[DNA reduplication pattern in the giant sex chromosomes of Microtus agrestis]

The behavior during pachynema of a normal and an inverted Y chromosome in Microtus agrestis

The pachytene behavior of the chromosomes of Microtus agrestis (L.) (Rodentia, Arvicolidae) males carrying either the standard, or the pericentrically inverted Lund Y chromosome have been examined by electron microscopy of microspread spermatocytes. There is no synapsis between the X and either the standard or the Lund Y chromosomes during any substage of pachynema. Since synapsis is generally considered a prerequisite for crossing over, there appears to be no opportunity for crossover or chiasma formation between the X and Y in this species. The G-, C- and NOR-banded mitotic karyotypes of animals carrying the standard and Lund Y are also presented.

Quantitative investigation of reproduction of gonosomal condensed chromatin during trophoblast cell polyploidization and endoreduplication in the East-European field vole Microtus rossiaemeridionalis

Simultaneous determinations of DNA content in cell nuclei and condensed chromatin bodies formed by heterochromatized regions of sex chromosomes (gonosomal chromatin bodies, GCB) have been performed in two trophoblast cell populations of the East-European field vole Microtus rossiaemeridionalis: in the proliferative population of trophoblast cells of the junctional zone of placenta and in the secondary giant trophoblast cells. One or two GCBs have been observed in trophoblast cell nuclei of all embryos studied (perhaps both male and female). In the proliferative trophoblast cell population characterized by low ploidy levels (2-16c) and in the highly polyploid population of secondary giant trophoblast cells (32-256c) the total DNA content in GCB increased proportionally to the ploidy level. In individual GCBs the DNA content also rose proportionally to the ploidy level in nuclei both with one and with two GCBs in both trophoblast cell populations. Some increase in percentage of nuclei with 2-3 GCBs was shown in nuclei of the placenta junctional zone; this may be accounted for by genome multiplication via uncompleted mitoses. In nuclei of the secondary giant trophoblast cells (16-256c) the number of GCBs did not exceed 2, and the fraction of nuclei with two GCBs did not increase, which suggests the polytene nature of sex chromosomes in these cells. In all classes of ploidy the DNA content in trophoblast cell nuclei with the single GCB was lower than in nuclei with two and more GCBs. This can indicate that the single GCB in many cases does not derive from fusion of two GCBs. The measurements in individual GCBs suggest that different heterochromatized regions of the X- and Y-chromosome may contribute in GCB formation.

The origin and distribution of the Lund Y chromosome in Microtus agrestis (Rodentia, Mammalia)

The Lund Y (Lu-Y) chromosome of the field vole (Microtus agrestis) is distinguished from the standard Y (St-Y) by its much longer short arm. G-banding revealed that the Lu-Y originated by a pericentric inversion in the St-Y. Chromosome analysis of 297 male field voles from 92 localities in Fennoscandia. Germany, and England, in addition to data from the literature, made it possible to map the distribution area of the Lu-Y. It is restricted to the south-western parts of Sweden. The question of when and where the Lund Y population originated is discussed. Adding data from a hybrid zone (Jaarola et al. 1997) and from females, totally 491 specimens from 120 localities were analyzed without detecting any variation in chromosome number and autosome morphology. Other cases of intraspecific Y chromosome polymorphism in mammals, and the use of Y chromosome variants as population genetic markers, are discussed.

5-Azadeoxycytidine induced undercondensation in the giant X chromosomes of Microtus agrestis

Fibroblasts of female Microtus agrestis were treated with 5-azadeoxycytidine (5-aza-dCyd) at a final concentration of 10(-5) M during the last 2 h of culture. This cytidine analogue induces distinct undercondensation of the constitutive heterochromatin in the giant X chromosomes. The undercondensed heterochromatic thread exhibits longitudinal segmentation reminiscent of a chromomere pattern. In the late-replicating X chromosome, 5-aza-dCyd also inhibits condensation of the genetically inactivated euchromatin (facultative heterochromatin). The described effects of 5-aza-dCyd on the X chromosome structure appear to be incorporation independent.

