Condensation anomalies and exclusion in micronuclei of rearranged chromosomes in human fibroblasts cultured in vitro

Anomalies of chromatin condensation, such as fragmentation, uncoiling and pulverization, were observed in XP9UV25, a xeroderma pigmentosum fibroblast clone in which a high proportion of cells carried an end-to-end dicentric chromosome, dic (5;16) (p15.2;q24), that gives rise during propagation in culture to a variety of dicentric and monocentric derivatives. The coiling anomaly affected exclusively part of a rearranged chromosome, in particular the region previously involved in breakage events. The heterochromatic 16q region, which is a preferential breakpoint in the formation of dicentric and monocentric derivatives, was consistently the limit of the uncoiled or pulverized regions. This observation suggests that the anomalous chromatin behavior could derive from alteration of a region relevant for the correct condensation of the chromosome. In XP9UV25 the frequency of nuclei with associated micronuclei increased with time in culture, in parallel with that of mitoses with dicentric chromosomes. In situ hybridization with DNA probes specific for chromosomes 5 and 16 revealed hybridization signals in about 40% of micronuclei. Since the frequency of micronuclei is about ten times less than that of dicentrics, it is probable that only the rearranged chromosomes undergoing coiling anomalies are excluded in micronuclei.     

CHROMOSOME PULVERIZATION IN HUMAN BINUCLEATE CELLS FOLLOWING COLCEMID TREATMENT

Under the influence of Colcemid, a substantial number of binucleate human cells from a line infected with herpes-like virus was found to possess pulverized chromosomes. Although this abnormality was also detected in untreated binucleate cells, the increase in the number of pulverized cells after the addition of Colcemid was too striking to be explained by accumulation of spontaneously occurring cells in response to the mitotic inhibition by Colcemid. Furthermore, the induction of pulverization may be dependent upon Colcemid concentration. These findings imply an involvement of Colcemid in the mechanism of pulverization induction in the system studied. When tritiated thymidine was added to the culture medium simultaneously with Colcemid, the majority of binucleate cells with an intact and a pulverized chromosome set incorporated this isotope into the pulverized set only. This obviously suggests that the nuclei in the binucleate cell are asynchronous in DNA synthesis, and that this asynchrony is intimately related to the induction of the pulverization phenomenon. It seems very probable that the late S phase in the late synthesizing nuclei represents a critical stage at which damage to the chromosomes most readily occurs.     

SYNTHETIC CAPACITIES OF CHROMOSOME FRAGMENTS CORRELATED WITH THEIR ABILITY TO MAINTAIN NUCLEOLAR MATERIAL

Onion (Allium cepa) and bean (Vicia faba) root tip cells containing many micronuclei, derived from x-ray-induced chromosome fragments, were exposed to H³-thymidine and H³-cytidine to determine the ability of such fragments to undergo DNA and RNA synthesis. Only a few micronuclei in onion and many in bean roots synthesize nucleic acid simultaneously with their main nuclei. A few micronuclei labeled with H³-thymidine undergo mitotic chromosome condensation along with the main nuclei, while the unlabeled ones never do so. The onset of nucleic acid synthesis as well as mitosis in micronuclei appears to be under generalized cellular control. Although all chromosomes and chromosome fragments at telophase give a positive reaction for a silver stainable nucleolar fraction, in the subsequent interphase only some micronuclei, derived from such chromosome fragments, are found to maintain nucleoli; others lose them with time. Those micronuclei which maintain nucleoli, perhaps due to the presence of specific chromosomal regions, are also active in DNA and RNA synthesis. These results are compatible with the concept that nucleoli and associated chromosome regions play an important role in the primary biosynthetic processes of the cell.     

Histone rearrangements accompany nuclear differentiation and dedifferentiation in Tetrahymena

