Under-replication of ribosomal cistrons in polytene chromosomes of Rhynchosciara

Dramatic nucleolar dispersion in the salivary gland of Schwenkfeldina sp. (Diptera: Sciaridae)

Micronucleoli are among the structures composing the peculiar scenario of the nucleolus in salivary gland nuclei of dipterans representative of Sciaridae. Micronucleolar bodies contain ribosomal DNA and RNA, are transcriptionally active and may appear free in the nucleoplasm or associated with specific chromosome regions in salivary gland nuclei. This report deals with an extreme case of nucleolar fragmentation/dispersion detected in the salivary gland of Schwenkfeldina sp. Such a phenomenon in this species was found to be restricted to cell types undergoing polyteny and seems to be differentially controlled according to the cell type. Furthermore, transcriptional activity was detected in virtually all the micronucleolar bodies generated in the salivary gland. The relative proportion of the rDNA in polytene and diploid tissues showed that rDNA under-replication did not occur in polytene nuclei suggesting that the nucleolar and concomitant rDNA dispersion in Schwenkfeldina sp. may reflect a previously hypothesised process in order to counterbalance the rDNA loss due to the under-replication. The chromosomal distribution of epigenetic markers for the heterochromatin agreed with early cytological observations in this species suggesting that heterochromatin is spread throughout the chromosome length of Schwenkfeldina sp. A comparison made with results from another sciarid species argues for a role played by the heterochromatin in the establishment of the rDNA topology in polytene nuclei of Sciaridae.

Chromatin structure of ribosomal RNA genes in dipterans and its relationship to the location of nucleolar organizers

Nucleoli, nuclear organelles in which ribosomal RNA is synthesized and processed, emerge from nucleolar organizers (NORs) located in distinct chromosomal regions. In polytene nuclei of dipterans, nucleoli of some species can be observed under light microscopy exhibiting distinctive morphology: Drosophila and chironomid species display well-formed nucleoli in contrast to the fragmented and dispersed nucleoli seen in sciarid flies. The available data show no apparent relationship between nucleolar morphology and location of NORs in Diptera. The regulation of rRNA transcription involves controlling both the transcription rate per gene as well as the proportion of rRNA genes adopting a proper chromatin structure for transcription, since active and inactive rRNA gene copies coexist in NORs. Transcription units organized in nucleosomes and those lacking canonical nucleosomes can be analyzed by the method termed psoralen gel retarding assay (PGRA), allowing inferences on the ratio of active to inactive rRNA gene copies. In this work, possible connections between chromosomal location of NORs and proportion of active rRNA genes were studied in Drosophila melanogaster, and in chironomid and sciarid species. The data suggested a link between location of NORs and proportion of active rRNA genes since the copy number showing nucleosomal organization predominates when NORs are located in the pericentric heterochromatin. The results presented in this work are in agreement with previous data on the chromatin structure of rRNA genes from distantly related eukaryotes, as assessed by the PGRA.

Whole genome amplification in large neurons of the terrestrial slug Limax

The brain of gastropod mollusks contains giant neurons whose nuclei are enlarged with a large amount of genomic DNA. Such DNA is produced by repeated endoreplication. We have previously demonstrated that the frequency of the neuronal DNA endoreplication is correlative to the body growth of the adult land slug and to the increase in the amount of transcripts within the neuron. However, it has long been controversial whether the neuronal DNA endoreplication entails whole genome amplification (polyploidy), or whether only the necessary genomic loci are amplified (polyteny, polysomy, or cis-amplification by unequal recombination). In the present study, we adopted two modern techniques - quantitative genomic PCR and 5'-bromodeoxyuridine labeling - to distinguish between these two possibilities. Our results demonstrated that multiple genomic loci were amplified to the same extent irrespective of the transcriptional activities at these loci. Moreover, the visceral giant cell, the biggest neuron in the slug's brain, was estimated to contain approximately 10 000-times as much genomic DNA as the haploid amount. The 5'-bromodeoxyuridine-labeling experiments also revealed a uniform DNA synthesis within the nucleus. These results strongly support the idea that the giant neurons contain a polyploid genome rather than a locus-specific amplified genome.

