[DNA replication patterns in salivary gland chromosomes of Chironomidae]

Chromosomal organization of the ribosomal RNA genes in the genus Chironomus (Diptera, Chironomidae)

Chromosomal localization of ribosomal RNA coding genes has been studied by using FISH (fluorescence in situ hybridization) in 21 species from the genus Chironomus Meigen, 1803. Analysis of the data has shown intra- and interspecific variation in number and location of 5.8S rDNA hybridization sites in 17 species from the subgenus Chironomus and 4 species from the subgenus Camptochironomus Kieffer, 1914. In the majority of studied species the location of rDNA sites coincided with the sites where active NORs (nucleolus organizer regions) were found. The number of hybridization sites in karyotypes of studied chironomids varied from 1 to 6. More than half of the species possessed only one NOR (12 out of 21). Two rDNA hybridization sites were found in karyotypes of five species, three - in two species, and five and six sites - in one species each. NORs were found in all chromosomal arms of species from the subgenus Chironomus with one of them always located on arm G. On the other hand, no hybridization sites were found on arm G in four studied species from the subgenus Camptochironomus. Two species from the subgenus Chironomus - Chironomusbalatonicus Devai, Wuelker & Scholl, 1983 and Chironomus "annularius" sensu Strenzke, 1959 - showed intraspecific variability in the number of hybridization signals. Possible mechanisms of origin of variability in number and location of rRNA genes in the karyotypes of species from the genus Chironomus are discussed.

Cytogenetic index and functional genome alterations in Chironomus piger Strenzke (Diptera, Chironomidae) in the assessment of sediment pollution: a case study of Bulgarian and UK rivers

The genotoxicity of trace metals in the sediments from a number of polluted sites on UK and Bulgarian rivers to Chironomus piger was assessed by an examination of genome instability as demonstrated by structural and functional changes to the salivary glands chromosomes. Based on the metal assays, the sediments were characterized to range from 'extremely' to 'strongly contaminated'. The cytogenetic index calculated on the basis of somatic structural chromosome alterations in the polytene chromosomes indicates a high level of pollution (0.07-0.06 in Bulgarian and 0.10-0.13 in UK stations). Exposure of C. piger to contaminated sediments resulted in a high level of chromosome damage as indicated by a somatic index of between 1.96 and 4.0. The transcription mechanism of the Balbiani rings and nucleolar organizer was damaged as their activity was either partially or completely suppressed. We have demonstrated that the C. piger genome is a sensitive sublethal indicator of sediment contamination, and is a highly suitable candidate for ecotoxicological monitoring of running waters.

Ribosomal genes as early targets of cadmium-induced toxicity in Chironomus riparius larvae

Cadmium is a widespread environmental pollutant that causes severe impacts in organisms. Although the effects of cadmium on aquatic insects have been studied in terms of their toxicity and changes in developmental parameters, little is known about its molecular and genetic effects. We have investigated the alterations in the pattern of gene expression provoked by acute exposure to cadmium in Chironomus riparius Mg. (Diptera, Chironomidae), a sentinel organism widely used in aquatic toxicity testing. The early cytotoxic effects were evaluated using immunocytochemistry and specific fluorescent probes in fourth instar larvae after 12 h of 10 mM cadmium treatments; under these conditions no significant effect on larvae mortality was detected until after 36 h of exposure. The changes in the pattern of gene expression were analysed by means of DNA/RNA hybrid antibodies in the polytene chromosomes from salivary gland cells. A decrease in the activity of the nucleolus is especially remarkable, accompanied by a significant reduction in size and the modification in nucleolar architecture, as shown by FISH. The inhibition of rDNA transcription was further evaluated by Northern blot analysis, which showed a marked decrease in the level of preribosomal rRNA (54% 45S 12 h). However, the BR genes, whose products are the giant polypeptides that constitute the silk-like secretion for constructing housing tubes, remain active. Simultaneously, decondensation and activation take place at some chromosomal regions, especially at the centromeres. The changes observed in the pattern of gene expression do not resemble those found after heat shock or other cell stressors. These data provide the first evidence that cadmium interacts with ribosomal genes and results in a drastic impairment of the functional activity of the nucleolus, an essential organelle for cellular survival. Therefore, the depletion of ribosomes would be a long-term effect of Cd-induced cellular damage. These findings may have important implications for understanding the adverse biological effects of cadmium and its toxic mechanism, as yet not clearly defined, and provide a sensitive biomarker of cadmium exposure.

