Constitutive heterochromatin of Indian muntjac chromosomes revealed by DNAse treatment and a C-banding technique

5-Methylcytosine-Rich Heterochromatin in the Indian Muntjac

Two 5-methylcytosine (5-MeC)-rich heterochromatic regions were demonstrated in metaphase chromosomes of the Indian muntjac by indirect immunofluorescence using a monoclonal anti-5-MeC antibody. The metaphases were obtained from diploid and triploid cell lines. A major region is located in the 'neck' of the 3;X fusion chromosome and can be detected after denaturation of the chromosomal DNA with UV-light irradiation for 1 h. It is located exactly at the border of the X chromosome and the translocated autosome 3. A minor region is found in the centromeric region of the free autosome 3 after denaturing the chromosomal DNA for 3 h or longer. The structure and possible function of the major hypermethylated region as barrier against spreading of the X-inactivation process into the autosome 3 is discussed.

DNA content measurements and an improved idiogram for the Indian muntjac

The Indian muntjac, an asiatic deer, has the lowest diploid chromosome number among mammals (female 2N = 6; male 2N = 7). Using flow cytometric quantification of propidium iodide-stained cells, we determined the DNA content of muntjac cells to be 94% that of human. This suggests that the muntjac may serve as a model for investigation of karyotypic evolution and rearrangement. In order to facilitate future comparative gene mapping studies, computer-aided analysis of digitized metaphase chromosomes allowed development of a detailed Indian muntjac G-banded idiogram incorporating both ISCN-type nomenclature and quantitative estimates of the size of each band and position.

Longitudinal differentiation of metaphase chromosomes of Indian muntjac as studied by restriction enzyme digestion, in situ hybridization with cloned DNA probes and distamycin A plus DAPI fluorescence staining

The longitudinal differentiation of metaphase chromosomes of the Indian muntjac was studied by digestion with restriction enzymes, in situ hybridization with cloned DNA probes and distamycin A plus DAPI (4'-6-diamidino-2-phenylindole) fluorescence staining. The centromeric regions of chromosomes 3 and 3 + X of a male Indian muntjac cell line were distinct from each other and different from those of other chromosomes. Digestion with a combination of EcoRI and Sau3A revealed a pattern corresponding to that of C-banding. Digestion with AluI, EcoRII or RsaI yielded a band specific to the centromeric region only in chromosomes 3 and 3 + X. Furthermore, HinfI digestion yielded only a band at the centromeric region of chromosome 3, whereas DA-DAPI staining revealed a single band limited to the extreme end of the C-band heterochromatin of the short arm of 3 + X. These results suggest that centromeres of Indian muntjac chromosomes contain at least four different types of repetitive DNA. Such diversity in heterochromatin was also confirmed by in situ hybridization using specific DNA probes isolated and cloned from highly repetitive DNA families. Heterozygosity between chromosome homologs was revealed by restriction enzyme banding. Evidence is presented for the presence of nucleolus organizer regions (NORs) on the long arm of chromosome 1 as well as on the secondary constrictions of 3 and 3 + X.

Immunological visualization of 5-methylcytosine-rich regions in Indian Muntjac metaphase chromosomes

Regions rich in 5-methylcytosine were localized in male metaphase chromosomes of the Indian muntjac deer (Muntiakus muntjak). Chromosomes were ultraviolet irradiated and subsequently photooxidized in the presence of methylene blue to induce maximum DNA denaturation. Following treatment with anti 5-methylcytosine antibody (anti 5-MeC), regions of antibody binding were visualized by an immunofluorescence or immunopreoxidase staining procedure. All chromosomes showed some level of antibody binding along their length and at centromeric regions, with intense binding evident in the centromere of chromosome 3 and the elongated centromeric "neck" of chromosome 3-X. The Y chromosome displayed low levels of antibody binding. The banding pattern observed with anti 5-MeC is the reverse of that obtained by quinacrine staining.

Kinetics of DNA replication in the Indian muntjac chromosomes as studied by quantitative autoradiography

DNA replication patterns of individual chromosomes and their various euchromatic and heterochromatic regions were analyzed by means of quantitative autoradiography. The cultured cells of the skin fibroblast of a male Indian muntjac were pulse labeled with 3H-thymidine and chromosome samples were prepared for the next 32 h at 1--2 h intervals. A typical late replication pattern widely observed in heterochromatin was not found in the muntjac chromosomes. The following points make the DNA replication of the muntjac chromosomes characteristics: (1) Heterochromatin replicated its DNA in a shorter period with a higher rate than euchromatin. (2) Two small euchromatic regions adjacent to centromeric heterochromatin behaved differently from other portions of euchromatin, possessing shorter Ts, higher DNA synthetic rates and starting much later and ending earlier their DNA replication. (3) Segmental replication patterns were observed in the chromosomes 2 and 3 during the entire S phase. (4) Both homologues of the chromosome 3 showed a synchronous DNA replication pattern throughout the S phase except in the distal portion of the long arms during the mid-S phase.

Cytological studies of heterochromatin function in the Drosophila melanogaster male: autosomal meiotic paring

In Drosophila melanogaster it is now documented that the different satellite DNA sequences make up the majority of the centromeric heterochromatin of all chromosomes. The most popular hypothesis on this class of DNA is that satellite DNA itself is important to the pairing processes of chromosomes. Evidence in support of such a hypothesis is, however, circumstantial. This hypothesis has been evaluated by direct cytological examination of the meiotic behaviour of heterochromatically and/or euchromatically rear-ranged autosomes in the male. It was found that neither substantial deletions nor rearrangements of the autosomal heterochromatin cause any disruption of meiotic pairing. Autosomal pairing depends on homologs retaining sufficient euchromatic homology. This is the first clear demonstration that the highly repeated satellite DNA sequences in the heterochromatin of the second, third and fourth chromosomes are not important in meiotic pairing, but rather than some euchromatic homology in the autosome is essential to ensure a regular meiotic process. These results on the autosomes, when taken in conjunction with our previous studies on sex chromosome pairing, clearly indicate that satellite DNA is not crucial for male meiotic chromosome pairing of any member of the D. melanogaster genome.