Transcriptional activity of constitutive heterochromatin in the mammal Microtus agrestis (Rodentia, Cricetidae)

Two cytological approaches were applied to the analysis of genetic activity in euchromatin and heterochromatin in the European field vole Microtus agrestis. The first is based on the transcriptional activity of prematurely condensed chromosomes, the second on in situ hybridization of labeled cellular RNA to metaphase chromosomes. The results show that in cultivated fibroblasts, the transcriptional activity of constitutive heterochromatin is in the same order of magnitude as that of euchromatin; its chromosomal pattern is nonrandom. It is concluded that--as has already been shown for insects and amphibians--also in mammals, constitutive heterochromatin is transcribed in its decondensed state but is genetically inactive when forming chromocenters at interphase.

The human inactivated X chromosome folds in early metaphase, prometaphase, and prophase

The inactivated X chromosome has a site of unusually frequent folding in region Xq1, whereas a fold in Xq1 is uncommon on the active X. We investigated the pattern of X chromosome folding in high-resolution GTG- and RBG-stained preparations from four women. In early metaphase cells, slightly more than 50% of late-replicating Xs folded at Xq1----Xq21, compared with about 6% of early replicating Xs. The late-replicating X folded in about 80% of prometaphase cells; the early, in only about 14% of these cells. and the late-replicating X folded in 19 of 20 prophase cells. Occasionally, one X had an omega-shaped loop or apparent physical connection between Xq13 and Xq21.1. It is possible that a segment of Xq1 never completely uncoils and may help to provide continuity for the Barr body from one interphase to the next.

DNase I sensitivity in facultative and constitutive heterochromatin

In situ nick translation allows the detection of DNase I sensitive and insensitive regions in fixed mammalian mitotic chromosomes. We have determined the difference in DNase I sensitivity between the active and inactive X chromosomes in Microtus agrestis (rodent) cells, along both their euchromatic and constitutive heterochromatic regions. In addition, we analysed the DNase I sensitivity of the constitutive heterochromatic regions in mouse chromosomes. In Microtus agrestis female cells the active X chromosome is sensitive to DNase I along its euchromatic region while the inactive X chromosome is insensitive except for an early replicating region at its distal end. The late replicating constitutive heterochromatic regions, however, in both the active and inactive X chromosome are sensitive to DNase I. In mouse cells on the other hand, the constitutive heterochromatin is insensitive to DNase I both in mitotic chromosomes and interphase nuclei.

Labelling of DNA and differential sister chromatid staining after BrdU treatment in vivo

A method of labelling DNA in vivo with 5-bromodeoxyuridine (BrdU) is described. After 6 h permanent subcutaneous infusion of BrdU in rodents (adult Microtus agrestis, pregnant NMRI-mice), cell nuclei which have undergone DNA synthesis during the BrdU treatment can be differentiated from the nuclei of other cycle stages by means of their altered staining behaviour after Giemsa. 24 h after the BrdU treatment, mitoses from both bone marrow of the adult animals and tissues from the fetuses showed a differential sister chromatid staining. In male M. agrestis, sister chromatid exchanges were most frequently found in the euchromatic part of the X and in the constitutive heterochromatin of both sex chromosomes.

Heteropyknosis of the chromosomes in liver cells of different ploidy: a nuclear image study

The chromatin densities of Feulgen-stained diploid, tetraploid, octoploid and binucleate cells of smear preparations of the liver of female field voles, Microtus agrestis, were examined by means of image analysis. The ratio of the areas of flattened nuclei of 2n, 4n and 8n ploidy was about 1 : 1.62 :2.60 and that of the relative DNA content 1 : 2 : 4. In flattened polyploid nuclei, the chromosomes are more densely arranged than in diploid. In diploid nuclei, absolutely and percentually smaller areas of dark chromatin particles were found than in polyploid nuclei. After correction of the grey values with a density factor, the frequency distribution curves of all ploidy classes were found to be nearly identical. The results show that the percentage of heteropyknotic chromosome material is the same in diploid, polyploid and binucleate liver cells.

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.

Mammalian X-chromosomes: change in patterns of DNA replication during embryogenesis

One arm of both X-chromosomes in female eight-cell embryos of the golden hamster replicates late in the period of DNA synthesis. Midgestation embryos and adult fibroblasts show an increase in late-replicating DNA. Here, one X-chromosome is labeled in one arm; the other is labeled throughout.