The salivary gland of Chironomus (Diptera): a model system for the study of cell differentiation

Diptera and Drosophila Karyotype Databases: A Useful Dataset to Guide Evolutionary and Genomic Studies

Karyotypes and chromosome data have been widely used in many subfields of biology over the last century. Unfortunately, this data is largely scattered among hundreds of articles, books, and theses, many of which are only available behind paywalls. This creates a barrier to new researchers wishing to use this data, especially those from smaller institutions or in countries lacking institutional access to much of the scientific literature. We solved this problem by building two datasets for true flies (Order: Diptera and one specific to Drosophila), These datasets are available via a public interactive database that allows users to explore, visualize and download all data. The Diptera karyotype databases currently contain a total of 3,474 karyotype records from 538 publications. Synthesizing this data, we show several groups are of particular interest for future investigations by whole genome sequencing.

Assessment of Cell Viability in Intact Glandular Tissue in Chironomus ramosus using Dye-exclusion and Colorimetric Assays

Conventionally, dye-exclusion test for determining cell viability has been restricted only for cells in suspension in tissue culture. In this paper, salivary gland of Chironomus has been proposed as a simple tissue model system where dye-exclusion test can be reliably employed for the intact gland. We have compared suitability of commonly used vital dyes and nigrosin was found suitable for the salivary gland cells. Biochemical tests using tetrazolium salts are also commonly used for determining quantitative indices of cell viability in metabolically active cells. Ours is the first attempt to extend the same technique for the whole tissue. We standardized the conditions and prepared a protocol for MTT-based colorimetric assay suitable for the salivary gland of Chironomus. A strong correlation (r(2) = 0.9893) was obtained where increasing O.D. correlated linearly with the number of live glands. We concluded that nigrosin dye-exclusion and MTT metabolic inclusion assays are suitable methods for the viability test of metabolically active intact salivary gland of Chironomus which can serve as a potential model for the assessment of cytotoxicity in future.

Evidence for a common ancestor sequence for the Balbiani ring 1 and Balbiani ring 2 genes in Chironomus tentans

The Balbiani ring (BR) 1 and BR 2 genes in Chironomus tentans are functionally related and are only expressed in the salivary gland cells. Here we reveal the principal structure of the BR 1 gene and analyze the structural and evolutionary relationship between the BR 1 and BR 2 genes. The properties of the BR 1 gene, 37 kilobases in size, are derived from the analysis of a cloned cDNA sequence, pCt 21. A considerable part of the BR 1 gene consists of one or a few blocks of a tandemly repeated 246-base-pair (bp) major repeat unit. About half of this major repeat unit is in turn built from four tandem repeats of a 33-bp sequence. This hierarchic arrangement of repetitive sequences within the BR 1 gene suggests that the gene has evolved through two major amplification steps, starting from a short primordial sequence. A similar evolutionary model has been put forward for the BR 2 gene [Sümegi, J., Wieslander, L. & Daneholt, B. (1982) Cell 30, 579-587]. The two putative primordial genes contain a similar, 102-bp-long sequence (86% nucleotide sequence homology), indicating that the BR 1 and BR 2 genes most likely arose from the same ancestor sequence. During the course of evolution the two genes diverged, mainly due to differences in the length and sequence of the gene segments involved in the two amplification steps. Moreover, at least one of the BR genes was translocated to another chromosomal locus.

Immunocharacterization of the DNA puff BhC4-1 protein of Bradysia hygida (Diptera: Sciaridae)

The DNA puff BhC4-1 gene is amplified and highly expressed in the salivary gland of Bradysia hygida late larvae. Using affinity-purified polyclonal antibodies we have identified the product of the BhC4-1 gene as a 43 kDa polypeptide which is present in extracts of salivary glands from late fourth instar larvae and in the corresponding gland secretion, but not in glands from earlier stages. We also demonstrate that this protein is produced mainly in the S1 and S3 regions of the salivary gland, where BhC4-1 amplification levels are more pronounced and larger amounts of mRNA are produced. By immunoelectron microscopy the BhC4-1 protein was detected in secretory granules of the S1 and S3 regions, and localized in fibrous structures present in the saliva.

