Inheritance of nuclear DNA content in leaf epidermal cells of Zea mays L.

 The nuclear DNA content (ploidy level) of maize leaf-epidermal cells was investigated by Feulgen cytophotometry in two lines, Illinois High Protein (IHP) and Illinois Low Protein (ILP), their reciprocal hybrids, and their F₂s. Epidermal cells have a 2C, 4C or 8C nuclear DNA content. The mean DNA content per nucleus in IHP was significantly higher than in ILP; the mean DNA content per nucleus in hybrids was intermediate between the parental lines, and the same DNA content was measured in reciprocal crosses. In F₂s the same mean DNA content as in F₁s was observed but with larger variability than in the F₁, possibly indicating genetic segregation. It is inferred that the ploidy level in the leaf epidermis is inherited, and incomplete dominance occurs in hybrids. The same behaviour in the different genotypes was observed for epidermal cell-surface area, except that an increase of mean surface area occurred in the F₁, probably due to heterotic effects. The difference in the accumulation of 4C and 8C nuclei in leaf epidermis parallels that reported between two genotypes for the endosperm tissue: to the greater chromosome endoreduplication found in the endosperm there were correspondingly higher frequencies of 4C and 8C nuclei in the leaf epidermis, indicating a higher general tendency to chromosome endoreduplication in IHP than in ILP. It is suggested that the accumulation of 4C nuclei (G₂-block) in the leaf epidermis may be regarded as the initial step of chromosome endoreduplication, the two phenomena being related to the control of the sequence DNA synthesis-mitosis, possibly involving the same genes in both endosperm and leaf. However, the inheritance of DNA content per nucleus in epidermal tissue seems to be different from that observed in endosperm tissue of the same genotypes, suggesting that differences may occur in the regulation of the activity of these genes. Received: 19 November 1996 / Accepted: 29 November 1996     

Nuclear size and DNA content of the embryo and endosperm during their initial stages of development in Glycine max (Fabaceae)

A technique was developed for isolating embryo sacs from ovules of soybean and for separating embryo from endosperm. Image analysis and cytophotometry were used to determine the relative mass of DNA and size of nuclei of endosperm and embryo cells. Analyses were done at the globular through late heart-shaped embryo stages to correlate ploidy level or nuclear size, and differentiation in these tissues. Mean size of embryo nuclei was fairly constant through all stages studied. Ploidy condition of the embryo was stable, 95%–99% of the nuclei were distributed in a bipolar pattern by relative mass at 2C and 4C. Few embryo nuclei (3%) had ploidy levels above 4C at the late heart-shaped embryo stage. Variability in size of endosperm nuclei seemed correlated with the morphological state of these nuclei (free-nuclear vs. cellular). Most endosperm cells did not show significant polyploidy with 84%–92% of nuclei in the expected 3C–6C range, but some nuclei with elevated ploidy levels were noted during endosperm cellularization. Endosperm senescence was correlated with nuclear DNA loss over time. Polyploidy seems to have no direct role in the early differentiation of the soybean embryo and endosperm, but these stable conditions may be necessary for the early establishment of the embryo.     

Analysis of chromatin and DNA during chromosome endoreduplication in the endosperm ofTriticum durum Desf.

Summary Chromatin structure was studied in nuclei of the endosperm of durum wheat (Triticum durum Desf., cv. Creso), where a large number of cells undergo chromosome endoreduplication during caryopsis development. Optical density profiles of interphase nuclei at different ploidy levels after Feulgen staining were determined cytophotometrically. It was observed that, within each development stage, polyploid nuclei (6–12C and 12–24C) show more condensed chromatin than euploid nuclei (3–6C): this should indicate that endoreduplication is accompanied by some reduction of nuclear activity. Within the same ploidy level, 3–6C and 6–12C nuclei become increasingly condensed with development (except for the last stage), while 12-24C nuclei are identical at all stages. DNA methylation at different stages of caryopsis development was then analyzed in genomic DNA, highly repeated sequences and ribosomal DNA, by digestion with cytosine-methylation-sensitive restriction enzymes. We observed that (i), depending on the enzyme, DNA from caryopses may show higher mean length than DNA from shoot apices and variations occur during endosperm development; (ii) highly repeated DNA sequences also show some variation in base methylation between apices and endosperms and among endosperm development stages, even though to a lesser extent than genomic DNA; (iii) rDNA shows variations only between endosperm and apices while no variation was observed among endosperm development stages in relation to chromosome endoreduplication. Our data may be explained by assuming the occurrence, during endosperm development, of processes of chromatin condensation possibly involved in silencing the activity of extra copies of DNA resulting from chromosome endoreduplication. At least in part, DNA methylation is involved in the process of chromatin condensation. rDNA shows no variation during endosperm development: this suggests that rDNA copies are actively transcribed in both triploid and endoreduplicated nuclei.     

