Number and DNA content of nuclei in the free-living nematode <Emphasis Type="Italic">Panagrellus silusiae</Emphasis> at each stage during postembryonic development

Nuclei from the four major tissues of the nematode Panagrellus silusiae were enumerated and examined using Feulgen microspectrophotometry at each stage during postembryonic development. The number of nuclei in the hypodermis, nerve, and intestine remains fairly constant during maturation, but there is a slight increase (～57%) in the number of muscle nuclei. Thus, this organism is not stringently eutelic. The total number of somatic nuclei is about 600. DNA values of hypodermis and nerve nuclei were unimodal and adult nuclei had 2C amounts of DNA. The DNA distribution of muscle nuclei reflects the pattern expected for a tissue in which a portion of the nuclei are undergoing DNA synthesis. Intestinal nuclei accumulated DNA in the absence of nuclear division and in the adult the nuclei fall into discrete DNA classes which correspond to a geometric series of the 2C value. It is concluded that chromatin diminution does not occur in this species. In addition, the relationship in the different tissues of nuclear DNA content to nuclear volume and cell size is discussed.     

Relative ribosomal RNA cistron multiplicity in oocytes and postembryonic stages of the eutelic nematodePanagrellus silusiae

Summary The number of ribosomal RNA cistrons has been measured in the total DNA extracted from L2 juvenile and adult stages of the free-living nematodePanagrellus silusiae. Saturation hybridization studies with homologous rRNA indicate that both stages have about 275 ribosomal genes per haploid equivalent. Using homologous¹²⁵I-labelled rRNA for in situ hybridization, the mean number of silver grains per DNA content for oocyte, hypodermis and gut nuclei was similar. The mean DNA contents of maturing oocyte, hypodermis and gut nuclei are about 20C, 2C, and 10C respectively. We conclude that rDNA amplification alone is insufficient to account for the variation in DNA content of oocytes and that postembryonic development in this eutelic organism occurs without a significant differential increase in the number of ribosomal cistrons per worm.Supported by the National Research Council of Canada     

A histological and histochemical study of the cotyledons ofPhaseolus vulgaris L. during germination

Summary The cotyledon ofPhaseolus vulgaris L. comprises four tissues: epidermis, abaxial hypodermis, storage parenchyma, and procambium. A complex intercellular space system is present throughout the storage tissue and comprises about 16% of the cotyledon volume. All the cells contain protein bodies, and the hypodermis and storage parenchyma also contain starch grains. The epidermal cells are at the 2 C level of DNA, those of the hypodermis at the 4 C level, and the storage cells vary from 8 C to 32 C. During germination stomata differentiate in the epidermis. Reserve mobilization begins in the cells furthest from the epidermis and from the vascular tissue. Protein is removed from these cells with little or no coalescence of protein bodies. The DNA content of the nuclei decreases. The cell walls swell and then decrease in thickness as material is removed. Finally the nuclei and cytoplasm disappear and the cells collapse. In the cells near vascular bundles the protein bodies coalesce before losing their protein. The DNA content of the nuclei declines but nuclei and cytoplasm are still present at abscission. These cells do not collapse. Cytoplasmic RNA content is highest near the abaxial surface. Most of the RNA is removed during the first three days of germination.     

Status of nuclear division in arbuscular mycorrhizal fungi during in vitro development

Summary The number of nuclei in spores and along hyphae of an arbuscular mycorrhizal fungiGigaspora margarita was measured in digital images of fluorescence arising from mithramycin stained cultures. Typical dormant spores (250 m diameter) contained 2000 nuclei. Eight hundred nuclei were mobilized during the first 3 days of germination. The number of nuclei in the spores nearly returned to the initial number after 22 days of hyphal growth. The average relative DNA content in the nuclei of dormant spores and in the nuclei of spores incubated for 22 days was comparable, as judged from fluorescence intensity. Hyphal elongation occurred with 460 nuclei per cm under a special set of in vitro conditions that promote extensive hyphal growth of arbuscular mycorrhizal fungi. We found an average total of 26000 hyphal nuclei per germinating spore after 22 days. The specific DNA polymerase inhibitor aphidicolin did not inhibit spore germination but it rapidly reduced the rate of hyphal growth and arrested growth after 4 days. No nuclei were produced de novo during this time. These results demonstrate thatG. margarita replicates nuclear DNA and undergoes nuclear division when grown in vitro even in the absence of a plant host.     