A cell-specific glycosylated silk protein from Chironomus thummi salivary glands. Cloning, chromosomal localization, and characterization of cDNA

Chironomid salivary glands contain 40 cells dedicated to the synthesis of a relatively small ensemble of silk proteins. Glands in some species contain a special lobe composed of 4 cells distinguishable from the others. We have cloned a special lobe-specific cDNA from Chironomus thummi salivary glands. Northern blots of salivary gland RNA demonstrated that the cDNA hybridizes to a 2.5-kilobase transcript present only in the special lobe. In situ hybridization mapped the gene encoding this cDNA to region A2b on polytene chromosome IV, the locus of the special lobe-specific Balbiani ring a. The deduced amino acid sequence encodes a protein with a calculated molecular mass of 77 kDa and numerous potential glycosylation sites; it appears unrelated to other known chironomid silk proteins. Polyclonal antibody, raised against a cDNA-encoded fusion protein, reacted exclusively with a special lobe-specific 160-kDa silk protein. Lectin binding studies indicate that the immunoreactive 160-kDa protein contains both N- and O-linked glycan moieties. We conclude that glycosylation most likely contributes to the difference between calculated and apparent molecular masses and that this cDNA encodes the special lobe-specific silk protein previously described as ssp160 (Kolesnikov, N. N., Karakin, E. I., Sebeleva, T. E., Meyer, L., and Serfling, E. (1981) Chromosoma 83, 661-677).

Secretory proteins of Chironomus salivary glands: structural motifs and assembly characteristics of a novel biopolymer

Salivary glands of Chironomus synthesize a family of at least ten secretory proteins that can be grouped into three size classes: the large (about 1000 kDa), intermediate (100- to 200 kDa), and small (less than 100 kDa). After synthesis, secretory proteins undergo a dramatic transformation to form a novel biopolymer. Secretory proteins accumulate in the central lumen of the gland, forming dissociable complexes that appear as a network of smooth fibrils and multistranded beaded fibers. When secretory protein complexes are extruded through the secretory duct, the fibers become oriented in parallel arrays; when these parallel arrays of fibers emerge from the mouth of larvae they are an insoluble, silk-like thread. Regulation of secretory protein-coding gene expression determines which secretory proteins are synthesized, thus, the composition of silk threads. At least two types of threads are produced: larval silk is used to construct tubes for protective housing and assist with feeding; prepupal silk is used to construct tubes for larval/pupal ecdysis (pupation). Variations in composition presumably contribute to different mechanical properties of larval and prepupal silk threads. Since the macroscopic physical properties of polymerized silk most likely reflect the microscopic structure and interaction of secretory proteins, it becomes important to learn the principles which govern secretory protein assembly at the molecular level. Which secretory proteins interact and what are the sites used for intraportein and protein-protein interactions during the assembly of this biopolymer? All eight secretory proteins characterized thus far contain tandemly repeated peptide sequences (ranging from 14-90 amino acids in length) and/or a periodic distribution of Cys residues. These motifs appear to be unique; no other biopolymer has either the repeated peptide sequences or composite structure of chironomid silk threads. The evolutionary conservation of motifs within repeats and among different secretory proteins suggests that the sequences and three-dimensional structures of the motifs may be important for assembly of secretory proteins into complexes, oriented fibers, and silk threads. Further study of secretory protein assembly will bring us closer to understanding how this silk assembles in vivo. By learning principles that nature employs to construct such a novel composite biopolymer, it may become feasible to design and produce new classes of fibers or biomolecular materials with distinctive properties that are currently unavailable.