Inherited and somatic cytogenetic variability in Palearctic populations of Chironomus riparius Meigen 1804 (Diptera, Chironomidae)

Inter- and intracytogenetic variability was analyzed in 13 natural Palearctic populations of Chironomus riparius Meigen 1804 (syn. Chironomus thummi) by examining hereditary and somatic aberrations (mainly inversions) of the salivary gland polytene chromosomes. In total, 77 different types of inherited inversion sequences and 184 different types of somatic inversions were found. The median percent frequency of inherited inversions was 1.4% and karyotypic divergence between populations was very low. Most hereditary inversions were endemic and always in a heterozygous state. Only six inversion sequences, each of them shared by two very distant populations, may be considered a relic of very ancient ancestral inversions. Unlike inherited inversions, occurrence of somatic aberrations seems to increase with the overall rise in the level of heavy metal pollution of the sediments from which larvae were sampled. In contrast with what occurs in populations of other chironomid species, populations of C. riparius do not seem to undergo a process of cytogenetic differentiation.

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.

Bromodeoxyuridine: a diagnostic tool in biology and medicine, Part III. Proliferation in normal, injured and diseased tissue, growth factors, differentiation, DNA replication sites and in situ hybridization

This paper is a continuation of parts I (history, methods and cell kinetics) and II (clinical applications and carcinogenesis) published previously (Dolbeare, 1995 Histochem. J. 27, 339, 923). Incorporation of bromodeoxyuridine (BrdUrd) into DNA is used to measure proliferation in normal, diseased and injured tissue and to follow the effect of growth factors. Immunochemical detection of BrdUrd can be used to determine proliferative characteristics of differentiating tissues and to obtain birth dates for actual differentiation events. Studies are also described in which BrdUrd is used to follow the order of DNA replication in specific chromosomes, DNA replication sites in the nucleus and to monitor DNA repair. BrdUrd incorporation has been used as a tool for in situ hybridization experiments.

The organization, localization and nucleotide sequence of the histone genes of the midge Chironomus thummi

Several histone gene repeating units containing the genes for histones H1, H2A, H2B, H3 and H4 were isolated by screening a genomic DNA library from the midge Chironomus thummi ssp. thummi. The nucleotide sequence of one complete histone gene repeating unit was determined. This repeating unit contains one copy of each of the five histone genes in the order and orientation mean value of H3 H4 mean value of H2A H2B H1 mean value of. The overall length is 6262 bp. The orientation, nucleotide sequence and inferred amino acid sequence as well as the chromosomal arrangement and localization are different from those reported for Drosophila melanogaster. The codon usage also shows marked differences between Chironomus and Drosophila. Thus the histone gene structure reported for Drosophila is not typical of all insects.

MEC: a transposable element from Chironomus thummi (diptera)

Two genomic clones, pC1.2 and p20D (containing inserts of 2.0 and 1.6 kb, respectively) were isolated from the A2b region to polytene chromosome IV of Chironomus thummi thummi salivary gland cells. Upon in situ hybridization to polytene chromosomes of C. thummi thummi and C. thummi piger, p20D DNA hybridized mainly over the A2b region of chromosome IV, whereas pC1.2 DNA hybridized to at least 90 sites distributed over all the chromosomes. A partial nucleotide sequence analysis showed that these clones were very similar and allowed the detection of a 596 bp insert in the pC1.2 clone. This insert possesses all of the essential features of a Class II transposable element and was called MEC. It carries a nearly perfect 107 bp terminal inverted repeat containing one mismatch and is flanked by a 5 bp direct repeat. The 372 bp central region contains a short open reading frame with a coding capacity of 58 amino acids.