Histone synthesis and deposition into specific classes of nuclei has been investigated in starved and conjugating Tetrahymena. During starvation and early stages of conjugation (between 0 and 5 hr after opposite mating types are mixed), micronuclei selectively lose preexisting micronuclear-specific histones α, β, γ, and H3^F. Of these histones, only α appears to accumulate in micronuclear chromatin through active synthesis and deposition during the mating process. Curiously, α is not observed (by stain or label) in young macronuclear anlagen (4C, 10 hr of conjugation). Thus, young macronuclear anlagen are missing all of the histones which are known to be specific to micronuclei of vegetative cells. By 14–16 hr of conjugation, we observe active synthesis and deposition of macronuclear-specific histones, hv1, hv2, and H1, into new macronuclear anlagen (8C). Thus macronuclear differentiation seems well underway by this time of conjugation. It is also in this time period (14–16 hr) that we first detect significant amounts of micronuclear-specific H1-like polypeptides β and γ in micronuclear extracts. These polypeptides do not seem to be synthesized during this period, which suggests that β and γ are derived from a precursor molecule(s). Since these micronuclear-specific histones do not appear in micronuclear chromatin until after other micronuclei have been selected to differentiate as macronuclei, we suspect that micronuclear differentiation is also an important process which occurs in 10–16 hr mating cells. Our results also suggest that proteolytic processing of micronuclear H3^S into H3^F (which occurs in a cell cycle dependent fashion during vegetative growth) is not operative during most if not all of conjugation. Thus micronuclei of mating cells contain only H3^S which also seems consistent with the fact that some micronuclei differentiate into new macronuclei (micronuclear H3^S is indistinguishable from macronuclear H3). Interestingly, the only H3 synthesized and deposited into the former macronucleus of mating cells is the relatively minor macronuclear-specific H3-like variant, hv2. These results demonstrate that significant histone rearrangements occur during conjugation in Tetrahymena in a manner consistent with the fact that during conjugation some micronuclei eventually differentiate into new macronuclei. Our results suggest that selective synthesis and deposition of specific histones (and histone variants) plays an important role in the nuclear differentiation process in Tetrahymena. The disappearance of specific histones also raises the possibility that developmentally regulated proteolytic processing of specific histones plays an important (and previously unsuspected) role in this system.     

Genotoxicity of 1,3- and 1,6-dinitropyrene: induction of micronuclei in a panel of mammalian test cell lines

1,3-Dinitropyrene (1,3-DNP) and 1,6-dinitropyrene (1,6-DNP) were assessed for their potential to increase the frequencies of micronuclei in a panel of test cell lines consisting of H4IIEC3/G, 5L, 5L/r-1,3-DNP1, 208F, V79, V79/r-1,6-DNP1, HepG2 and BWI-J cells, which have been partially characterized for their expression of xenobiotic metabolising enzymes. The micronuclei were analyzed for the presence or absence of kinetochores indicating the occurrence of aneuploidy or chromosome breakage, respectively. 1,3-DNP caused a substantial increase in the frequency of micronuclei only in V79 cells. 1,6-DNP was strongly genotoxic in lines H4IIEC3/G⁻, 208F, V79 and, to a minor degree, in 5L/r-1,3-DNP1. It caused the formation of kinetochore-positive as well as kinetochore-negative micronuclei in V79 cells but only of kinetochore-negative micronuclei in H4IIEC3/G⁻ and 208F cells. 1,6-DNP-induced formation of micronuclei was paralleled by the appearance of multinucleated cells. Treatment of V79 cells with 1,3-DNP resulted in the same types of damage as treatment with 1,6-DNP, although considerably higher concentrations were required.The results show that 1,6-DNP can be highly genotoxic in mammalian cells, whereas, at least in the panel of test cell lines used, 1,3-DNP possesses only a low genotoxic activity. 1,3-DNP appears to be activated to genotoxic products in V79 cells by the same pathway(s) as 1,6-DNP.     

Mammalian cell fusion

The behaviour of heterochromatin during premature chromosome condensation (PCC) was studied in a cell line of Microtus agrestis after fusion with mitotic HeLa cells. In the G₁- and G₂-PCC, the heterochromatic nature of the X-chromosomes was detectable by their intense staining. The pulverized appearance of the S-phase PCC was correlated with incorporation of ³H TdR into the DNA. Three types of S-PCC were observed. PCC with a pulverized appearance of: (a) only the autosomes (early S); (b) autosomes and X-chromosomes (mid S); and (c) only the X-chromosomes (late S). The behaviour of heterochromatin during replication, as observed by the PCC method, was no different from that of euchromatin. The data on the sequence of chromosome replication indicate that the centromeric regions of the X-chromosomes were the last segments to replicate. The completion of DNA synthesis in the X-chromosomes appears to be followed by progressive chromosome condensation during G₂ even before the actual initiation of prophase.     