A new member of a secretory protein gene family in the dipteran Chironomus tentans has a variant repeat structure

We describe the structure of a gene expressed in the salivary gland cells of the dipteran Chironomus tentans and show that it encodes 1 of the approximately 15 secretory proteins exported by the gland cells. This sp115,140 gene consists of approximately 65 copies of a 42-bp sequence in a central uninterrupted core block, surrounded by short nonrepetitive regions. The repeats within the gene are highly similar to each other, but divergent repeats are present in a pattern which suggests that the repeat structure has been remodeled during evolution. The 42-bp repeat in the gene is a simple variant of the more complex repeat unit present in the Balbiani ring genes, encoding four of the other secretory proteins. The structure of the sp115,140 gene suggests that related repeat structures have evolved from a common origin and resulted in the set of genes whose secretory proteins interact in the assembly of the secreted protein fibers.

A BR 1 gene in Chironomus tentans has a composite structure: a large repetitive core block is separated from a short unrelated 3'-terminal domain by a small intron

The large Balbiani ring (BR) genes in the dipteran genus Chironomus have been considered to be homogeneous repetitive structures. Analysis of a genomic DNA segment now reveals that a BR 1 gene in C. tentans is a composite gene, consisting of two different types of sequences. A 15-20 kb core block of tandemly arranged repeat units extends close to the 3' end of the BR 1 gene and ends in repetitive structures partly different from the repeat units in the core block. A 55 bp long intron separates the core block, which probably constitutes a single exon, from a non-related 3'-exon, comprising the final 332 bp of the translated part of the gene. According to hydrophobicity and secondary structure predictions, the 3'-exon encoded peptide is distinctly different from the repetitive core block domain and attains a globular structure. The carboxyl-terminal peptide domain is likely to be a general feature of BR encoded proteins and may have important functions in the excretion and polymerisation of the secretory proteins.

A novel giant secretion polypeptide in Chironomus salivary glands: implications for another Balbiani ring gene

Chironomus salivary glands contain a family of high Mr (approximately 1,000 X 10(3)) secretion polypeptides thought to consist of three components: sp-Ia, sp-Ib, and sp-Ic. The use of a new extraction protocol revealed a novel high Mr component, sp-Id. Results of a survey of individual salivary glands indicated that sp-Id was widespread in more than a dozen strains of C. tentans and C. pallidivittatus. Sp-Id was phosphorylated at Ser residues, and a comparison of cyanogen bromide and tryptic peptide maps of 32P-labeled polypeptides suggested that sp-Ia, sp-Ib, and sp-Id are comprised of similar but nonidentical tandemly repeated amino acid sequences. We concluded that sp-Id is encoded by an mRNA whose size and nucleotide sequence organization are similar to Balbiani ring (BR) mRNAs that code for the other sp-I components. Furthermore, parallel repression of sp-Ib and sp-Id synthesis by galactose led us to hypothesize that both of their genes exist within Balbiani ring 2.

Conserved and nonconserved structures in the secretory proteins encoded in the Balbiani ring genes of Chironomus tentans

The large, repetitive Balbiani ring (BR) genes, BR 1, 2, and 6, in Chironomus tentans originated from a short ancestral sequence and have all evolved according to analogous amplification schemes. We analyzed the structures of the BR-encoded secretory proteins and defined the parts that have been conserved during the evolutionary process. The BR products show striking similarities, with the BR 1 and BR 2 products being more similar to each other than to the BR 6 product. In the constant (C) region of the repeat units, 7 of the 30 amino acid residues are strictly conserved; 4 of these are the cysteine residues. The subrepeat (SR) regions of all the BR products are dominated by repeated tripeptide elements rich in proline and charged amino acid residues. Most of the amino acid replacements in both regions are conservative. Secondary structure predictions suggested that the C regions of the BR 1 and BR 2 products have several elements of secondary structure: an alpha-helix, a beta-strand, and one or two reverse turns, as in "globular structures." The prediction for the C region of the BR 6 product is similar but lacks a beta-strand. The predictions for the intervening SR regions appear less conclusive, but are clearly different from those for the C regions, and suggest regular structures not differing in their conformational elements. The SR regions evolved from an ancestor sequence similar to the C region; thus, the BR products seem to represent an example of evolution from one structure to two differently folded products.(ABSTRACT TRUNCATED AT 250 WORDS)