A tissue-specific puff (Balbiani ring a) in Chironomus thummi may contain a gene encoding a 67-kDa protein which exhibits non-tissue-specific expression

A 2.3-kb genomic clone has been isolated from the region where the tissue-specific puff, Balbiani ring a (BRa), is found on chromosome IV of the special lobe of Chironomus thummi salivary gland cells. The clone was characterized by nucleotide sequence analysis. Two clusters of direct tandem repeats were identified, as well as large and small open reading frames (ORFs). The large ORF was fused to an Escherichia coli lacZ gene. Antibodies against the beta-galactosidase/ORF fusion protein reacted selectively on Western blots with a 67-kDa protein. Western-blot analysis and immunoelectron microscopy showed that this protein was distributed in the cells of all larval tissues examined. We concluded that BRa, a tissue-specific puff, whose activity correlates with the synthesis of 160-kDa secretory protein [Kolesnikov et al., Chromosoma 83 (1981) 661-677], may also contain a gene which is not expressed in a tissue-specific manner.

New foldback transposable element TFB1 found in histone genes of the midge Chironomus thummi

A new Foldback transposable element (TFB1) has been found in the histone H1-H3 intergenic region in the midge Chironomus thummi thummi. TFB1 has long terminal inverted repeats, composed of short, degenerate subrepeats and is flanked by nine or ten base-pair "target site" duplications. TFB1 is present in at least two adjacent histone gene units in Ch. th. thummi, indicating a homogenization of histone gene repeats. The copy number and chromosomal distribution of TFB1 are different in the closely related subspecies Ch. th. thummi and Ch. th. piger. showing that amplification, elimination and transposition of TFB1 have occurred recently during evolution.

The nucleotide sequence and in situ localization of a gene for a dimeric haemoglobin from the midge Chironomus thummi piger

A cluster containing at least four globin genes was isolated by screening an lambda EMBL3 genomic DNA library of the midge Chironomus thummi piger (Ctp) with a heterologous haemoglobin (Hb) gene IV (HbIV) probe from Chironomus thummi thummi (Ctt). This globin gene cluster was localized by in situ hybridization to chromosome II. One globin gene together with its 5'- and 3'-flanking regions has been sequenced. It can be deduced from the sequence that it is a new member of the dimeric HbVIIB family. The Ctp HbVIIB-5 gene displays 91.8% nucleotide sequence homology to a HbVIIB cDNA sequence, reported previously. There is no evidence for intron/exon structure in the Ctp HbVIIB-5 gene.

Cotransposition of a highly repetitive DNA element with flanking sequences in the genome of the midge Chironomus thummi

A family of highly repetitive DNA elements, the Cla-elements, is present in the genomes of the two sibling species Chironomus th. thummi and Ch. th. piger. These Cla-elements are organized in large tandem repetitive clusters as well as occurring as interspersed monomeric elements, in both subspecies. The analysis of a monomeric Cla-element and several kilobases of its flanking sequences from Ch. th. piger revealed that the short Cla-elements are cotransposed together with adjacent DNA. We found the same association of Cla-elements with specific flanking DNA in clones obtained from the rDNA of Ch. th. thummi and from nonribosomal Cla-DNA of Ch. th. piger. The Cla-element-flanking DNA is clearly also repetitive, but mainly of interspersed organization.

