Relation of puffing to bristle and footpad differentiation in Calliphora and Sarcophaga

Polytene Chromosomes: 70 Years of Genetic Research

Polytene chromosomes were described in 1881 and since 1934 they have served as an outstanding model for a variety of genetic experiments. Using the polytene chromosomes, numerous biological phenomena were discovered. First the polytene chromosomes served as a model of the interphase chromosomes in general. In polytene chromosomes, condensed (bands), decondensed (interbands), genetically active (puffs), and silent (pericentric and intercalary heterochromatin as well as regions subject to position effect variegation) regions were found and their features were described in detail. Analysis of the general organization of replication and transcription at the cytological level has become possible using polytene chromosomes. In studies of sequential puff formation it was found for the first time that the steroid hormone (ecdysone) exerts its action through gene activation, and that the process of gene activation upon ecdysone proceeds as a cascade. Namely on the polytene chromosomes a new phenomenon of cellular stress response (heat shock) was discovered. Subsequently chromatin boundaries (insulators) were discovered to flank the heat shock puffs. Major progress in solving the problems of dosage compensation and position effect variegation phenomena was mainly related to studies on polytene chromosomes. This review summarizes the current status of studies of polytene chromosomes and of various phenomena described using this successful model.

Heat shock responses in polytene foot pad cells of Sarcophaga bullata

Heat shock induces a single large puff (hs puff) near the tip of chromosome arm EL in polytene foot pad cells of fly pupae (Sarcophaga bullata). The inducible hs locus is constitutively active, invariably forming a small puff, which can be maximally activated in cells of the dorsal epidermis or in trichogen cells at any time during the lifetime of mature polytene chromosomes. Both in vivo and in cultured food pads, maximal puff induction occurs at 37 degrees C. At the same temperature, normal development of puffing patterns continues undisrupted for several days. A few specific hs proteins are vigorously induced at 37 degrees C, also without disrupting patterns of normal protein synthesis. Rates of normal protein synthesis in cultured food pads and rates of pupal development are enhanced up to about 39 degrees C. During heat shock at 41 degrees-44 degrees C protein synthesis becomes completely dominated by the production of hs proteins. The severe or complete suppression of most of the proteins normally made is followed by developmental arrest. There is also a decline of transcription (chromosomal uridine incorporation) between 37 degrees and 44 degrees C, which appears to affect all chromosomal loci proportionally, including the hs locus. The hs puff is no longer maximally induced at 41 degrees-44 degrees C, but the expanded puff now persists indefinitely, whereas below 39 degrees C, initial puff expansion is always followed by at least partial puff regression. The control of the duration of the puffing response appears to be entirely independent of protein synthesis, e.g., complete inhibition of protein synthesis by cycloheximide fails to prolong transient puffing responses. Canavanine also has no effect on puff regression. Heat shock above 45 degrees C arrests all RNA and protein synthesis within 30 min. RNA synthesis is resumed immediately after shift-down to 25 degrees C, not only at the hs locus, but at most or all previously active loci. Protein synthesis is also resumed immediately, but it is almost completely restricted to the production of the major hs protein (hsp-65, equivalent to hsp-70 of Drosophila melanogaster). Extreme heat shock also triggers maximal puffing responses at the hs locus, but actual puff expansion is delayed and only occurs hours after shift-down in the wake of a surge of hsp-65 synthesis. Following these delayed hs responses pupal thermotolerance starts increasing and protein synthesis returns to normal.

Induction of a heat shock puff by hypoxia in polytene foot pad chromosomes of Sarcophaga bullata

The single large heat-responsive puff (hs puff) in polytene foot pad cells of fly pupae (Sarcophaga bullata) is shown to be inducible by oxygen deprivation but not, as in other systems, by reoxygenation following an hypoxic treatment. The ambient oxygen concentration must drop below 2% for the hs puff to be maximally induced but the puff is fully inducible and transcriptionally active even in the complete absence of oxygen. Lack of oxygen is also compatible with continued transport of puff materials (formation and dissipation of puff droplets at the hs locus). Hypoxia-induced hs puffs persist indefinitely (greater than 2 days) at maximal or intermediate size and only regress completely after oxygen is resupplied. The induction of the hs puff during hypoxia is highly specific and does not seem to involve activation of any other chromosomal loci, yet the reaction is not confined to the giant foot pad cells or to specific developmental stages. Azide poisoning of cultured foot pads simulates the in vivo effects of hypoxia. The induction of the hs puff by azide, heat, or other means is inhibited by sulfhydryl reagents (iodoacetamide, arsenite) and fluoride, but not by an inhibitor of substrate-linked phosphorylation (arsenate). Instead, arsenate, like other uncouplers (2,4-dinitrophenol) is an inducer of the hs locus. The hs puff can be fully induced by hypoxia at any temperature between 2 degrees and 45 degrees C. The rate of puff expansion is strictly temperature dependent and the temperature characteristics of this process are remarkably similar to those of a promoter RNA polymerase association.(ABSTRACT TRUNCATED AT 250 WORDS)

Polytene chromosomes in pupal and adult blackflies (Diptera: Simuliidae)

A number of pupal and adult tissues of eight Australian blackfly species representing three genera, Austrosimulium, Cnephia and Simulium, were examined for the presence of polytene chromosomes. Banded polytene chromosomes were found in malpighian tubules, hind gut, fat body, and ovary, but only those from the malpighian tubules of female adults and pupae were of good quality. A detailed comparison of polytene chromosomes from larval salivary glands and adult malpighian tubules was made in S. ornatipes and, to a limited extent, in S. melatum. The banding patterns of chromosomes from both tissues were found to be identical with minor differences in puffing patterns in S. ornatipes and chromocenter characteristics in S. melatum. A survey of the remaining six species shows five of them to have malpighian chromosomes suitable for detailed cytological analysis. Simultaneous studies of larval, pupal and adult polytene chromosome systems offer a novel approach to the analysis of population problems in blackflies. The ability to recognise sibling species in adults also has potential practical significance in efforts to control vectors of onchocerciasis.

Proposals to the problem of structural and functional organization of polytene chromosomes

The concept of the structural and functional organization of polytene chromosomes is formulated. It is based on the following:Interbands are transcriptional active chromosome regions; they seem to be responsible for the base metabolism of the cell. The stability of these metabolic processes in the cells of different organs results in constancy of chromosome banding pattern. Thus, each functional state of the cell possesses its own banding pattern which may vary depending on activity level and degree of condensation of interbands.The band is considered not as a structural or functional unit, but as a region of the chromosome not involved in transcription at a certain point of time; a band may contain one or several genes.