Balbiani ring DNA: sequence comparisons and evolutionary history of a family of hierarchically repetitive protein-coding genes

All known types of Balbiani ring (BR) genes consist of multiple, tandemly arranged, ca. 180 to 300-bp repeat units that can be divided into a constant region and a subrepeat region. The latter region includes short tandem subrepeats (SRs). Comparison of all available BR sequences using computer methods has enabled us (a) to define more precisely the constant and subrepeat regions, (b) to infer the evolutionary relationships among the various types of BR repeats, (c) to derive a consensus approximation of an ancestral sequence from a small segment of which the highly diverse present-day SRs may have originated, and (d) to detect an underlying substructure in the constant region, evident in the consensus but not in the present-day sequences and possibly corresponding to an original 39-bp DNA segment from which the extant, giant BR sequences may have evolved. We discuss the processes of reduplication, diversification, and homogenization within the hierarchically repetitive BR sequences as examples of how a simple DNA element may evolve into a diverse family of large, protein-coding genes.

Isolation and characterization of the two giant secretory proteins in salivary gland of Chironomus tentans

The two giant secretory proteins, sp-Ia and sp-Ib, in salivary-gland cells of the larva of the fly Chironomus tentans, were isolated by preparative gel electrophoresis and characterized chemically. Their amino acid compositions are dominated by polar amino acids, with about 30% of basic amino acid residues. Crossed immunoelectrophoresis of sp-Ia and sp-Ib provided evidence that they share antigenic determinants. They also have major methionine-containing tryptic peptides in common. CNBr cleavage of sp-Ib gives a small number of low-Mr fragments, indicating that this protein has a repetitive structure.

Self-complementary DNA sequences within the BRc gene of Chironomus thummi

The Balbiani ring c (BRc) DNA of Chironomus thummi consists of tandemly arranged 249-base pair (bp) repeats, which represent the major part of the gene (Bäumlein et al. 1982a) and are transcribed and translated in a periodic polypeptide of unusual size. To obtain further information on the DNA sequence organization of that gene a recombinant phage (lambda CthBRc-1) with a relatively long insert (containing predominantly 249-bp repeats) was studied by electron microscopy (EM). lambda CthBRc-1 was found to undergo specific sequence elimination of BRc DNA resulting in heterogeneous size distribution of insert length within the limits of the cloning capacity of the phage with a maximum around 15 kilobase pairs (kb). The EM analysis of R loops formed between recombinant molecules and poly(A)+RNA (containing the transcripts of BRc and BRb) revealed the existence of self-complementary inverted and direct repeats as further sequence elements of BRc DNA scattered throughout a long portion of the BRc transcription unit. Different intrastrand structures (stems, hairpins, complex loops) originate from the renaturation of several sets of self-complementary repeats. Most double-stranded regions fell into one main-size class with an average length of 0.1 kb. The overall data suggest that self-complementary repeats belong to the same DNA sequence family and are able to cooperate in the formation of loops of different size and complexity. The results are discussed in relation to the functional significance of self-complementary inverted repeats (palindromes) for BRc expression.

[Do repetitive DNA sequences have a biological function?]

By DNA reassociation kinetics it is known that the eucaryotic genome consists of non-repetitive DNA, middle-repetitive DNA and highly repetitive DNA. Whereas the majority of protein-coding genes is located on non-repetitive DNA, repetitive DNA forms a constitutive part of eucaryotic DNA and its amount in most cases equals or even substantially exceeds that of non-repetitive DNA. During the past years a large body of data on repetitive DNA has accumulated and these have prompted speculations ranging from specific roles in the regulation of gene expression to that of a selfish entity with inconsequential functions. The following article summarizes recent findings on structural, transcriptional and evolutionary aspects and, although by no means being proven, some possible biological functions are discussed.

