distribution and size of DNA-positive 

 centers (i.e. chromocenters) within interphase 

 nuclei of Hymenolepis diminuta appears to be a 

 function of the tissue in which the nuclei are 

 located. Chromocenters of nuclei in early blasto- 

 meres and embryonic tissues are numerous and 

 small while those in fully differentiated tissues tend 

 to be larger and less numerous. Intranuclear 

 contraction centers accentuated in freshly iso- 

 lated nuclei by 10 -2 m arginine correspond 

 morphologically to foci which are positive for 

 DNA and basic nucleoprotein in fixed material. 



It is tempting to conjecture that variation in 

 distribution and size of intranuclear chromo- 

 centers in different tissues has physiological signi- 

 ficance; and, for example, it might be that the 

 imposition of a confining microscopic geometry 

 on specific regions of a submicroscopic com- 

 ponent (e.g. DNA double helices) would alter 

 the phenotypic expression of a genome in such a 

 way as to provide, via coding, for different cyto- 

 plasmic and nuclear enzymes in different tissues. 



6.22. Functional Changes in the Polytene Chromo- 

 somes of Drosophila melanogaster. B. M. Sli- 

 zynski (Edinburgh, Great Britain). 



A recessive autosomal mutant gene in Dro- 

 sophila melanogaster located in the second chro- 

 mosome at locus 12.0, called "fat" symbol ft, 

 produces besides its described effects (short 

 abdomen, thorax and wings with some abnor- 

 malities in the morphology of the posterior cross- 

 vein) also numerous vacuolae in the cells of 

 salivary gland. The vacuolae are formed at the 

 end of the second instar. The formation appears 

 to proceed in a wave starting from the distal end 

 of the gland. Later the vacuolae disappear and 

 still later new vacuolae are being formed. The 

 formation of vacuolae is accompanied by varying 

 degree of puff formation in the band or bands 

 corresponding to the cytological location of the 

 gene, namely in the subdivision 24D/E of the 

 cytological map of salivary gland chromosome. 

 The neck cells of the gland do not form any 



6.23. Pulse Labeling Studies of Nucleoprotein Syn- 

 thesis on Drosophila Polytene Chromosomes. 



W. Plaut (Madison, U.S.A.). 



Drosophila salivary glands excised from ad- 

 vanced larvae have been incubated in modified 

 Ringer's medium with tritiated thymidine, ly- 

 sine, uridine, and cytidine. Results to date indi- 

 cate that autoradiographically detectable chro- 

 mosomal labeling can be obtained in 2} min of 

 incubation. The labeling patterns resulting from 

 brief incubations in DNA, RNA, and protein 

 precursors are strikingly different. DNA labeling, 

 in particular, is frequently discontinuous along 

 the chromosome. The possible significance of 

 these synthesis patterns relative to chromosomal 

 structure and function will be discussed. 



6.24. Variability of RNA Synthesis in Polytene 

 Tissues. Claus Pelling (Tubingen, Germany). 



Investigations of RNA synthesis in salivary 

 gland chromosomes of Chironomus larvae have 

 demonstrated that the rate of uridine H 3 in- 

 corporation into nuclear structures is highly 

 variable. 



Larvae of the same batch, taken at approxi- 

 mately the same time under the same experi- 

 mental conditions, may sometimes differ by 

 more than 3 orders of magnitude in the amount 

 of incorporated uridine determined by grain 

 counts of a given nuclear structure (Balbiani 

 Ring II). 



Different batches, grown up in different 

 culture vessels, may also differ in the overall 

 level of uridine uptake. 



Salivary glands are not the only polytene tissue 

 showing variation in RNA synthesis. Autoradio- 

 graphy of the anterior part of the intestine, the 

 malpighian tubules, and the rectum also reveal 

 large alterations in uridine incorporation from 

 animal to animal. Activities in different tissues 

 vary independently. Striking differences have 

 also been observed between cells within the same 

 tissue. A mosaic-like pattern of nuclear uridine 

 incorporation is typical for some areas of the 

 intestine, whereas activity gradients seem to 

 occur along the malpighian tubules. 



In contrast to the polytene tissues, embryonic 

 diploid nuclei seem to incorporate at a more or 

 less constant rate. 



All observations point to the conclusion that 



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