Evidence of DNA replication in an arbuscular mycorrhizal fungus in the absence of the host plant

Summary This study provides evidence thatGigaspora margarita replicates its nuclear DNA, even in the absence of a host plant. Three experimental approaches were used: (i) static cytofluorimetry to quantify the DNA content, (ii) pulse treatments with bromodeoxyuridine (BrdU), which is an analogue of thymidine, to reveal nuclei undergoing DNA synthesis, and (iii) ultrastructural observations to study changes in chromatin morphology during the fungal cell cycle. A slight second peak of approximately twice the value of a major peak was found by cytofluorimetry, showing that a small number of nuclei had entered in cycle during in vitro development. Nuclei which had incorporated BrdU were observed after pulses of 24 h; nuclei with condensed chromatin were also apparent at this time. The results demonstrate thatG. margarita has all the metabolic pathways needed to replicate its nuclear DNA even in the absence of the host, suggesting that more complex mechanisms inhibit the extended growth in vitro of arbuscular mycorrhizal fungi.Abbreviations AM-fungi arbuscular mycorrhizal fungi - A.U. arbitrary units - BrdU 5-bromo-2-deoxyuridine - DAPI 4,6-diamidino-2-phenylindole - UV ultraviolet light     

contribution to the study of nuclear total proteins and DNA during the mitotic cycle in fibroblasts cultivated in vitro and in Ehrlich ascites cells

Summary The dry weight and total protein content of nuclei has been measured by interferometry in living or fixed cells cultivated in vitro (freshly prepared chick, mouse or rat embryo fibroblasts) and in fixed Ehrlich ascites tumor cells of the mouse growing in vivo. The DNA content was estimated by cytophotometry after Feulgen reaction in the same nuclei. The dry weight of nucleoli in fibroblasts and the dry weight and DNA content of chromosomes in dividing fibroblasts and Ehrlich tumor cells have also been measured.During the interphase in fibroblasts, the dry weight of the living nucleus and the nuclear total protein content as measured in fixed cells doubles during the preparation for mitosis, as the DNA content does. In chick and mammal fibroblasts and within the limits of accuracy of our measurements, the synthesis curves for nuclear proteins and DNA do not seem to be necessarily identical.In our fibroblasts, the nucleolar total dry weight per nucleus doubles during the interphase (nucleolar preparation for mitosis); it increases in proportion to the nuclear total protein content, even in polyploid nuclei.During the mitosis, the chromosomes contain all the DNA of the nucleus but some nuclear proteins (non chromosomal proteins) seem to move into the cytoplasm during the mitosis and return into the nucleus at the post-telophase.According to our observations, Ehrlich ascites mouse tumor cells are near-tetraploid as far as the number of chromosomes, nuclear total protein content and DNA content are concerned. During the preparation for mitosis, these amounts double but no necessary close time relation seems to link these premitotic syntheses. Prom this point of view, our results show no clear-cut differences between these tumor cells and the fibroblasts. Except the polyploidy, the behaviour of nuclear proteins and DNA during mitosis in the tumor cells is the same as that observed in our fibroblasts.The effects of various antimitotic agents on rat fibroblasts cultivated in vitro have also been studied with our cytochemical methods. Our measurements of nuclear protein, DNA and nucleolar material content have been made in cells in which mitosis was prevented by alkylating agents, beryllium sulphate, RNase or neutral DNase. The effects of colchicine on these cellular parameters have also been studied.     

Nuclear conditions in haploid Pelargonium in vivo and in vitro

In the shoot apices of the haploid Pelargonium cultivar Kleine Liebling, all mitoses are haploid (n = 9); however, ca. 20% of the interphase nuclei have DNA contents greater than 2C (up to 4C), indicating a tendency to chromosome endoreduplication in this material. — In internodes in vivo, the few mitoses present are haploid (quite probably, cambium cells); in addition to haploid interphases (1C to 2C DNA contents), endoreduplicated (endopolyploid) nuclei and nuclei in the course of endoreduplication occur with a frequency of ca. 40–50% (DNA contents up to 8C). — When internodes are cultured in vitro, differentiated cells are stimulated to divide, thus forming a population of diploid and tetraploid mitoses in addition to the preexistent meristem (haploid) cell population. In the process of time, diploid and tetraploid mitoses continue to be present in the callus, whilst haploid mitoses may decrease in number and eventually disappear. All mitoses analyzed had euploid chromosome numbers (9, 18 and 36) and their DNA contents were correspondingly 2C, 4C and 8C. Since no extensive chromosome counts were made, aneuploidy in the cultured material cannot be excluded; but, if occurring, it should be rather rare. — Under the experimental conditions used, prolonged culture in vitro leads to the production of nuclei with DNA contents (16C and 32C) greater than those occurring in vivo (8C), due to one and two additional DNA replications respectively beyond the limits attained in vivo. Even in these cultures, however, a population of the meristematic haploid cell line (DNA values 1C to 2C) is still present. — The present results are discussed in their relations with previous works on nuclear conditions in vivo and in vitro and on regeneration processes in cultured tissues in plants.     