Mapping of the pattern of DNA replication in polytene chromosome from Chironomus thummi using monoclonal anti-bromodeoxyuridine antibodies

We present results from a nonautoradiographic study of DNA replication in polytene chromosomes from dipteran larvae. Monoclonal antibodies with specificity for 5-bromodeoxyuridine (BrdUrd) were used to localize by indirect immunofluorescence the sites of BrdUrd incorporation and to follow the dynamics of DNA synthesis in salivary gland cells of 4th instar Chironomus thummi larvae. This technique presents numerous advantages over autoradiographic procedures and allows mapping of DNA synthesis patterns at the level of resolution of one chromosomal band. Several replication patterns were observed, classified according to characteristic features, and tentatively assigned to specific periods of the S-phase. In early S-phase, DNA synthesis is first detectable in puffs and interbands, later in bands. Most chromosomal bands appear to initiate DNA synthesis synchronously; however, in bands within centromeric and heterochromatic regions the start of synthesis is delayed. At mid S-phase, all the bands show uniform staining. Subsequent staining patterns are increasingly differential with the bands displaying characteristic fluorescence intensities. As replication progresses through the late S-phase period, the chromosomes show a decreasing number of fluorescent bands. The last bands to terminate replication are located in centromeric and heterochromatic DNA-rich regions and a few bands of low DNA content in region IIAa-c.

Laser scanning microscopy of surface spread polytene chromosomes

A new type of a Laser Scan Microscope (Zeiss) was used for the analysis of the band-interband pattern of polytene chromosomes in Chironomus. In contrast to the previously used techniques of transmission light and electron microscopy, we used differential interference contrast (DIC) in incident light to depict the pattern. Instead of using common squash preparations, we carried out this investigation with surface spread polytene (SSP) chromosome preparations of salivary glands. The combination of techniques used enabled a more detailed light microscopic presentation of polytene structures in individual preparations than conventional techniques used so far for chromosome mapping.

Application of 7-amino-actinomycin D for the fluorescence microscopical analysis of DNA in cells and polytene chromosomes

The cytochemical properties of a guanine-specific synthetic fluorescent analogue of actinomycin D, 7-amino-actinomycin D, have been studied in fixed and living preparations of L cells and polytene chromosomes of salivary glands of Chironomus thummi thummi and Drosophila lummei (Hackman). 7-Amino-actinomycin D has been shown to bind to DNA-containing structures, thereby inducing in them a bright red fluorescence. No specific fluorescence has been found in RNA-containing structures treated with this fluorescent probe. The fluorescence pattern of some regions of polytene chromosomes with a known nucleotide composition was analysed. It has been established that 7-amino-actinomycin D induces a very weak fluorescence in GC-poor chromosome regions of the Drosophila lummei toromere structure. Data indicating a nonlinear dependence between the fluorescence intensity of a stained chromosome region and the GC content in its DNA have been obtained. The influence of DNA nucleotide composition in a chromosome region on the fluorescence of 7-amino-actinomycin D is discussed. In combination with quinacrine staining and the Feulgen fluorescence reaction, treatment with 7-amino-actinomycin D provides useful information about the distribution of GC base pairs in the chromosome region under study.

Left-handed Z-DNA in bands of acid-fixed polytene chromosomes

Antibodies to DNA in the left-handed (Z) conformation bind to acid-fixed polytene chromosomes of both Chironomus thummi and Drosophila melanogaster, as shown by direct and indirect immunofluorescence. Comparison of the phase-contrast, immunofluorescence, and DNA staining patterns shows a predominant localization of the antibody to the regions of high contrast and DNA density, the bands. The immunofluorescence is completely abolished by competition with polynucleotides in the Z conformation but not by those in the B form. DNase but not RNase treatment eliminates the antibody staining. Actinomycin D inhibits binding, whereas mithramycin has no effect. The highly reproducible immunofluorescence patterns obtained with the anti-Z-DNA antibodies demonstrate variations in fluorescence intensity between particular bands, which can be quantitated by laser scanning and photon counting techniques. The telomeric regions and DNA strands associated with end-to-end chromosome linkage and ectopic pairing are exceptionally bright. At saturation, average values of 1 IgG molecule per 3,000 base pairs and 1 per 15,000 base pairs are found in the intensely and weakly staining regions, respectively. An alternative statement is that the left-handed Z-DNA conformation is present at a frequency of 0.02-0.1%. The measured differences reflect variations in the local density of Z-DNA sites and not in the affinity for the specific antibody, which appears to be relatively constant throughout the chromosomes (Kd approximately equal to 10 nM). These observations taken together with results of biophysical studies on the properties of Z-DNA in solution suggest that regions of DNA in the left-handed conformation could be involved in higher-order structural organization of chromosomes and possibly in modulation of their functional state.