Curiously composite structures of a retrotransposon and a complex repeat associated with chromosome ends of Rhynchosciara americana (Diptera: Sciaridae)

In Drosophila, telomere retrotransposons counterbalance the loss of telomeric DNA. The exceptional mechanism of telomere recovery characterized in Drosophila has not been found in lower dipterans (Nematocera). However, a retroelement resembling a telomere transposon and termed "RaTART" has been described in the nematoceran Rhynchosciara americana. In this work, DNA and protein sequence analyses, DNA cloning, and chromosomal localization of probes obtained either by PCR or by screening a genomic library were carried out in order to examine additional features of this retroelement. The analyses performed raise the possibility that RaTART represents a genomic clone composed of distinct repetitive elements, one of which is likely to be responsible for its apparent enrichment at chromosome ends. RaTART sequence in addition allowed to assess a novel subtelomeric region of R. americana chromosomes that was analyzed in this work after subcloning a DNA fragment from a phage insert. It contains a complex repeat that is located in the vicinity of simple and complex tandem repeats characterized previously. Quantification data suggest that the copy number of the repeat is significantly lower than that observed for the ribosomal DNA in the salivary gland of R. americana. A short insertion of the RaTART was identified in the cloned segment, which hybridized preferentially to subtelomeres. Like RaTART, it displays truncated sequences related to distinct retrotransposons, one of which has a conceptual translation product with significant identity with an endonuclease from a lepidopteran retrotransposon. The composite structure of this DNA stretch probably reflects mobile element activity in the subtelomeric region analyzed in this work.

The localization of ribosomal DNA in Sciaridae (Diptera: Nematocera) reassessed

The chromosomal localization of ribosomal DNA (rDNA) was studied in polytene and diploid tissues of four sciarid species, Trichosia pubescens, Rhynchosciara americana, R. milleri and Schwenkfeldina sp. While hybridization to mitotic chromosomes showed the existence of a single rDNA locus, ribosomal probes hybridized to more than one polytene chromosome region in all the species analyzed as a result of micronucleolar attachment to specific chromosome sites. Micronucleoli are small, round bodies containing transcriptionally active, probably extrachromosomal rDNA. In T. pubescens the rDNA is predominantly localized in chromosome sections X-10 and X-8. In R. americana the rDNA is frequently found associated with centromeric heterochromatin of the chromosomes X, C, B and A, and also with sections X-1 and B-13. Ribosomal probes in R. milleri hybridized with high frequency to pericentric and telomeric regions of its polytene complement. Schwfenkfeldina sp. displays a remarkably unusual distribution of rDNA in polytene nuclei, characterized by the attachment of micronucleoli to many chromosome regions. The results showed that micronucleoli preferentially associate with intercalary or terminal heterochromatin of all sciarid flies analyzed and, depending on the species, are attached to a few (Trichosia), moderate (Rhynchosciara) or a large (Schwenkfeldina sp.) number of polytene chromosome sites.

Ribosomal RNA genes in mosquitoes: localization by fluorescence in situ hybridization (FISH)

Fluorescence in situ hybridization (FISH) was used to localize the 18S-28S ribosomal RNA gene clusters on the chromosomes of 15 mosquito species belonging to the Anophelinae and Culicinae subfamilies. In the genus Anopheles the rRNA genes are localized on the heterochromatic arm of both sex chromosomes. The association between rRNA genes and sex determining chromosomes also applies to the homomorphic karyotype of Culicinae mosquitoes, at least in those cases in which localization of the sex locus/loci has been determined. In these species ribosomal genes are often found within or adjacent to heterochromatic regions (C bands). Differences in the location of rRNA genes among and within genera suggest the occurrence of several chromosomal rearrangements during the evolution of mosquitoes.

Evidence for differential DNA endoreplication during the development of a molluscan brain

The DNA content of neurons in the cerebral ganglion of Achatina fulica was determined by the two-wavelength method of microspectrophotometry using Feulgen-stained sections. DNA measurements of mouse hepatocytes were used as a control of the methods. All sampled neurons with a nuclear diameter greater than 7 microns were polyploid. The shape of the frequency histogram of DNA contents is not compatible with an interpretation that assumes one or more simple duplications of the genome. It is suggested, instead, that the results are due to either the underreplication of some DNA sequences or the selective amplification of genes. Additional experiments using [3H]-thymidine autoradiography showed that the incidence of neuronal DNA synthesis is highest during the period of the animal's greatest growth and then declines rapidly near the onset of sexual maturity. The mesocerebrum incorporated greater amounts of [3H]-thymidine than the rest of the brain, whereas the procerebrum remained diploid throughout the course of the study.