Initiation of DNA synthesis in the prematurely condensed chromosomes of G1 cells

The objective of this study was to investigate whether G1 cells could enter S phase after premature chromosome condensation resulting from fusion with mitotic cells. HeLa cell synchronized in early G1, mid-G1, late G1, and G2 and human diploid fibroblasts synchronized in G0 and G1 phases were separately fused by use of UV-inactivated Sendai virus with mitotic HeLa cells. After cell fusion and premature chromosome condensation, the fused cells were incubated in culture medium containing Colcemid (0.05 micrograms/ml) and [3H]thymidine ([3H]ThdR) (0.5 microCi/ml; sp act, 6.7 Ci/mM). At 0, 2, 4, and 6 h after fusion, cell samples were taken to determine the initation of DNA synthesis in the prematurely condensed chromosomes (PCC) on the basis of their morphology and labeling index. The results of this study indicate that PCC from G0, G1, and G2 cells reach the maximum degree of compaction or condensation at 2 h after PCC induction. In addition, the G1-PCC from normal and transformed cells initiated DNA synthesis, as indicated by their "pulverized" appearance and incorporation of [3H]ThdR. Further, the initiation of DNA synthesis in G1-PCC occurred significantly earlier than in the mononucleate G1 cells. Neither pulverization nor incorporation of label was observed in the PCC of G0 and G2 cells. These findings suggest that chromosome decondensation, although not controlling the timing of a cell's entry into S phase, is an important step for the initiation of DNA synthesis. These data also suggest that the entry of a S phase may be regulated by cell cycle phase-specific changes in the permeability of the nuclear envelope to the inducers of DNA synthesis present in the cytoplasm.     

EVIDENCE FROM THYMIDINE-3H-LABELED MERISTEMS OF VICIA FABA OF TWO CELL POPULATIONS

Treatments with tritiated thymidine (TdR-³H) have revealed the existence of two populations of mitotically active cells in meristems of lateral roots of Vicia faba. A rapidly dividing population, with a cycle time of 14 hr, constitutes about half the cells in the meristem. A second population of cells, with a cycle time in excess of 30 hr, is also present. Estimates of the relative size of this slowly dividing population are more difficult to make, but we calculate that this population includes 27–43% of meristem cells. The remaining fraction of the meristem is made up of cells that divide rarely or not at all. Since, at all times, both populations contribute to the mitotic index, the curve of the percentage of labeled mitoses that can be determined after a pulse label with TdR-³H differs from the curve expected of an ideal population in an important way: the peak value of the curve of the percentage of labeled mitoses is always less than 100%, usually between 75 and 80%. This heterogeneity within a meristem must be borne in mind in terms of the response of meristems to disruptive treatments, the mechanisms controlling mitotic cycle duration, and the spatial organization of a heterogeneous population in an organ that shows polarized growth.     

Inhibition of the M r 70,000 S6 kinase pathway by rapamycin results in chromosome malsegregation in yeast and mammalian cells

The antifungal and immunosuppressive drug rapamycin arrests the cell cycle in G1-phase in both yeast and mammalian cells. In mammalian cells, rapamycin selectively inhibits phosphorylation and activation of p70 S6 kinase (p70^S6K), a protein involved in the translation of a subset of mRNAs, without affecting other known kinases. We now report that rapamycin causes chromosome malsegregation in mammalian and yeast cells. Chromosome malsegregation was determined by metaphase chromosome analysis of human lymphocytes and lymphoblasts, detection of CREST-positive micronuclei in human lymphoblasts and Chinese hamster embryonic fibroblast (CHEF) cells, and selection of doubly prototrophic cells in a specially constructed yeast strain. The number of ana-telophases with displaced chromosomes and interphase and mitotic cells with an irregular number of centrosomes was also determined in CHEF cells. In quiescent mammalian cells (human lymphocytes and CHEF cells) induced with growth factor to re-enter the cell cycle, rapamycin was effective when cells were exposed at the time of p70^S6K activation. In yeast, rapamycin was more effective when treatment was started in G1- than in G2-synchronized cells. Cells from ataxia telangiectasia (A-T) patients are characterized by chromosome instability and have recently been found to be resistant to the growth-inhibiting effect of rapamycin. We found that an A-T lymphoblastoid cell line was also resistant to the induction of chromosome malsegregation by rapamycin, but the level of spontaneous aneuploidy was higher than in normal cells. In yeast, the induction of chromosome malsegregation was dependent on the presence of a wild-type TUB2 gene, encoding the β-subunit of tubulin. The finding that rapamycin acts in different cell types and organisms suggests that the drug affects a conserved step important for proper segregation of chromosomes. One or more proteins required for chromosome segregation could be under the control of the rapamycin-sensitive pathway. Received: 3 August 1998 / Accepted: 20 August 1998     