Selective exposure of antigenic determinants in chromosomal proteins upon gene activation in polytene chromosomes

The immunological accessibility of the nucleosomal core protein H3 and non-histone protein, HMG-1, was studied in transcriptionally active regions of Chironomus thummi polytene chromosomes. Chromosomal loci were decondensed by incubating isolated salivary glands in various salt solutions or hemolymph. Indirect immunofluorescence studies on these chromosomes using anti-sera to histone H3 revealed that the puffed regions were depleted of fluorescence. The lack of fluorescence could be correlated with the degree of puffing and the level of transcriptional activity. The puffed regions fluoresce after anti-H3 addition if the chromosomes are not cross-linked with formaldehyde, and if prior to the addition of antibodies the chromosomes are treated with 45% acetic acid. We conclude that, whereas histone H3 is present in the puffed regions, its antigenic determinants are sterically hindered by components which are extractable by 45% acetic acid. On the other hand, the antigenic determinants of protein HMG-1 are always available to antibody binding in puffed regions, as well as other chromosomal areas.

Chromosomal replication in Drosophila virilis. III. Organization of active origins in the highly polytene salivary gland cells

Using DNA fiber autoradiography, the rate of fork movement in D. virilis salivary glands was determined to be 0.1 micron/ml (25 degrees C). This value is 3.5 times slower than the replication rate determined in the diploid brain cells using the same experimental conditions (Steinemann, 1981). Replicon lengths in the polytene cells span from 5 to 203 micron, with a mean of approximately x = 46.7 micron and a median of approximately x = 39.5 micron. The polytene replicon length is about the same as that for diploid brain cells (31.0 micron). Dividing the haploid genome of D. virilis by the estimated number of 5,000 bands and taking the resulting 13.6 micron as average DNA length per chromomere, the average replicon length covers 3.4 chromomere units. This result does not support the concept that "one-band-plus interband" behaves as a replicating unit (Pelling, 1966). From the positive skew of the replicon length distributions, combined with the results derived from the chromosomal labelling patterns, the following schedule of chromosomal euchromatin doubling is infered: A short initiation period (discontinuous phase I), asynchronous with respect to individual origins, is followed by a phase of common replication activity at all inducible origins (continuous phase). The discontinuous II pattern is formed by few, long replicons which are still replicating while the shorter ones have already finished (weak points are not considered). Replicons, distributed in the main peak of the histogram and smaller than about 100 micron, are then responsible for the replication of the bulk of chromosomal euchromatin, the continuous phase. the interpretation is in conflict with the hypothesis assuming a clustered organization of the replicons to explain spot labelling (reviewed in Hand 1978). The diploid karyotype of D. virilis contains 45% satellite sequences, located in the alpha-heterochromatin (Gall et al., 1971). They do not replicate in the highly polytene salivary gland cells of 3rd instar larvae. Comparison of DNA fiber autoradiograph patterns from salivary glands and brains suggests that these satellite sequences replicate in short (less than 10 micron), to some extent irregularly spaced replicons.

Localization of chromosomal protein HMG-1 in polytene chromosomes of Chironomus thummi

The distribution of accessible antigenic sites in the chromosomal protein high mobility group one (HMG-1) in Chironomus thummi polytene chromosomes is visualized by immunofluorescence. The results indicate that (a) HMG-1 is distributed in a distinct banding pattern along the entire length of the chromosomes; (b) the banding pattern obtained with fluorescent antibody does not strictly correspond to that observed by phase-contrast microscopy; and (c) the amount of HMG-1 increases, and the fluorescent banding pattern changes, during the development of the organism. Our findings suggest that the protein may be involved in the modulation of the structure of selected loci in the chromosome.