Changes of lysosomal enzymes during hereditary degeneration and histogenesis of retina in mice

Summary In the normal histogenesis of mouse retina localized distribution of acid phosphatase positive granules has been seen around the photoreceptor cell nuclei along the outer limiting membrane. These granules disappear during the development of the rod elements. Temporarily increased activity is also seen along the nuclei of the inner layer adjacent to and in the course of the development of the outer and the inner plexiform layers. Within the inner nuclear layer, the cells at the outer and inner rows develop localized acid phosphatase positive granules which persist in the adult retina. Ganglion cells and the layer of nerve fibres show little change. In the pigment epithelium the enzyme gradually increases. In mice, homozygous for the retinal degeneration gene, degenerating photoreceptor cell nuclei, characterized by perinuclear acid phosphatase staining, can be detected before morphological signs of degeneration. Increased frequency of such nuclei and intensity of staining are recorded with the progress of degeneration. Enzyme activity in the photoreceptor cells, within the inner nuclear layer and in the degenerating photoreceptor cell nuclei is demonstrable using naphthol substrates but not -glycerophosphate. Positive reaction with -glycerophosphate is obtained in these sites in the presence of dimethyl sulphoxide. Existence of differential permeability among the retinal lysosomes is tentatively suggested.     

Polyamines in liver and their influence on chromatin condensation after 17-β estradiol treatment of Atlantic salmon

Atlantic salmon (Salmo salar) were treated with 17- estradiol to induce vitellogenin synthesis in liver. This led to an increase in liver wet weight and total DNA. After incubation with micrococcal nuclease (EC 3.1.31.1) less soluble chromatin was obtained from nuclei of the estradiol treated than the control fish, but active gene regions were solubilized by the nuclease. Thus, in the estradiol treated fish soluble mononucleosomes contained hybridizable vitellogenin gene sequences. As a result of estradiol treatment the content in total liver of putrescine rose 3-fold, that of spermidine 2-fold, while spermine was unchanged. In muscle no significant changes were observed. The regulatory functions of polyamines during gene expression were investigated by binding (¹⁴C)spermine to isolated liver nuclei depleted of endogenous polyamines. The number of binding sites was higher in nuclei of estradiol treated than control fish. (^14C)spermine associated preferentially with micrococcal nuclease insensitive chromatin. Thus, the high content of putrescine and spermidine in liver supported the view of polyamine accumulation in proliferating tissues. The preferential binding to condensed chromatin indicated a stabilizing effect of polyamines on the organization of inactive chromatin structures.Abbreviations MNase micrococcal nuclease - PMSF phenylmethylsulfonylfluoride     

Location of heitz's zerstäubungsstadium (dispersion phase) in the mitotic cycle ofPhaseolus coccineus and the concept of Angiosperm endomitosis

