Repeated sequences in the DNA of Drosophila and their localization in giant chromosomes

In situ hybridization of ribosomal DNA labelled with 125iodine to metaphase and lampbrush chromosomes from newts

Methods are described for in situ hybridization of ribosomal DNA from Xenopus laevis, labelled in vitro with 125iodine, to mitotic and lampbrush chromosomes from Triturus cristatus carnifex. The hybridization reaction was carried out in a mixture containing 50% formamide, 4 X SSC, 0.1 M KI, at 37 degrees C, or in 2 X SSC, 0.1 M KI at 65 degrees C. Autoradiographs of mitotic metaphases from 2 males showed labelling over the middle of the short arm of one chromosome IX in each metaphase. In some cases, a region near the end of a longer chromosome was also labelled. In a lampbrush preparations, labelling was confined to a region identified as about 53 units, near the middle of the short arm of both halves of bivalent IX. The usefulness of the technique and the significance of the labelling of only 1 of the 2 chromosomes IX in mitotic preparations are discussed.

Heterochromatic chromosomes and satellite DNAs of Drosophila nasutoides

Drosophila nasutoides is distinguished from other Drosophila species in that the metaphase karyotype shows a pair of very large V-shaped chromosomes. With Giemsa, a distinctive C-banding pattern is revealed along the arms of this large chromosome, indicating a largely heterochromatic nature. Furthermore, the banding patterns of the arms are symmetrical, indicating that it is an iso-chromosome. A comparison between the metaphase karyotype and polytene chromosomes suggests that the large V chromosome appears as the dot chromosome in polytene squash. One autosome has twice the arm length of typical Drosophila polytene chromosomes and arose either by centric fusion and a pericentric inversion, or by translocation connecting distal ends with a subsequent loss of one centromere. This chromosome appears to have a short arm which ectopically pairs with the proximal region of the long arm, representing a duplication of about ten bands. When the nuclear DNA is examined by neutral CsCl gradient, four satellites are observed. As much as sixty percent of the total DNA appears as satellites in the lysate of larval brains. No satellite was detectable in the lysate of salivary glands. These observations led us to suggest that the heterochromatic nature of the large V chromosome is due to the presence of all four satellites in this chromosome and that this large chromosome appears as the dot because of the under-reduplication of the satellites during polytenization.

Chromosome structure in Chilocorus (Coleoptera: Coccinellidae). I. Fluorescent and Giemsa banding patterns

Chromosomes of six species of Chilocorus have been examined for their reaction to Quinacrine (Q), and to Giemsa (G) after a variety of 'Denaturation-Renaturation' schedules and digestion with trypsin. Four types of chromatin can be distinguished with these techniques: 1, moderately stained with both Q and G; 2, brightly stained with Q and darkly stained with G; 3, unstained with Q but darkly stained with G; 4, unstained with Q but moderately stained with G. The last three of these types are restricted in chromosomally monomorphic species to the pericentric heterochromatin, but are variably distributed in the heterochromatic arms of C. stigma. Euchromatin per se does not react differentially. The relationship between karyotype stability and uniformity of banding patterns is discussed.

Under-replication of ribosomal cistrons in polytene chromosomes of Rhynchosciara

DNA preparations obtained from several tissues of Rhynchosciara americana and two related species, R. milleri and R. papaveroi, were hybridized to R. americana rRNA. The percentage of hybridization was found to be higher in tissues with low polyteny than in tissues with high polyteny, suggesting a relationship between the amount of rDNA and the tissue polyteny. This could be explained by under-replication of ribosomal cistrons in polytene cells, such as those from the salivary gland. Only slight tissue-dependent changes in the percentages of hybridization can be observed in heterologous hybridization using Xenopus laevis rRNA. The possibility that these experiments could not detect differences in the amount of ribosomal cistrons among tissues is discussed. The female:male ratio for the percentages of hybridization in the salivary gland of R. americana agrees with the results obtained by in situ hybridization experiments (16, 17) which have shown that the rRNA cistrons are distributed among chromosomes other than chromosome X.