Allocyclic behaviour and underreplication of the nucleolus chromosome in Pseudodiamesa (Chironomidae)

The chironomid midge Pseudodiamesa branickii shows three polytene, banded elements in salivary gland and Malpighian tubule cells, none of which carry a nucleolar organizer. The present investigation revealed four chromosome pairs in mitotic metaphases, the nucleolus was localized in the tiny chromosome 4. Microphotometric determinations of the DNA contents in mitotic and salivary gland nuclei of Ps. branickii yielded a 2C value of 0.23 pg DNA and a maximum level of 12 endoreplications. In both salivary glands and Malpighian tubules, chromosome 4 appeared as a nonbanded network of chromatin in Feulgen preparations. Indirect immunofluorescent staining with antibodies specific for RNA X DNA hybrids indicated that this structure was transcriptionally active. Its reaction to heavy metal staining suggested that it included the nucleolar organizer, and this was conclusively demonstrated by in situ hybridization with 125I-rRNA. Chromosome pair 4 comprises about 11.4% of the total DNA in metaphase, but only 3.7% in the highly polytene salivary gland complement. According to a numerical simulation, the degree of underrepresentation suggests that about 70% of chromosome 4 is precluded from polytenization. The allocyclic aspect is probably due to this underreplication, with the nucleolar activity causing a structural loosening of the rest of the chromosome. The possible nature of the underreplicated sequences is discussed in the light of recent work on molecular aspects of rDNA structure in various Diptera.

Gene amplification in Rhynchosciara salivary gland chromosomes

Late in the fourth larval instar, several regions of the Rhynchosciara americana salivary gland chromosomes undergo "DNA puffing." We have constructed a library of cloned cDNAs synthesized from poly(A)+RNA isolated from salivary glands during the period of development when the DNA puffs are active. From this library we have studied clones representative of three genes active during this period but not active at earlier developmental periods of the gland. One of these genes is not amplified during the developmental process and encodes a 0.6-kilobase RNA molecule. The other two genes are located within the DNA-puff sites C3 and C8 and encode 1.25-kilobase and 1.95-kilobase RNA molecules, respectively. We estimate from the quantitation of transfer hybridization experiments that each of these genes undergoes 16-fold amplification during DNA puffing.

Localization and characterization of rDNA in Drosophila tumiditarsus

Using in situ nucleic acid hybridizations, the genes that code for 28, 18 and 5S rRNA have been localized in the polytene chromosomes of Drosophila tumiditarsus. The 5S genes are found at a single site near the centromere of the second chromosome, whereas the 28 and 18S genes are found at the nucleolar organizer region of the dot chromosome. The dot chromosome has been previously described as alpha-heterochromatic. However, our cytochemical and autoradiographic results do not support such a conclusion. The autoradiographic results reveal that the dot chromosome is transcriptionally active and is not late-replicating, as is expected of alpha-heterochromatin. Further, the dot chromosomes possess none of the usual staining characteristics of heterochromatin except for its lack of polytene bands. Using rRNA-DNA filter hybridizations, we find that the rDNA of D. tumiditarsus salivary glands is under-replicated. This is the first species of Drosophila where the rDNA in not found on the sex chromosomes, and is the first report of an under-replicated autosomal locus which is not located in heterochromatic blocks.

Further studies on the ribosomal RNA cistrons of Sciara coprophila (Diptera)

Additional experiments with homologous as well as heterologous hybridization confirmed our previous finding in Sciara coprophila that XX females have nearly twice the number of ribosomal RNA cistrons as XO males. A comparison between two different X' chromosomes revealed that only the one carrying the irradiation-induced Wavy mutation has a deletion of 70% of its ribosomal RNA cistrons as compared to the standard X. The deletion is relatively stable, and the remaining ribosomal RNA cistrons donot appear to undergo disproportionate replication or magnification as in Drosophila. Homologous hybridization experiments revealed an unusually low reiteration of ribosomal RNA cistrons in this fly, 45 gene copies per X chromosome. The question is raised as to whether such a low number of cistrons may be related to the unusual nucleolar condition encountered in the Sciaridae.

Independent replication of the ribosomal RNA genes in the polytrophic-meroistic ovaries of Calliphora erythrocephala, Drosophila hydei, and Sarcophaga barbata

By filter saturation hybridizations the ribosomal (r)DNA contents of the ovaries Calliphora erythrocephala, Drosophila hydei, and Sarcophaga barbata have been measured in comparison to the rDNA percentages of their diploid brains. The measurements of the ovarian rDNA have been carried out on ovaries where the nurse cells in the distal egg chamber of the ovarioles had reached their highest ploidy level. The diploid rDNA content of each of the respective species was chosen as a 100% standard and the rDNA amounts of the ovaries were related to this 100% level. The results show that the ovaries of C. erythrocephala contain 135% rDNA whereas the rDNA contents in the ovaries of D. hydei and S. barbata are only 51% and 47%, respectively. Measurements carried out on isolated nuclei of the nurse cells and follicle cells in D. hydei show that both have a reduced rDNA content in comparison to the brains (45% and 70%, respectively). The data are discussed in relation to the problem of an rDNA amplification in the germ cells and an rDNA underreplication in polyploid nuclei.