Larva Chromosome Polytene

A large number of different food media have been used for the culture of D. melanogaster. Several are described by Ashburner and Thompson (1978), Ashburner (1989b), Roberts and Standen (1998), and on the Bloomington Stock Center Web Site http //fly.bio.indi-ana.edu/media.recipes.htm. Only two are described below a general-purpose medium, and one suitable for growing larvae for polytene chromosome preparations. Table 35.2 lists the sources of ingredients used in the preparation of these media. In addition, instant media are available commercially (see Table 35.2). 

Figure 36-4. Illustration of the tight correlation between the presence of RNA polymerase II and RNA synthesis. A number of genes are activated when Chirono-mus tentans larvae are subjected to heat shock (39 °C for 30 minutes). A Distribution of RNA polymerase II (also called type B) in isolated chromosome IV from the salivary gland (at arrows). The enzyme was detected by immunofluorescence using an antibody directed against the polymerase. The 5C and BR3 are specific bands of chromosome IV, and the arrows indicate puffs. B Autoradiogram of a chromosome IV that was incubated in H-uridine to label the RNA. Note the correspondence of the immunofluorescence and presence of the radioactive RNA (black dots). Bar = 7 pm. (Reproduced, with permission, from Sass H RNA polymerase B in polytene chromosomes. Cell 1982 28 274. Copyright 1982 by the Massachusetts Institute of Technology.)...

Ecdysone stimulates the synthesis of RNA in tissues. Visual demonstration of the effect is provided by its action on polytene chromosomes of fly larvae (Fig. 26-14).361 Fifteen minutes after the application of ecdysone, a puff is induced on one band of the chromosome a second puff forms at a later time while a preexisting puff diminishes. Thus, like steroid hormones in mammals, ecdysone appears to have a direct controlling effect on transcription. The cuticle-shedding process (ecdysis) is initiated by the brain peptide eclosian. However, the brain may be responding to the ecdysis-triggeiing hormone, a peptide that is secreted by a series of epitracheal glands located in various segments of the body.362... 

In the same way that amphibian oocytes are an accessible scource of giant lampbrush chromosomes (as well as nuclear envelopes), the salivary glands of dipteran larvae are a source of giant polytene chromosomes, which are formed by repeated endomitotic reduplication of the DNA, while retaining lateral association of chromatids. In Chironomus tentans, polytene chromosomes in salivary... 

The puff activation of polytene chromosomes in the salivary glands of Diptera larvas occurs under the control of the steroid hormone ecdysone. The genes which control the formation of steroid metabolic enzymes are supposedly active in the gland cells where this hormone is synthesized. We can consider the regulation of salivary gland functions as a result of the indirect interactions... 

These workers incubated the salivary gland of larvae of various species of Drosophila in a solution containing -thymidine for a period of about 10-15 min, which is between one-fortieth and one-fiftieth of the total DNA replication time in polytene chromosome (10-12 h). In this case DNA synthesis was not observed... 

Investigations to study the effect of somatic radiation-induced mutations in the salivary gland cells of Drosophila after irradiation of embryos and larvae in the early stages of development (before polytenization of the chromosomes) have now been commenced in our laboratory (Lychev and Medvedev, 1967 Lychev, 1967). Recently several new species of insects have begun to be used as objects for study of the dynamics of puffs in connection with morphogenesis and with particular (especially secretory) functions (Whitten, 1964), and it must be anticipated that this extension of the research must soon lead to the discovery of new underlying principles. 

Note Female larvae have better polytene chromosomes than males and also have twice the amount of X chromosome DNA. 

---