DNA replication of a polytene chromosome section in Drosophila prior to and during puffing

Polytene chromosome sections 63E1-6 of 3L in Drosophila melanogaster were studied by 3H-uridine and 3H-thymidine autoradiography in late third instar larvae and prepupae. In late third instar larvae 63E does not incorporate 3H-uridine. In prepupae, however, a large puff is formed in 63E which is most active in RNA synthesis. 3h-thymidine labeling patterns and frequencies of regions 61A-64C were analysed and the nonpuffed and puffed 63E sections were compared with reference sections. Both in late third instar larvae and in prepupae 63E shows late replication behavior. It is concluded that the decondensation of chromosome bands does not necessarily entail earlier and/or faster DNA replication.

Co-replication of satellite DNA of Chironomus melanotus with mainband DNA during polytenization

The DNAs of five Chironomus species, C. plumosus, C. nuditarsis, C. thummi thummi, C. melanotus, and Camptochironomus pallidivittatus, were investigated in analytical neutral isopycnic CsCl density gradients. DNA was isolated both from larval brains ("diploid-DNA") and salivary glands ("polytene-DNA"). The buoyant densities of mainband DNAs were 1.692 g/cm3, with the exception of C. melanotus whose mainband had a density of 1.693 g/cm3. The densities correspond to a calculated GC content of 33% and 34% respectively. Only in C. melanotus was the DNA clearly resolved into mainband DNA and two distinct satellite shoulders: Satellite I, with a buoyant density of 1.684 g/cm3 (25% GC, calculated) and satellite II of 1.679 g/cm3 (19% GC, calculated). The two satellites comprise 15% of the total DNA in the "diploid-DNA"' and they also occur in the "polytene-DNA", where they make up 11%. The results are discussed in the general context of under- and overreplication in polyploid and polytene cells.

Differential staining of Chironomus thummi giant chromosomes by treatment with acridine orange after mild acid hydrolysis

The fluorescence of Chironomus thummi giant chromosomes stained by acridine orange after mild acid hydrolysis (1N HCl at 37 degrees) has been studied. After very short hydrolysis as well as in control preparations (untreated by HCl) all chromosome regions show green fluorescence. After long hydrolysis (10 min) all chromosome bands fluoresce red. Intermediate time of hydrolysis (2 min) give in all transcriptionally inactive bands including centromeric ones red fluorescence whereas transcriptionally active puffing regions are green. The possible mechanisms of differential staining of individual chromosome regions are discussed. It is suggested that transcriptionally active chromosome regions are less susceptible to the action of acid or that the difference in chromatin stainability after acid-AO treatment is due to the difference in chromatin packing. The applicability of this method to the study of functional states of chromatin, at least in giant chromosomes, is discussed.

Differential staining of polytene chromosome bands in Chironomus by Giemsa banding methods

Two Giemsa banding methods (C banding and RB banding) are described which selectively stain the centromere bands of polytene salivary gland chromosomes in a number of Chironomus species. - By the C banding method the polytene chromosome appearance is changed grossly. Chromosome bands, as far as they are identifiable, are stained pale with the exception of the centromere bands and in some cases telomeres, which then are intensely stained reddish blue. - By the RB method the centromere bands are stained bright blue, whereas the remainder of the polytene bands stain red to red-violet. - Contrary to all other species examined, in Chironomus th. thummi numerous interstitial polytene chromosome bands, in addition to the centromere regions, are positively C banded and blue stained by RB banding. In the hybrid of Ch. th. thummi X Ch. th. piger only those interstitial thummi bands which are known to have a greater DNA content than their homologous piger bands are C banding positive and blue stained by the RB method whereas the homologous piger bands are C banding negative and red stained by RB banding. Ch. thummi and piger bands with an equal amount of DNA both show no C banding and stain red by RB banding. - It seems that the Giemsa banding methods used are capable of demonstrating, in addition to centromeric heterochromatin, heterochromatin in those interstitial polytene chromosome bands whose DNA content has been increased during chromosome evolution.

