Balbiani ring activities in acricotopus lucidus

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.

Patterns of puffing activity and chromosomal polymorphism in Drosophila subobscura. IV. Position effect at the boundaries of the E12 inversion

The puffing patterns in polytene E chromosomes of Drosophila subobscura were followed in third-instar larvae and throughout the prepupa period. Two gene arrangements, Est and E1+2+9+12 were studied. A majority of puffs exhibit a similar pattern, but the puffs 61AC and 67AB behave differently in the two chromosomal arrangements, both in homozygotes and in heterozygotes. These two puffs are located at the end of the E12 inversion. This position effect is an interesting phenomenon that probably is not due to a heterochromatinization effect.

Photoinduced bonding of endogenous ecdysterone to salivary gland chromosomes of Chironomus tentans

Endogenous ecdysterone has been bonded to chromosomal loci by irradiation of Ch. tentans salivary glands. The hormone has been localized on the polytene chromosomes by indirect immunofluorescence microscopy. Hormone binding to chromosomes is stage-specific. Seven chromosomal loci could be identified which specifically bound hormone in larval salivary glands, and 21 chromosomal loci which specifically bound hormone in prepupal salivary glands. All puffs that have been described by Clever (1961) as being inducible by ecdysterone have been found to contain irreversibly bound ecdysterone in prepupal salivary gland chromosomes. A small number of puff sites in larval salivary gland chromosomes exhibited varying amounts of bound ecdysterone, (as judged by fluorescence intensity) most notably 117B and Balbiani rings 1 and 3 on chromosome IV. In addition to stage specific binding sites, there were many others showing equal binding of the hormone in both, larval and prepupal, stages of development.--Fluorescence intensities (reflecting the amount of bonded hormone) at puff sites along the tip section of the prepupal salivary gland chromosome arm IR have been computed indicating that differences between fluorescence intensities of different puffs can be expressed as multiples of a basic fluorescence intensity. Thus, the amount of fluorescence intensity (bonded hormone) in the various puffs may be quantized.--The data indicate that in Ch. tentans salivary glands ecdysterone acts, at the chromosomal level. The development of larvae into prepupae generates more puff sites and more hormone binding. This is discussed in the light of current models of hormone-receptor function.

Two transcripts of an individual Balbiani ring from the salivary gland cells of Acricotopus lucidus (Diptera, Chironomidae)

The polytene chromosomes in main lobe cells of the Acricotopus salivary gland carry two large Balbiani rings (BR1, BR2), which BR1 exhibits the higher incorporation rate of tritiated nucleosides. The size of BR1 varies in conjunction with a subterminal inversion which includes the BR1 site. BR1 in both the homozygous and heterozygous inverted arrangement (BR1A and BR1B, respectively) is consistently smaller than in the homozygous standard arrangement (BR1C). The RNA content of BR1A and BR1C (3.1 and 6.7 pg, respectively) corresponds well with their relative size.--Using the method of micromanipulation of squashed salivary gland chromosomes the RNA transcripts of BR1A and BR1C have been extracted from microdissected Balbiani rings and fractionated by electrophoresis on 1% agarose gels. In BR1A one RNA fraction with a molecular weight of about 40 x 10(6) D is transcribed. The gel electrophoretic pattern of BR1C is characterized by an additional RNA fraction of 5 x 10(6) D. The main part of BR1 RNA is bound to oligo(dT)-cellulose suggesting the adenylation at the chromosome of Balbiani ring RNA.--In the profile of poly(A)-containing cytoplasmic RNA obtained after in vivo labelling two peaks can be detected which correspond in size to the BR1 RNA. These putative BR1 transcripts accumulate to a high concentration in the cytoplasm and show a relatively low stability during in vivo chase experiments.

Puffing patterns during the fourth larval instar in Chironomus pallidivittatus salivary glands

The puffing pattern of polytene chromosomes in salivary glands from Chironomus pallidivittatus larvae and prepupae has been studied in glutaraldehyde-acetic acid fixed, lactic acid flattened preparations. Some observations were also made on F1 hybrid species C. pallidivittatus X C. tentans. Concerning the situation of puffing in Balbiani rings (BR), 2.783 chromosomes IV from 188 animals were scored. In standard 4th instar larvae, BR2 appears expanded, BR3 smaller but not collapsed and BRI either reduced of collapsed. During the first days following the red-head stage, which signals the beginning of the 4th instar, larvae show a large BR1; later it reduces and tends to collapse. At the end of the 4th instar, prepupae again may present an expanded BR1. On the contrary, the size of BR2 and BR3 remains unchanged from the red-head stage to the prepupa. A variable accumulation of droplets has been observed to occur in BR2 and BR1 from dated larvae and prepupae.--A characteristic pattern of puffing was found in prepupae, which consisted in the appearance of conspicuous puffs at regions I-6B, I-7B, I-7B, I-18C, III-9B and IV-4B. Puffs at I-2B, I-3B, I-9A,I-11C,II-4A, and IV-4B were observed during most of the 4th larval instar, as well as in late larvae and prepupae. Among all these puffs, those at I-7B, I-9A, I-17B, and IV-4B frequently showed variable amounts of droplets.