Summary DNA microdensitometry and autoradiography after treatment with³H-thymidine were used to study the phase of dispersion of chromocenters (Z phase) in parallel with chromocentric nuclei inPhaseolus coccineus. In all materials studied, two types of chromocentric nuclei were present.In radicle apices of dry seeds, two classes of nuclear DNA contents were measured, 2 C (G₁) and 4 C (G₂). The 2 C DNA class comprised all chromocentric type I nuclei, the 4 C class included Z phases and chromocentric type II nuclei. The 4 C (G₂) condition of Z phases implies that Z phases maintain their nuclear structure for some time after the end of DNA replication. Shoot apices also contain 2 C (G₁) and 4 C (G₂) nuclei but 4 C nuclei (Z phases and chromocentric type II nuclei) are rare.In seedling root apices, Z phases are from 1.02 to 4.08 times as frequent as prophases. This excludes that Z phase is a very early prophase. DNA microdensitometry shows that the chromocentric type I includes 2 C (G₁) nuclei and nuclei in the first part of the S phase, Z phases include 4 C (G₂) nuclei and nuclei in the last stage of the S phase and chromocentric type II includes mainly 4 C (G₂) nuclei and nuclei in the second part of S. After 90 minutes of treatment with³H-thymidine all Z phase nuclei are labeled. This result and the microdensitometric data unequivocally demonstrate that Z phase is located at the end of S.The present results and those of previous authors on Z phase are discussed in relation to Geitler's concept of Angiosperm endomitosis. It is concluded that the term Angiosperm endomitosis must be abandoned and substituted by the term chromosome endoreduplication.     

Cell-specific endopolyploidy in developing Artemia

Summary Cells in developing Artemia franciscana SFB demonstrated tissue-specific differences in DNA content, as determined by fluorescence intensity of bisbenzimide-stained nuclei and by nuclear area. The general epidermis comprised proliferating diploid (2C) cells. The setal cells had 4C–8C DNA content and did not divide during the first two instars. Salt gland cells were polyploid (>8C) and also did not undergo mitosis. Neural cells in the brain were diploid and were replicating. Cells in the thorax region of the gut had a 4C–8C DNA content and were proliferating. The muscle cells in the cephalic appendages contained 2C non-replicating nuclei. Only diploid epidermal cells were involved in segment morphogenesis. There was no difference in number of chromosomes (n=42) in the epidermal cells and the gut cells, indicating that the tissue-specific endopolyploidy was due to endoreduplication.     

Determination of ploidy level and nuclear DNA content in blueberry by flow cytometry

The technique of DNA flow cytometry was used to study variation in DNA content among different ploidy levels, as well as among diploid species, of Vaccinium section Cyanococcus. In a sample of plants of varying ploidy level, the relative fluorescence intensity (RFI) of nuclei stained with propidium iodide was a function of the number of chromosome sets (x), as represented by the linear equation RFI=3.7x-2.3 (r²=95%). The data indicated that DNA flow cytometry could be useful for the determination of ploidy level at the seedling stage in blueberry. They also suggest that conventional polyploid evolution has occurred in this section of the genus Vaccinium with an increase in nuclear DNA content concurrent with the increase in chromosome number. The nuclear DNA content of diploid species of Vaccinium section Cyanococcus was estimated from the relationship of the observed RFI to an internal known DNA standard (trout red blood cells). A nested analysis of variance indicated significant variation among species, as well as among populations within species, in nuclear DNA content, although this variation was small compared to the variation among ploidy levels. The variation in nuclear DNA content corresponded to the phylogenetic relationships among species determined from previous studies.     

Analysis of cell ploidy in histological sections of mouse tissues by DNA-DNA in situ hybridization with digoxigenin-labelled probes

Summary DNA-DNA in situ hybridization, with two digoxigenin-labelled, chromosome-specific DNA probes, was used to determine the number of copies of a given chromosome in interphase nuclei and so identify putatively polyploid nuclei in histological sections of several mouse tissues. One hybridization site per diploid genome was expected for tissues with hemizygous markers: male mice hybridized with a Y chromosome probe (pY353/B) or hemizygous transgenic mice hybridized with a -globin probe (pM02). Nuclei with more than one hybridization site were considered putative polyploids. Three groups of experiments were undertaken: (1) evaluation of the method, using mouse liver sections; (2) studies of tissues already known to contain polyploid nuclei, and (3) studies that resulted in the discovery that the mouse ovary contains polyploid nuclei. First, control studies showed that the ability to detect the target DNA sequences was affected by section thickness. Studies of nuclear ploidy in the developing mouse liver revealed a pattern similar to that established by previous studies using DNA content as a criterion for ploidy. At birth, only about 5% of the liver nuclei were polyploid; this increased to 10–15% by 10–20 days and was followed by a sharp increase in the frequency of tetraploid nuclei between 20 and 40 days (to about 35%) and a more gradual increase in higher order polyploid nuclei. Secondly, this technique was used to confirm that polyploid (mostly tetraploid) nuclei were present in the bladder epithelium, heart, uterine decidua and placental trophoblast. Higher order polyploidy was seen in large bone marrow cells (megakaryocytes) but not in the even larger trophoblast giant cells of the placenta, thus confirming previous claims that these cells are polytene rather than polyploid. Thirdly, putatively tetraploid nuclei were found in the ovarian follicle and corpus luteum. As far as we are aware, this is the first time polyploid nuclei have been reported for the mouse ovary.     