Unexpected DNA content in the endosperm of Cupressus dupreziana A. Camus seeds and its implications in the reproductive process

 Nuclear DNA content of embryo and endosperm from mature and immature Cupressus dupreziana A. Camus seeds was estimated using laser flow cytometry. Relative DNA content of endosperm nuclei corresponded to four ploidy levels: 2C, 4C, 6C and 8C. The embryo nuclei invariably exhibited a diploid pattern. In all endosperm tissue analyzed no haploid nucleus was found. This is problematic since, in gymnosperms, endosperm and female gametes originate from one functional haploid megaspore produced by meiosis. The possible origin and derivation of C. dupreziana endosperm are discussed in light of previous results concerning the two other Mediterranean cypresses, C. sempervirens and C. atlantica. Received: 15 January 1998 / Revision accepted: 27 March 1998     

Flow cytometric evidence for multiple ploidy levels in the endosperm of some gymnosperm species

 Considered to be haploid tissue, the endosperm of coniferous trees has been extensively used by forest geneticists. Using laser flow cytometry, we show that endosperm ploidy level depends on the systematic position. The Abies, Cedrus and Pinus species tested exhibited uniform haploid endosperm compared to the diploid DNA content of the corresponding embryo. Endosperm of Cupressaceae contained multiple ploidy levels: Cupressus arizonica, Juniperus oxycedrus and Thuja orientalis endosperms exhibited a mixture of haploid–diploid nuclei, while C. atlantica and C. sempervirens endosperms contained six ploidy levels: 1C, 2C, 3C, 4C, 5C and 6C. Physiological and genetic implications of this original feature are discussed. Received: 17 August 1996 / Accepted: 18 October 1996     

Endoreduplication of nuclear DNA in the developing maize endosperm

Multiparametric flow cytometry was used to analyze the development of the endosperm in Zea mays L. during the period from 8 to 20 days after pollination (dap). Nuclear size, DNA content per nucleus, and frequencies of nuclei with varying properties were measured in preparations that included all of the endosperm nuclei of single kernels of the inbred strain Al88. Characteristics of nuclear populations from different kernels on the same ear showed minimal variation. The dynamic changes of non-mitotic cells involved in endosperm development consisted of alternating periods of DNA replication with non-replication. Seven rounds of DNA replication had occurred in some nuclei in the later developmental stages with the rate averaging approximately one round per 24-hour period. Analysis of the DNA levels in the nuclei showed an exact doubling pattern indicating an endoreduplication process, that is, replication of the entire genome during each round. The loosely organized polytenization of the chromatin occurred to varying extents among the nuclei within an endosperm. A weak positive correlation existed between DNA content and size of nuclei suggesting that DNA increases and nuclear growth may not be highly coordinated in this tissue. Increased proportions of the larger nuclei occurred in the later stages of endosperm development. Considering the entire endosperm, the average DNA content per nucleus at the 15-dap peak level was approximately 12.8 C constituting a 2.7-fold overall increase from 8 dap.     

Quantitative investigation of reproduction of gonosomal condensed chromatin during trophoblast cell polyploidization and endoreduplication in the east-european field vole <Emphasis Type="Italic">Microtus rossiaemeridionalis</Emphasis>

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.     