Multiple gene sites for 5S and 18 plus 28S RNA on chromosomes of Glyptotendipes barbipes (Staeger)

Ribosomal RNAs (28 + 18S and 5S) and 4S RNA extracted from the chironomid Glyptotendipes barbipes were iodinated in vitro with (125)I and hybridized to the salivary gland chromosomes of G. barbipes and Drosophila melanogaster. Iodinated 18 + 28 S RNA labeled three puffed sites with associated nucleoli on chromosomes IR, IIL, and IIIL of G. barbipes and the nucleolar organizer of Drosophila. Labeled 5S RNA hybridized to three sites on chromosome IIIR, two sites on chromosome IIR and one site in a Balbiani ring on chromosome IV of Glyptotendipes. Most of the label produced by this RNA was localized seven bands away from the centromere on the right arm of chromosome III, and we consider this to be the main site complementary to 5S RNA in the chironomid. This same RNA preparation specifically labeled the 56 EF region of chromosome IIR of Drosophila which has been shown previously to be the only site labeled when hybridized with homologous 5S RNA. Hybridization of G. barbipes chromosomes with iodinated 4S RNA produced no clearly localized labeled sites over the exposure periods studied.

Relatedness of mouse satellite deoxyribonucleic acid to deoxyribonucleic acid of various Mus species

Mouse (Mus musculus musculus) satellite DNA is able to reassociate with repeated DNA sequences of Mus caroli and Mus cervicolor, but low thermal stability of the products indicates significant differences between satellite and related DNAs of these two Mus species. There appear to be several satellite-related populations in M. caroli DNA, each of which forms hybrids of low thermal stability with repeated sequences of M. cervicolor DNA. DNAs from the subspecies Mus musculus molossinus and Mus musculus castaneus reassociate with mouse satellite to form hybrids of very high thermal stability, but the satellite content of M. m. musculus DNA is only about 60% that of M. m. musculus DNA. Reassociation of M. m. musculus nonrepeated DNA with M. m. molossinus DNA reveals no detectable differences between them; reassociation with M. caroli (or M. cervicolor) DNA yields a product whose melting temperature depression relative to homologous DNA is about 5 degrees .

A family of three related satellite DNAs in Drosophila virilis

Isolated single DNA strands of satellites II or III of Drosophila virilis form hybrid duplexes with the complementary single strands of satellite I. The hybrids denature at a higher temperature than controls, renature rapidly, and form bands of hybrid density in neutral CsCl. Isolated single strands of satellite II do not form clear-cut duplexes with the complementary strands of satellite III. Mixtures of satellites II and III denature at the same temperature as controls, and do not form bands in neutral CsCl. Therefore, satellite I-II and I-III complexes are extensively base paired, while satellite II-III complexes are minimally base paired. These experiments demonstrate partial homology among the three satellites, and suggest an evolutionary relationship among them. They also suggest that satellites and other repeated sequences from related species, which do not form hybrids in vitro, could have a common evolutionary origin, but have accumulated enough base substitutions to lose interspecific homology demonstrable by hybrid duplex formation.

Repeated DNA sequences in the heterochromatic Y chromosome of adult Drosophila melanogaster

DNA isolated from Drosophila melanogaster adults (XX, XY, XXY, XYY) was used in DNA-DNA hybridization experiments on nitrocellulose filters. Filter-bound DNA of "high-heterochromatin" flies (those with one or more Y chromosomes) is more effective in forming hybrid duplexes than is XX DNA. The quantitative difference in hybridization efficiency is due primarily to molecules with a relatively high thermal stability (duplexes that dissociate in the temperature range 80-90 degrees ). Hybridization experiments with DNA samples that have been fractionated into reiterated and unique portions show that the majority of the hybrid duplexes formed involve reiterated DNA. A small, but highly specific, interaction of unique DNA sequences has also been detected in our experiments. These data indicate that a class of repeated DNA sequences is associated with the constitutive heterochromatin, specifically with the Y chromosome of D. melanogaster. Evidence is presented that this same class of molecules, or very similar ones, is to be found in other portions of the genome, presumably in X-chromosomal or centromeric heterochromatin.