The basic repeat unit of a Chironomus Balbiani ring gene

A clone derived from the Balbiani ring b (BRb) gene of Chironomus thummi has been used to study the internal organization of that gene. Much of the gene consists of approximately 80 copies of a ca. 300 bp repeat unit, which are tandemly organized. The BRb clone contains a major part of that unit (242 bp). Sequence analysis shows that approximately 60% of the unit corresponds to short, tandemly organized subsequences, which encode peptides 8 to 11 residues long. In turn, each subsequence consists of even shorter internal repeats, corresponding to a tripeptide (consensus Proline. Serine. Lysine.). The remainder of the ca. 300 bp unit probably does not have obvious repetitive substructure.

Modulation of 75S RNA synthesis in the Balbiani rings of Chironomus tentans with galactose treatment

Galactose has been used as a tool to modify gene activity in the giant puffs Balbiani ring 2 (BR2) and Balbiani ring 1 (BR1) on chromosome IV in the salivary glands of Chironomus tentans. BR2 decreased gradually and was absent or almost absent after a four day galactose treatment. Concomitant with this morphological change, the labelling of the population of growing 75S RNA molecules in BR2 decreased, and was essentially abolished after four days in galactose. Since the elongation rate at the 75S RNA genes proved to be the same in the galactose treated glands as in the control glands, the decreased labelling in BR2 was likely to correspond to a decreased production of 75S RNA. No changes in the size distribution of the growing 75S RNA molecules were noted during the galactose treatment, suggesting that the modulation of the activity was most likely accomplished at the initiation level, but regulation of a very early premature termination could not be excluded. When galactose was removed from the medium, BR2 attained its normal size and its ordinary RNA labelling. BR1 was studied in parallel with BR2 and it behaved strikingly different: BR1 expanded during the galactose treatment and the amount of growing 75S RNA increased, indicating an enhanced production of this 75S RNA species. Also the modulation of BR1 RNA synthesis was reversible. During the galactose treatment no changes in the labelling of chromosome I-III and of nucleolar RNA were observed suggesting that during the four day treatment, galactose exerted its effect mainly on the synthesis of BR2 and BR1 transcription products. The significance of these observations are considered in relation to the information available on the synthesis of the corresponding secretory polypeptides and the formation of the tube-like burrows. We also discuss the implications of the results for models of the regulation of gene activity and of the puffing process.

Cell-specific synthesis and glycosylation of secretory proteins in larval salivary glands of Chironomus thummi

Synthesis and glycosylation of larval salivary gland secretory proteins of Chironomus thummi were analyzed with respect to cell specific differences in the Balbiani ring (BR) pattern and glycoprotein composition of secretion formerly detected by histochemical staining procedures. In the secretion of a special cell type in salivary glands, which is characterized by the appearance of an additional BR, and additional polypeptide with a relative molecular weight (Mr) of 160 kD was found differing in its antigenic properties and tryptic fingerprint pattern from main cell secretion proteins. This so-called ssp-160 component is preferentially synthesized and glycosylated in the special cells. In the same cells, both the synthesis and glycosylation of all other major secretory proteins was found to be diminished or even repressed. In contrast to the conspicuous cell-specific differences at the level of protein synthesis. RNA analyses show the prominent synthesis of 75S RNA in both main and special cells and gave no clear indication of the synthesis of a smaller RNA fraction as expected from the size of ssp-160 component.--These and further data on synthesis and properties of secretory proteins as well as expression of BR DNA are discussed with regard to the assumption that at least some of the eight major secretory polypeptides are coded for by BR DNA. The BR gene(s) might have originated by manifold duplications and modifications of short repetitive prototype DNA sequences, which are coordinatively expressed.

Transplantation of insect giant chromosome nuclei into amphibian oocytes

Polytene salivary gland nuclei of Chironomus pallidivittatus were transplanted into oocytes of Xenopus laevis which were then cultured in vitro for 18 h. The giant chromosomes and nucleoli as well as the entire nuclei enlarged considerably in volume during this time. The polyteny and specific chromomere pattern of the chromosomes were maintained, and the puffing of the salivary gland-specific Balbiani rings was not noticeably changed. - Polytene nuclei from differentiated insect cells transplanted into Xenopus oocytes thus appear suited for exposing giant chromosomes in vivo to purified factors such as regulatory molecules.