Endopolyploidy in vertebrate liver: An evolutionary perspective

Hepatocyte nuclei in several species of vertebrates were examined, to establish the frequency of polyploidy and related parameters along evolutionary lines. Nuclei were compared in terms of volume, DNA content, ploidy ranges and internal organization. Several trends emerged. When present, heterochromatin occupied 20–25% of nuclear profile areas, irrespective of nuclear volume and ploidy; the volume of heterochromatin, however, increased in direct proportion to ploidy level. Regardless of internal organization, ploidy and species, a direct correlation emerged between the volumes of nuclei and their absolute DNA content. Results are discussed in terms of structural and genic DNA.     

DNA content of the nuclei of secondary giant cells of the rat trophoblast at different phases of the polytene nucleus cycle

A cytophotometric study of DNA content has been made for secondary trophoblastic giant cells, which differ morphologically in relation to the stage of the cycle of the polytene nucleus. The ploidy rate varying from 16c to 512c. It is shown that the DNA content of the nuclei with polytene chromosomes in phase G is more stable, corresponding to the 2c multiple DNA content. Unlike, reticular nuclei in phase S do not present clear-cut peaks on a histogram of DNA. Ratios of nuclei with unequal ploidy differ depending on the structure of these nuclei.     

Changes in Nuclear DNA Content of Endosperm Cells During Grain Development in Rice (Oryza sativa)

Morphological, cytological and quantitative DNA changes associatedwith endosperm development in rice caryopsis were investigated.Following a brief free-nuclear phase, the endosperm became cellularby the 4th d after anthesis. While the mean length and breadthof grain attained maximum values at about 10, d after anthesis,f. wt of the whole grain, and of the endosperm separately, continuedto increase until about 16 d after anthesis. Cell divisionsin the endosperm continued until 10 d and following stabilizationof the cell number, the nuclei attained irregular shapes. Thesize of the nuclei and nuclei and the amount of nuclear DNAvaried considerably during endosperm development. The endospermnuclei did not retain the expected 3C–6C DNA level afterthe first few rounds of division and nuclei having more than30C DNA were frequent 8 d past anthesis. The highest C valuerecorded was 74C in a 16-d-old endosperm cell. Oryza sativa, rice, caryopsis, endosperm, cell number, nuclear area, nuclear DNA content, endoreduplication     

Cell Production and DNA Accumulation in the Wheat Endosperm, and their Association with Grain Weight

Nuclear DNA content was measured in developing endosperm cellsof two wheat varieties, Chinese Spring and Spica. 3C, 6C, 12Cand 24C nuclei were detected, indicating that some form of endoreduplicationand/or endopolyploidization was occurring. The total amountof DNA in the endosperm continued to increase until 24 dayspost anthesis. This accumulation of DNA resulted both from productionof new nuclei and also from increases in the DNA content ofexisting nuclei. Estimates of endosperm cell numbers were made from the totalDNA content per endosperm and the mean DNA content per endospermnucleus for a range of genotypes differing in mature grain weight.Endosperm DNA content and cell number were both positively associatedwith mature grain weight among the genotypes examined. However,not all of the variation in grain weight could be attributedto variation in cell number because of differences in mean dryweight per endosperm cell. The large-grained variety, Spica, had a greater mean weightper endosperm cell than Chinese Spring and this difference aroseafter cell production in the endosperm had ceased. Triticum aestivum, grain weight, cell size, cell number, DNA content     

Nuclear fusions contribute to polyploidization of the gigantic nuclei in the chalazal endosperm of<Emphasis Type="Italic"> Arabidopsis</Emphasis>

Somatic polyploidization is recognized as a means to increase gene expression levels in highly active metabolic cells. The most common mechanisms are endoreplication, endomitosis and cell fusion. In animals and plants the nuclei of multinucleate cells are usually prevented from fusing. Here, we report that the nuclei from the syncytial cyst of the chalazal endosperm of Arabidopsis thaliana (L.) Heynh. are polyploid with some intermediate ploidy levels that cannot be attributed to endoreplication, suggesting nuclear fusion. Analysis of isolated nuclei, together with fluorescent in situ hybridization (FISH), revealed that nuclei from the chalazal endosperm are two or three times bigger than the nuclei from the peripheral endosperm and have a corresponding increase in ploidy. Together with the consistent observation of adjoined nuclei, we propose that nuclear fusion contributes, at least in part, to the process of polyploidization in the chalazal endosperm. Confocal analysis of intact seeds further suggested that free nuclei from the peripheral endosperm get incorporated into the chalazal cyst and likely participate in nuclear fusions.Abbreviations BAC Bacterial artificial chromosome - CZE Chalazal endosperm - DAPI 4,6-Diamino-2-phenylindole - FISH Fluorescent in situ hybridization - NOR Nucleolar organizing region - NCD Nuclear cytoplasmic domain - PEN Peripheral endosperm     