Euploid somatic recombinants with two active X or XY (1)Y(I) chromosomes isolated from cultured male Indian Muntjac cells after HVJ virus fusion, and their use for gene assignment

Four diploid somatic recombinants were isolated from hybrids either between or within two diploid cell lines of a male Indian muntjac after HVJ virus-mediated cell fusion. Both parental lines had a normal male karyotype, 7,X,Y1,Y2, in which the largest autosomal pair was heteromorphic with respect to the size of the secondary constriction (1h+/1h-), C bands, and nucleolar organizers. Of the four recombinants, three showed a 6,XX,1h+/1h+ or 1h+/1h- karyotype, the remaining one a 7,XY1Y2,1h+/1h+. No late-replicating X chromosome was found in the XX recombinants, although it was demonstrated in the natural XX line, suggesting the presence of two active X chromosomes in the former. The G6PD, PGK, and HPRT activities were proportional to the number of active X chromosomes present in all cell types examined, indicating their X-linkage, whereas the same level of activities obtained for LDH and 6PGD indicated their autosomal linkage.

Visualization of nucleolar organizer regions im mammalian chromosomes using silver staining

A simple ammoniacal silver staining procedure, designated Ag-AS, differentially stains the chromosomal locations of ribosomal DNA in certain mammalian species. This was critically demonstrated by Ag-AS staining of the nucleolus organizer regions in karyotypes of the same species and cell lines used for locating the ribosomal cistrons by DNA/RNA in situ hybridization. With Ag-AS, silver stained NORs (Ag-NORs) are visualized as black spherical bodies on yellow-brown chromosome arms. Ag-NORs were visualized throughout mitosis at the secondary constrictions in the rat kangaroo, Seba's fruit bat, Indian muntjac, and Rhesus monkey. The Chinese hamster and cattle have telomeric Ag-NORs, the mouse subcentromeric Ag-NORs, and the field vole Ag-NORs as minute short arms or choromosomal satellites. Ag-NORs occur at both secondary constrictions and at telomeres in the cotton rat. Variability in Ag-NOR pattern included differences in the number of Ag-NORs per cell within a cell population, size of Ag-NORs among chromosomes of a complement, and presence of Ag-NOR on particular chromosomes in two cell lines of the Chinese hamster. The available cytochemical data suggest that the Ag-AS reaction stains chromosomal proteins at the NOR rather than the rDNA itself.

Distribution of 18+28S ribosomal genes in mammalian genomes

In situ hybridization with 3H 18S and 28S ribosomal RNA from Xenopus laevis has been used to study the distribution of DNA sequences coding for these RNAs (the nucleolus organizing regions) in the genomes of six mammals. Several patterns of distribution have been found: 1) A single major site (rat kangaroo, Seba's fruit bat), 2) Two major sites (Indian muntjac), 3) Multiple sites in centromeric heterochromatin (field vole), 4) Multiple sites in heterochromatic short arms (Peromyscus eremicus), 5) Multiple sites in telomeric regions (Chinese hamster). - The chromosomal sites which bind 3H 18S and 28S ribosomal RNA correspond closely to the sites of secondary constrictions where these are known. However, the correlation is not absolute. Some secondary constrictions do not appear to bind 3H ribosomal RNA. Some regions which bind ribosomal RNA do not appear as secondary constrictions in metaphase chromosomes. - Although the nucleolus organizing regions of most mammalian karyotypes are found on the autosomes, the X chromosomes in Carollia perspicillata and C. castanea carry large clusters of sequences complementary to ribosomal RNA. In situ hybridization shows that the Y chromosome in C. castanea also has a large nucleolus organizing region.

Variation of C-bands in the chromosomes of several subspecies of Rattus rattus

All subspecies of black rats (Rattus rattus) used in the present study are characterized by having large and clear C-bands at the centromeric region. The appearance of the bands, however, is different in the subspecies. Chromosome pair No. 1 in Asian type black rats (2n=42), which are characterized by an acrocentric and subtelocentric polymorphism, showed C-band polymorphism. In Phillipine rats (R. rattus mindanensis) the pair was subtelocentric with C-bands, but in Malayan black rats (R. rattus diardii) it was usually acrocentric with C-bands. In Hong-Kong (R. rattus flavipectus) and Japanese black rats (R. rattus tanezumi) it was polymorphic with respect to the presence of acrocentrics with C-bands or subtelocentrics without C-bands. The other chromosomes pairs showed clear C-bands, but in Hong-Kong black rats the pairs No. 2 and 5 were polymorphic with and without C-bands. In Japanese black rats, 6 chromosome pairs (No. 3, 4, 7, 9, 11 and 13) were polymorphic in regard to presence and absence of C-bands, but the other 5 chromosome pairs (No. 2, 5, 6, 8 and 10) showed always absence of C-bands. Only pair No. 12 usually showed C-bands. C-bands in small metacentric pairs (No. 14 to 20) in Asian type black rats generally large in size, but those in the Oceanian (2n=38) and Ceylon type black rats (2n=40) were small. In the hybrids between Asian and Oceanian type rats, heteromorphic C-bands, one large and the other small, were observed. Based on the consideration of karyotype evolution in the black rats, the C-band is suggested to have a tendency toward the diminution as far as the related species are concerned.