Cell-free synthesis and immunological characterization of salivary proteins from Chironomus tentans

We have isolated the salivary proteins of the larva of the harlequin fly Chironomus tentans, and characterized its constituents by gel electrophoresis and immunological techniques. The detailed composition of saliva from individual animals is shown to be very variable, but four main protein groups can be defined. The largest, Fraction A, comprises up to five species, with molecular weights of between 820,000 and 700,000 Daltons. It includes at least two distinct antigenic species. This finding is discussed in the context of the known heterogeneity of the 75S RNA fraction which is transcribed in the Balbiani rings 1 and 2. -- The other prominent protein classes in isolated saliva range in size from 230,000 down to less than 20,000 Daltons. -- We have also employed antiserum against salivary proteins to investigate the products of in vitro translation of salivary gland RNA in the rabbit reticulocyte lysate system. A broad spectrum of polypeptide species is obtained which are immunologically related to salivary components, including species of over 300,000 Daltons. These latter are interpreted as unfinished Fraction A polypeptides resulting from incomplete translation of 75S RNA from BR1 and BR2. Evidence is presented to demonstrate that other salivary proteins, apart from Fraction A, are faithfully translated in the reticulocyte lysate.

Periodicities and tandem repeats in a Balbiani ring gene

The Balbiani ring (BR) DNAs show prominent periodicities of restriction enzyme sites. Studies using a cloned fragment of the BRc gene strongly suggest that these periodicities reflect the existence of tandemly repetitive sequences within BR DNA. Tandem repeats measuring 54-58 bp have been demonstrated by partial sequence analysis of the BRc clone; the restriction site periodicities suggest the existence of additional 175 (= 3 X 58) and 1050 (= 6 X 175) bp repeat units. The short, medium and long repeats (58, 175 and 1050 bp, respectively) show sequence homology. Constrained unequal crossing over (resulting from misalignment of repeat arrays, usually by one repeat) is proposed as the mechanism for evolution of short, medium and long repeats from each other, in a manner analogous to evolution of satellite DNA sequences. Paradoxically, the dominant restriction site periodicities appear to be more conservative than might be expected on the basis of the overall sequence divergence between the sequenced repeats. This may be a consequence of functionally important, long-range amino acid or oligopeptide periodicities (for example, Asp x Ser or Glu x Ser corresponding to Hinf I sites) in the BRc protein product, in conjunction with preferential use of certain synonymous codons.

Large sized nascent protein as dominating component during protein synthesis in Chironomus salivary glands

The main secretory protein fractions from Chironomus tentans have been investigated with particular emphasis on the dominant fraction, component 1, here designated I (Grossbach, 1969). This polypeptide was suggested to be the translatory product of 75SS RNA from Balbiani ring 2 (BR2) because of its size and quantitative prominence. Its molecular weight was estimated by gel filtration in 8 M urea at 850,000 + 101,000 D. During short pulses with radioactive amino acids a large fraction of the label was found in a population of polypeptide chains suggestive of molecules continuously growing to the size of compoenet I. Populations of nascent large protein chains of similar size distribution were dominant in the polysomes and constituted the only population present in the largest polysomes, known to contain 75S RNA from BR2 (and BR1) as predominant or only component (Daneholt et al., 1977; Wieslander and Daneholt, 1977). These data indicate strongly that the large size of component I is not a result of posttranslational modifications. No sequence similarities, using limited proteolysis, were found between component I and component II, both of which have been considered to the BR2 products. There was, furthermore, no detectable immunological identity between component I and smaller secretory protein fractions. The data support Grossbach's and Daneholt's suggestion that component I is closely related to the primary translation product of 75S RNA from the large Balbiani rings.

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.