Chromosome endoreduplication in endosperm cells of two maize genotypes and their progenies

Summary Chromosome endoreduplication is a very common process in higher plants but its function and genetic control are still to be clarified. In our experiments we analyzed, by Feulgen cytophotometry, chromosome endoreduplication in endosperm cells of two maize genotypes, IHP and ILP, having high and low protein content in their seed, respectively. Chromosome endoreduplication occurs in both lines within 24 days after pollination, attaining a maximum ploidy level of 384C (7 DNA replication rounds) in IHP and of 192C (6 replication rounds) in ILP. In the mature seed, endosperms of the two lines show different mean ploidy level. In reciprocal crosses between IHP and ILP the f₁ endosperms have mean ploidy levels analogous to that of the maternal parent, showing that the difference in ploidy level between the two genotypes is maintained. After selfing of the f₁ plants, the difference in ploidy level between the two F₂ populations is reduced. In F₂ the mean ploidy level is as variable as in f₁, indicating the absence of genetic segregation. From our data, it is apparent that both the genetic constitution (cytoplasmic and nuclear) of the maternal parent and the genotype of the individual endosperms influence the ploidy level. An analysis of the protein content in endosperms carried out on the same seed sample as analyzed cytophotometrically showed that the protein content increases, during seed development, parallel to chromosome endoreduplication and varies, in the two lines, in reciprocal crosses and their progeny, according to the same trend as mean ploidy level, suggesting a correlation between the two parameters.     

DNA endoreduplication in maize endosperm cells: the effect of exposure to short-term high temperature

DNA endoreduplication in Zea mays L. (cv. A619 × W64A) endosperm peaks between 16 and 18 d after pollination (DAP). The physiological function of DNA endoreduplication is not known but it is believed to be important in maize kernel development. In the present study, we investigated how 2, 4 or 6 d of high temperature (35 °C) affected DNA endoreduplication and maize kernel development in comparison with control kernels grown at 25 °C. Data were collected on fresh weight (FW), nuclei number, mitotic index, and DNA endoreduplication. Maize endosperm FW and nuclei number were reduced by exposure to 4 or 6 d of high temperature. At 18 DAP, the 2 d high temperature treatment (HTT) caused a reduction in FW and nuclei number, but had no effect on DNA endoreduplication and average DNA content per endosperm. However, when the exposure to high temperature was increased to 4 or 6 d, FW, nuclei number and the magnitude of DNA endoreduplication were progressively reduced, and the peak mitotic index was delayed compared with the control endosperm. At 18 DAP, the 4 d treatment showed 54·7% of the cells were 3 or 6 C, whereas only 41·2% were 12 C or higher. Six days of high temperature also resulted in a reduction in endosperm FW, nuclei number and a delay in the peak of mitotic index. DNA endoreduplication occurred in the kernels exposed to this treatment, although the magnitude was severely reduced compared with the control kernels. Nuclear DNA content was highly correlated (r = 0·93) with kernel FW, suggesting an important role of DNA endoreduplication in determining endosperm FW. The data suggest that high temperature during endosperm cell division exerted negative effects on DNA endoreduplication by dramatically reducing the nuclei number, leaving fewer nuclei available for DNA endoreduplication. However, the data also suggest that prolonged exposure to high temperature restricts entry of mitotic cells into the endoreduplication phase of the cell cycle.     

Plastid DNA during grain filling in wheat

During grain filling in wheat (Triticum aestivum L.) there is a progressive increase in the number of amyloplasts in the endosperm, as well as in cell number, DNA content and nuclear ploidy as the grain increases in size. The plastid DNA content also rises initially, and then there is a levelling off in the amount, with the percentage plastid DNA finally making up approximately 0.9% of the total endosperm DNA.     