A modified immunohistochemical procedure for the detection of estrogen receptor in mouse tissues

Summary A horseradish peroxidase (HRP) labeled antibody method was developed for use with a monoclonal antibody to detect estrogen receptor (ER) in mouse tissue. Combined use of HRP labeled F(ab)₂ fragment absorbed with mouse liver protein to minimize background staining and imidazol-DAB reaction gave the most reliable and sensitive immunostaining.The method was applied to uterine, vaginal, pituitary and liver tissues in ovariectomized adult mice. In uterus and vagina, ER was recognized in nuclei of epithelial cells, stromal cells and smooth muscle cells of the muscle layer and blood vessels. Liver tissue showed positive nuclear immunostaining in parenchymal cells; however, no reaction was present in endothelial cells, Kupffer cells, bile ductal cells, and smooth muscle cells of blood vessels. ER was localized in the nuclei of anterior pituitary cells while weak reaction was also recognized in cells of the intermediate lobe. No staining was detected in the posterior pituitary.Results demonstrate that both occupied and unoccupied ER are localized in the cell nucleus from several target tissues. Weak immunostaining in samples could not be enhanced by multiple procedures. It is suggested that nuclear ER is partially hidden by nuclear components such as nuclei acid and chromatin proteins.     

Microspectrophotometric and ultrastructural analyses of a case of cell differentiation without endopolyploidisation: the pith ofCrepis capillaris (L.) Wallr.

Summary DNA content and fine structure ofCrepis capillaris nuclei have been investigated during pith cell differentiation. The ploidy level of young or old pith cells, belonging to juvenile or mature plants, remained constant and near the 2 C reference value. However, modifications in the nuclear state such as a progressive decondensation of the chromatin, an increase in nuclear volumes and a decrease of the nucleolar volumes were observed.This study was aided by grants from the Centre National de la Recherche Scientifique (E.R.A. no. 616).     

β-Tubulin accumulation and DNA synthesis are sequentially resumed in embryo organs of cucumber (Cucumis sativus L.) seeds during germination

Summary Resumption of DNA synthetic activities and -tubulin accumulation was studied in embryo organs of germinating cucumber (Cucumis sativus L.) seeds. Flow-cytometric analysis indicated the existence of 2C, 4C, and 8C nuclei in the radicle of mature embryos, whereas in cotyledons most of the cells contained nuclei with 2C DNA content. Upon imbibition of water, nuclear DNA replication was initiated in the radicle within 15 h, subsequently spreading towards the cotyledons. Bromodeoxyuridine incorporation preceded detectable changes in the relative amounts of DNA, implying the occurrence of putative DNA repair. Organellar DNA synthesis occurred independently of the nuclear DNA synthetic cycle. Western blotting and immunohistochemical localization demonstrated that the constitutive level of -tubulin originated from preserved -tubulin granules. During imbibition, disappearance of fluorescent tubulin granules, accumulation of -tubulin, and formation of microtubular cytoskeleton were found in the radicle, but not in the cotyledon areas. Mitosis only occurred after radicle protrusion at 21 h of imbibition. It is concluded that the differences in the initiation and progress of these cellular and molecular events are associated with the discrete behaviors of the radicle and the cotyledons upon imbibition. The formation of cortical microtubular cytoskeleton and the accumulation of tubulins are important features in preparation of radicle protrusion, whereas DNA synthesis may contribute to postgerminative growth.     

Callus formation from the suspensor ofPhaseolus coccineus in hormone-free medium: a cytological and DNA cytophotometric study

Summary Intact embryos (with suspensor) ofPhaseolus coccineus (2 n=2 x=22) in different stages of development were grownin vitro on hormone-free medium under conditions favoring callus formation from the portion of the suspensor proximal to the embryo (handle portion). It was shown that callus formation was dependent upon the developmental stage of the embryo and the integrity of the embryo-suspensor system.Callus formation from the handle portion of the suspensor, whose cells have nuclei with DNA values from 4 C to 128 C, was initiated by amitosis (nuclear fragmentation) which led to binucleate and multinucleate cells. Amitosis was followed by mitosis of both nuclei resulting from a previous amitosis and nuclei of euploid cells (mostly diploid) pre-existing in the suspensor. The majority of mitoses (88%) had either a reduced chromosome number (hypodiploid, haploid, and hypohaploid) or the diploid number (22 chromosomes).The very high incidence of cells with reduced chromosome number in the suspensor callus is discussed in relation to its mechanism of origin and to its possible exploitation in plant regeneration experiments.     