Anomalous development and differential DNA replication in the X-chromosome of a Drosophila hybrid

Male hybrids of the cross D. azteca X D. athabasca are larger (hybrid giant males) than their parents, whereas hybrid females are of the same size as the parental species. Microspectrophotometric measurements have shown that the larval polytene salivary gland chromosomes of hybrid giant males undergo one more endoreplication than those of their sisters or parents. Replication patterns of the larval salivary gland chromosomes were compared after pulse labeling with 3H-thymidine and autoradiography. In females of either species as well as of hybrids X-chromosomes and autosomes are equally labeled, i.e. all chromosome arms replicate synchronously. In males, however. often fewer sites are labeled on the X-chromosome than on the autosomes. In addition, in a significant number of nuclei from D. athabasca males and also from hybrid giant males the converse can also be observed: i.e. more sites are labeled on the X-chromosome than on the autosomes. The modified labeling patterns are interpreted as an indication of a time-shift in the replication of hemizygous X-chromosomes in males, in relation to the autosomes.

RNA, RIBOSOMAL/*BIOSYNon of ribosomal 5S RNA genes in Chironomus thummi by in situ hybridization of iodinated 5S RNA

5 S RNA of Chironomus thummi larvae was purified from total phenol extracted RNA by gel filtration and labelled to about 10(7) dpm/mug with carrier-free iodine-125. After hybridization in situ of 125I-5 S RNA and autoradiography only region B3c-e (containing two "normal" and two very faint bands) of chromosome II of salivary gland cells was highly labelled. In chromosomes of an animal showing pairing discontinuities a clearly "heterozygous" labelling of the 5 S RNA region was found. Region B3c-e shows no clearcut morphological signs of puffing or autoradiographically detectable 3H-uridine incorportion in spite of a continuous synthesis of 5 S RNA in salivary gland cells.

Prolongation of replication time after doublings of the DNA content of polytene chromosome bands of Chironomus

Using 3H-thymidine autoradiography, labeling frequency of homologous asynapsed chromosome bands of the hybrid of Chironomus th. thummi and Chironomus th. piger has been studied. In a number of these bands the DNA content of the thummi bands if 2, 4, 8 or 16 times as large as that of the homologous piger bands (Keyl, 1965). Those bands of CH. TH. thummi which show one doubling of their DNA content in comparison with the homologous piger bands are also labeled two times more frequently than piger. In contrast to this such a correlation between increase of labeling frequency (i.e. prolongation of replication time) and doubling of the DNA content is not observed, when thummi bands have 4, 8 or 16 times more DNA than their homologues in piger. In these cases replication time is also prolonged after each doubling. Duration of DNA synthesis increases linearly but always by a smaller factor as the corresponding DNA content is increased.

Electron microscopic studies on the polytene chromosomes of Chironomus thummi salivary glands. I. Ultrastructural mapping

The method of ultrathin sections of unsquashed salivary gland polytene chromosomes of Ch. thummi was applied to their ultrastrual mapping. There was a good agreement between electron micrographs and Hägele's light microscopic map (1970) with respect to the pattern and number of bands. 94% of bands were identified in larval and prepupal chromosomes. In Ch. thummi, band thickness varied from 0.05-0.5 mum. Most characteristic were 0.2-0.3 mum bands. Morphologically, bands were classified as: continuous (frequently with holes and gaps), discrete, dotted and continuous-discrete, discrete-dotted. Band morphology is related to band size, such that smaller bands, as a rule, were also dotted. Bands beginning to puff likewise became dotted. Interbands in unsquashed chromosome sections were from 0.05-0.15 mum. The smallest interbands contained only fibrils, in the larger interbands few granules could be observed. This makes interbands distinguishable from a typical puff with many such granules.