Ultramicrochemical determination of nucleic acids in individual cells using the Zeiss UMSP-I microspectrophotometer. Application to isolated rat hepatocytes of different ploidy classes

Synopsis Edström's method for the ultramicrochemical determination of RNA and DNA in individual cells was modified for the measurement of extinction in u.v. light with the aid of the Zeiss scanning microspectrophotometer UMSP-I. With this new procedure, nucleic acids down to about 3 pg RNA or about 4 pg DNA can be measured with a very high accuracy.The method was applied to enzymatically isolated rat liver parenchymal cells. A mean DNA content of 6.52 pg was found for diploid cells. The DNA content of mononuclear cells of different ploidy levels and of binuclear cells showed a close proportionality with the nuclear ploidy and the number of nuclei per cell. The RNA content of mononuclear diploid cells amounted to 33.4 pg, yielding an RNA/DNA ratio of 5.12. The RNA/DNA ratio was similar for binuclear and mononuclear cells of the same ploidy level but decreased considerably with increasing nuclear ploidy.     

Chromosome organization in wheat endosperm and embryo

We have analysed the chromosome organization in endosperm and embryo of bread wheat (Triticum aestivum L.), in order to compare these tissues with developing anthers, in which the centromeres associate, and the developing root xylem vessel cells, in which the chromosomes endoreduplicate to become polytene and associate via their centromeres. Both endosperm and embryo showed a typical Rabl configuration and a degree of non-homologous centromere association and the endosperm also showed extensive telomere association. Wheat endosperm is initially triploid and during its development a percentage of the nuclei increase their DNA content to 6C and 12C. 6C nuclei showed twice as many centromeres as 3C nuclei and the centromere number increased further in 12C nuclei. The higher the C-content of a nucleus the more the telomeres associated in endosperm. The vast majority of 12C nuclei showed six rye chromosome arms, although a few showed three associated groups of rye chromosome arms. This means that during endosperm development wheat nuclei show both polyploidization and polytenization.     

Cytofluorimetric research on the changes in the DNA content in the nuclei of Amoeba proteus during prolonged starvation and after refeeding]

Dividing amoebae were manually selected from the culture of Amoeba proteus, and so groups of synchronously dividing (synchronized) amoebae were obtained. These synchronized amoebae were maintained without food. In spite of starvation, individual amoebae in some particular groups were seen to divide, whereas in other groups of amoebae there was no division at all. The starving amoebae died not earlier than 2 weeks after the last division. A relative DNA content in isolated nuclei has been determined cytofluorometrically for each of 6 groups of synchronized starving amoebae, unable to divide. The nuclei were isolated in different intervals after division (after the feeding was ceased): 1.0-1.5 h, 1 day and up to 13 days with 1-2 day intervals. In the all groups of amoebae DNA synthesis occurred on the first 1-2 days after division. The nuclear DNA content in amoebae of 3 groups increased more than two-fold as compared with the 1 h level, in other 3 groups the nuclear DNA content did not exceed the doubled 1 h level, but probably exceeded the doubled postmitotic level. Later on, the nuclear DNA content in starving amoebae of each group was seen to decrease by 16-20%. Amoebae of 3 of the 6 groups were given the food organisms (Tetrahymena pyriformis) 8 days after division (after cessation of feeding). 2-3 days after refeeding some of these amoebae divided, and the nuclear DNA content of the refed amoebae proved to be higher than that in amoebae that continued to starve. It is suggested that the decrease of DNA content in the nuclei of starving amoebae and the increase of DNA quantity in the nuclei of refed amoebae may result from degradation and induction of synthesis of specific extra DNA synthesized in amoeba nuclei during each cell cycle.     