The histology and ultrastructure of a nuclear polyhedrosis virus of the webbing clothes moth, Tineola bisselliella

A histological and ultrastructural study of a nuclear polyhedrosis virus in the webbing clothes moth, Tineola bisselliella, was conducted. Polyhedral development was observed in nuclei of cells of the foregut, cardiac valve, midgut, pyloric valve, hindgut, Malpighian tubules, ganglia of the ventral nerve cord, muscle, tracheae, fat, and hypodermis. Observations made with the electron microscope suggest that virions from the gut lumen are transported in vesicles through the cytoplasm into the nuclei of the columnar cells. Here they are released, replicate, take on membrane, and ultimately become multiply occluded in polyhedral protein. Polyhedra observed in nuclei of other tissues appeared identical to those in the gut.     

Presence of DNase γ-Like Endonuclease in Nuclei of Neuronal Differentiated PC12 Cells

DNase , which cleaves chromosomal DNA into nucleosomal units (DNA ladder formation), has been suggested to be the critical component of apoptotic machinery. Using rat pheochromocytoma PC12 cells, which are differentiated to sympathetic neurons by nerve growth factor (NGF), we investigated whether DNase -like enzyme is present in neuronal cells and is involved in neuronal cell death. The nuclear auto-digestion assay for DNase catalyzing internucleosomal DNA cleavage revealed that nuclei from neuronal differentiated PC12 cells contain acidic and neutral endonucleases, while nuclei from undifferentiated PC12 cells have only acidic endonuclease. The DNA ladder formation observed in isolated nuclei from neuronal differentiated PC12 cells at neutral pH requires both Ca²⁺ and Mg²⁺, and is sensitive to Zn²⁺. The molecular mass of the neutral endonuclease present in neuronal differentiated PC12 cell nuclei is 32000 as determined by activity gel analysis (zymography). The properties of the neuronal endonuclease present in neuronal differentiated PC12 cell nuclei were similar to those of purified DNase from rat thymocytes and splenocytes. Interestingly, in neuronal differentiated PC12 cells, internucleosomal DNA fragmentation is observed following NGF deprivation, whereas undifferentiated PC12 cells fail to exhibit DNA ladder formation during cell death by serum starvation. These results suggest that the DNase -like endonuclease present in neuronal differentiated PC12 cell nuclei is involved in internucleosomal DNA fragmentation during apoptosis, induced by NGF deprivation.     

Polyploid tissues in the nematode Caenorhabditis elegans

During larval development, the number of somatic nuclei in C. elegans hermaphrodites increases from 558 to 959 (J. E. Sulston and H. R. Horvitz, Dev. Biol. 56, 110-156, 1977; J. E. Sulston et al., Dev. Biol. 100, 64-119, 1983). At the same time, the animals increase about 60-fold in volume. We have measured the DNA contents of several classes of nuclei by quantitating the fluorescence of Hoescht 33258 stained DNA (D. G. Albertson et al., Dev. Biol. 63, 165-178, 1978). Probably all embryonic nuclei, including those of neurons, muscles, hypodermis, and intestine, are diploid at hatching. Neurons, muscles, and nondividing hypodermal nuclei remain diploid throughout larval development. The DNA content of the intestinal nuclei doubles at the end of each larval stage, reaching 32C by the adult stage. New hypodermal cells, generated by division of seam cells in the larval stages, undergo an additional round of DNA replication before fusing with the major syncytium (hyp7, Sulston et al., 1983). Thus the larval hyp7 syncytium comprises a fixed number of diploid embryonic nuclei plus an increasing number of tetraploid postembryonic nuclei. Some of the endoreduplications that occur in the intestinal and hypodermal lineages of C. elegans may correspond to nuclear or cellular divisions in another nematode Panagrellus redivivus (P. W. Sternberg and H. R. Horvitz, Dev. Biol. 93, 181-205, 1982).