Satellite DNAs in the embryos of various species of the genus Drosophila

A tentative evolutionary pattern has been found for two classes of the multiple satellite DNA's found in the genus Drosophila. The satellite DNA's from five Drosophila species (D. melanogaster, D. simulans, D. nasuta, D. virilis and D. hydei) were analyzed and found to fall into three arbitrary CsCl buoyant density classes: Class I, rho = 1.661-1.669 g cm(-3), DNA molecules composed of primarily dA and dT moieties; Class II, rho = 1.685 and rho = 1.692, DNA molecules of low GC content; and Class III, rho = 1.711, a DNA of high GC composition. The dAT satellite DNA's appear in all the species studied except D. hydei, the species of most recent evolutionary divergence, whereas the heavy satellite appears only in the two species of most recent divergence, D. virilis and D. hydei.

In situ demonstration of DNA hybridizing with chromosomal and nuclear sap RNA in Chironomus tentans

Cytological hybridization combined with microdissection of Chironomus tentans salivary gland cells was used to locate DNA complementary to newly synthesized RNA from chromosomes and nuclear sap and from a single chromosomal puff, the Balbiani ring 2 (BR 2). Salivary glands were incubated with tritiated nucleosides. The labeled RNA was extracted from microdissected nuclei and hybridized to denatured squash preparations of salivary gland cells under conditions which primarily allow repeated sequences to interact. The bound RNA, resistant to ribonuclease treatment, was detected radioautographically. It was found that BR 2 RNA hybridizes specifically with the BR 2 region of chromosome IV. Nuclear sap RNA was fractionated into high and low molecular-weight RNA; the former hybridizes with the BR 2 region of chromosome IV, the latter in a diffuse distribution over the whole chromosome set. RNA from chromosome I hybridizes diffusely with all chromosomes. Nucleolar RNA hybridizes specifically with the nucleolar organizers, contained in chromosomes II and III. It is concluded that the BR 2 region of chromosome IV contains repeated DNA sequences and that nuclear sap contains BR 2 RNA.

Heterochromatin, satellite DNA, and cell function. Structural DNA of eucaryotes may support and protect genes and aid in speciation

With the assumption that a portion that comprises some 10 percent of the genomes in higher organisms cannot be without a raison d'être, an extensive review led us to conclude that a certain amount of constitutive heterochromatin is essential in multicellular organisms at two levels of organization, chromosomal and nuclear. At the chromosomal level, constitutive heterochromatin is present around vital areas within the chromosomes. Around the centromeres, for example, heterochromatin is believed to confer protection and strength to the centromeric chromatin. Around secondary constrictions, heterochromatic blocks may ensure against evolutionary change of ribosomal cistrons by decreasing the frequency of crossing-over in these cistrons in meiosis and absorbing the effects of mutagenic agents. During meiosis heterochromatin may aid in the initial alignment of chromosomes prior to synapsis and may facilitate speciation by allowing chromosomal rearrangement and providing, through the species specificity of its DNA, barriers against cross-fertilization. At the nuclear level of organization, constitutive heterochromatin may help maintain the proper spatial relationships necessary for the efficient operation of the cell through the stages of mitosis and meiosis. In the unicellular procaryotes, the presence of a small amount of genetic information in one chromosome obviates the need for constitutive heterochromatin and a nuclear membrane. At higher levels of organization, with an increase in the size of the genome and with evolution of cellular and sexual differentiation, the need for compartmentalization and structural components in the nucleus became imminent. The portion of the genome that was concerned with synthesis of ribosomal RNA was enlarged and localized in specific chromosomes, and the centromere became part of each chromosome when the mitotic spindle was developed in evolution. Concomitant with these changes in the genome, repetitive sequences in the form of constitutive heterochromatin appeared, probably as a result of large-scale duplication. The repetitive DNA's were kept through natural selection because of their importance in preserving these vital regions and in maintaining the structural and functional integrity of the nucleus. The association of satellite (or highly repetitive) DNA with constitutive heterochromatin is understandable, since it stresses the importance of the structural rather than transcriptional roles of these entities. Nuclear satellite DNA's have one property in common despite their species specificity, namely heterochromatization. In this sense the apparent species specificity of satellite DNA may be the result of natural selection for duplicated short polynucleotide segments that are nontranscriptional and can be utilized in specific structural roles.