DNA Synthesis, Mitosis and Ploidy of Dividing Cells in Early Crown Gall

DNA synthesis, mitosis and ploidy of dividing cells were studied during 30 h after wounding around wounds inoculated with Agrobacterium tumefaciens, around sterile wounds and in control stems of Vicia faba. DNA synthesis was examined by counting nuclei labelled with ³H-thymidine in slides in which mitoses had been counted and analyzed before autoradiography. The main results were that around inoculated wounds, DNA synthesis and the number of mitoses showed a peak between 12 and 22.5 h. Both types of wounding induced mitoses, many of them polyploid (DNA content higher than 4C), both in the pith and the cortex, whereas in the control stems only diploid mitoses, mostly in the stelar area, were seen. The first polyploid (8C) mitoses around the inoculated wounds took place at 12 h and at 15.5 h 32C mitoses were seen; around the sterile wounds the first 8C divisions occurred at 26 h. The frequency of polyploid mitoses and their degree of ploidy continued to be considerably higher around the crown gall than around the control wounds. When a cell with a higher than 4C content is induced to divide, the 12 chromosomes, as a rule, consist of four, eight, 16 or 32 chromatids, instead of the normal two. The early division of highly polyploid cells around the inoculated wounds is obviously caused by growth factors which are known to be produced by the bacteria. It appears possible that this ability to synthesize excessive amounts of growth factors is subsequently transferred to the host cells through bacterial DNA.     

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

Simultaneous measurement of DNA content in cell nuclei and condensed chromatin bodies formed by heterochromatized regions of sex chromosomes (gonosomal chromatin bodies, GCB) has been performed in two trophoblast cell populations of the East-european field vole Microtus rossiaemeridionalis, namely in the proliferative population of trophoblast cells of the junctional zone of placenta and in the secondary giant trophoblast cells. One or two gonosomal chromatin bodies 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 (2c-16c), and in the highly polyploid population of secondary giant trophoblast cells (16c-256c), the total DNA content in GCB increased proportionally to the ploidy level. In separate bodies, the DNA content rose also in direct proportion with the ploidy level seen in the nuclei with both one and two GCBs in the two trophoblast cell populations. A certain increase in percentage of the nuclei with 2-3 GCBs was shown in the nuclei of the junctional zone of placenta; this may be accounted for by genome multiplication via uncompleted mitoses. In the secondary giant trophoblast cell nuclei (16c-256c), the number of GCBs did not exceed 2, and the share of nuclei with two GCBs did not increase, thus suggesting the polytene nature of sex chromosome in these cells. At different poloidy levels, the ratio of DNA content in the nucleus to the total DNA content in GCB did not change significantly giving evidence of a regular replication of sex chromosomes in each cycle of genome reproduction. In all classes of ploidy, the mean total DNA content in trophoblast cell nuclei with single heterochromatic body was less than in the nuclei with two and more GCBs. This may indicate that a single GCB in many cases does not derive from the fusion of two GCBs. To put it another way, in the nuclei with one GCB and in those with two or more GCBs, different chromosome regions may undergo heterochromatization. The regularities observed here are, most probably, associated with the peculiarities in the structure of X- and Y-chromosomes in a range of species of Microtus (M. agrestis, M. rossiaemeridionalis, M. transcaspicus). As a result, gonosomal chromatin bodies may include large blocks of both constitutive heterochromatin of X- and Y-chromosomes (in male and female embryos) and inactivated euchromatin of "lyonized" X-chromosome in female embryos. Therefore the presence of two or more GCBs in trophoblast cells of M. rossiaemeridionalis may be accounted for by both polyploidy and functional state of the nucleus, in which gonosomal constitutive heterochromatin and inactivated euchromatin form two large chromocenters rather than one. The differences in DNA content in GCBs in the nuclei with one and two GCBs seem to be an indirect indication that the two chromocenters may be formed by two different gonosomes, with the extent of their heterochromatization being higher than that in the nuclei with one GCB. GCBs in the trophoblast cells of M. rossiaemeridionalis are observed not only at the early developmental stages, as it was observed in rat at the first half of pregnancy (Zybina and Mosjan, 1967), but also at the later stages, up to the 17th day of gestation. At these stages, the nuclei with non-classical polytene chromosomes rearrange to those with a great number of endochromosomes, probably because of disintegration of chromosomes into oligotene fibrils. However, it does not seem unlikely that this process may involve heterochromatized gonosomal bodies, since only one or two large GCBs can be seen in the nuclei as before. The presence of prominent blocks of constitutive heterochromatin seems to favor a closer association of sister chromatids in polytene chromosomes, which prevents their dissociation into endochromosomes with the result that polyteny of sex chromosomes in the field vole trophoblast is probably retained during a longer period of embryonic development.