Patterns of incorporation of tritiated thymidine by the dorsal polytene foot-pad nuclei of Sarcophaga bullata (Sarcophagidae:Diptera)

The incorporation of tritiated thymidine into the DNA of the dorsal foot-pad nuclei of Sarcophaga bullata, during pharate adult development, was studied by scintillation counting and autoradiography. Incorporation was maximal on day 4 and showed a progressive temporal decline on days 5 to 8. Autoradiographs of chromosomal arms A1, A2 and D1, from late stages of the prolonged S period, showed discontinuous incorporation. On days 7 and 8 loci which incorporated the radio-labeled precursor were almost exclusively associated with DNA granules. DNA granules were associated with 8 specific loci in chromosomal arm A1, 10 in A2, and 8 in D1. The relationship between patterns of incorporation of tritiated thymidine and DNA granules is discussed.

Molecular characterization of an insect genome: Chironomus thummi

DNA extracted from Chironomus thummi larvae was studied by isopycnic centrifugation in CsCl, thermal denaturation and DNA-DNA reassociation techniques. The mean G+C content of the C. thummi DNA is 28-29% as indicated both by centrifugation in CsCl and thermal denaturation. According to optical reassociation analysis of total DNA and of isolated DNA fractions the C. thummi genome is composed of at least four components. About 80% of the DNA is classified as unique with a kinetic complexity of nearly 7 X 10(10) daltons. 6-8% intermediate DNA exhibits a kinetic complexity slightly above 10(8) daltons with a mean repetition frequency of 35. 11-13% fast-reassociating DNA has a kinetic complexity slightly above 10(6) daltons with a mean repetition frequency of 6000. 3-5% of the DNA cannot be properly studied by the optical reassociation technique and probably contains inverted repeats. The thermal denaturation behaviour of isolated DNA fractions indicated that most of the repetitive sequences in the C. thummi genome are tightly interspersed.

Quantitative analysis of DNA content in the salivary gland chromosomes of Chironomus thummi at larval and prepupal stages

Relative DNA content during the polytenization of the salivary gland nuclei of Chironomus thummi was measured by cytophotometric and cytofluorometric methods. To estimate the degree of polyteny, the DNA content was calculated in hemocyte and spermatocyte nuclei. Chromosome polytenization is associated with 10 to 12 replication rounds. There are 4-5 replication rounds in 1st instar, 2-3 rounds in 2nd instar; 3rd and 4th instars have 1-2 rounds each. From early postembryonic development, larvae already have salivary gland nuclei representing two polyteny classes (2-3 - 2-4C); A similar heterogeneity is retained in all instars. The approximate DNA content is 0.51-0.58 picogram per diploid set.

DNA and RNA content of Chironomus thummi polytene chromosomes determined by micro column gel filtration

A new microtechnique was used to determine the nucleic acid content of Chironomus polytene chromosomes. The method based on UV-microspectrophotometric measurement of alkali digested chromosome samples before and after gel filtration through Sephadex microcolumns permits the simultaneous estimation of DNA as well as RNA amount of single chromosomes and nucleoli. The DNA and RNA content of C. thummi salivary gland chromosomes I-III carrying a large number of puffs (but no nucleoli or Balbiani rings) corresponds roughly to their relative lengths. Chromosomes of the most frequent polyteny level (class B; probably 8192 C) contain 418 (Chromosome I), 372 (II), and 307 times 10-12 g DNA (III) and 116, 120 and 69 times 10-12 g RNA, respectively. The 4th chromosome, which carries the nucleolus and two Balbiani rings, contains about 150 times 10-12 g DNA and 328 times 10-12 g RNA. As the result of one of the last replication steps both DNA and RNA content of chromosomes I-III becomes duplicated whereas nucleolar RNA content is increased to a smaller extent.