Effect of tablet geometrical structure on the dehydration of creatine monohydrate tablets,and their pharmaceutical properties

The effects of compression and pulverization on the dehydration kinetics and hardness of creatine monohydrate tablets were studied using a variety of kinetic equations and physical models. The dehydration behavior of unpulverized and pulverized tablets was investigated by using differential scanning calorimetry (DSC) and powder X-ray diffraction (PXRD). The hardness of both unpulverized and pulverized monohydrate tablets was significantly decreased after dehydration. The relationship between the degree of dehydration and the tablet hardness of both unpulverized and pulverized monohydrate tablets formed a straight line. The results suggest that the reduction in tablet hardness is dependent on the dehydration of crystal water, and the values of the slopes indicate that the bonding energy of the unpulverized sample was stronger than that of the pulverized sample. The dehydration kinetics of the unpulverized and pulverized monohydrate tablets were evaluated by analyzing the fit of the isothermal DSC data using a variety of solid-state kinetic models. The dehydration of the unpulverized tablets at various levels of compression pressure followed the 3-dimensional growth of nuclei mechanism. In contrast, although the dehydration kinetics of pulverized monohydrate tablets compressed at 500 and 750 kg/cm² followed the 3-dimensional diffusion mechanism, those compressed at 1000 kg/cm² followed the 3-dimensional growth of nuclei mechanism. The PXRD analysis indicated that the diffraction intensity of the pulverized monohydrate powder was significantly lower than that of the unpulverized powder. The diffraction peaks of the (h00) planes and the micropore structure of the unpulverized monohydrate tablets were affected by pulverization and compression force, respectively. Published: October 26, 2005     

Possible relationship between micronucleated and binucleated cells induced by cisplatin in cultured CHO cells

Chinese hamster ovary (CHO) cells were treated with a single dose (10 μg/ml) of cis-diammino-dichloroplatinum (II) (cisplatin) for 1 h and the effect of the drug on the kinetics of proliferation of the cultures was studied. It was found that the drug produces a delay in the proliferation rates of the treated cultures.The induction of micronuclei and binucleated cells (BC) at different times after treatment have also been studied, and the ability of these cells to undergo DNA synthesis (measured as the ability to incorporate [³H]thymidine) is shown.It was found that cisplatin induced a particular type of BC that contains one or more micronuclei rather than a pure population of BC. The results obtained show a possible relationship between micronuclei and BC. The possibility that some of the micronucleated cells evolve in subsequent cell divisions to BC with micronuclei is suggested.     

X chromosome inactivation and micronuclei in normal and Turner individuals

Studies on aneuploidy have shown that the X is the most frequently lost chromosome in females, and that the number of X chromosome-positive micronuclei increases with age in women. Recently, we showed that the inactive X chromosome is incorporated preferentially in micronuclei. The objectives of the current study were, firstly, to determine the incidence of X chromosome incorporation into micronuclei in males and, secondly, to determine the incidence of X chromosome incorporation into micronuclei of females with Turner syndrome. Blood samples were obtained from 18 male newborns and 35 normal adult males ranging in age from 22 to 79 years and from seven women with non-mosaic Turner syndrome aged 11–39 years. Isolated lymphocytes were cultured in the presence of cytochalasin B and 2000 binucleated cells per subject were scored for micronuclei. Cells were then hybridized with the biotinylated X centromere-specific probe, pBamX7, and visualized with fluorescein-conjugated avidin. All micronucleated cells were relocated and evaluated for the presence or absence of the X chromosome. Of the 335 micronuclei observed, 6.6% (22/335) contained an X chromosome. Analysis of variance shows a statistically significant increase, for both males and Turner females, in the number of X chromosome-positive micronuclei with age (P < 0.001). These data also show that the X chromosome is included in micronuclei from males more often than would be expected by chance (P < 0.005; χ² analysis, 15 df). Here we show that there is a tenfold difference in the frequency of X chromosome-positive micronuclei in 46,XX females compared to 46,XY males and 45,X females, providing further support to our previous finding that the X chromosome in micronuclei is the inactive chromosome. Received: 29 April 1997 / Accepted: 9 May 1997     

Nucleolus-like bodies in micronuclei of cultured Xenopus cells

Two of the 36 chromosomes in Xenopus laevis are known to carry nucleolar organizer loci. Partitioning of the chromosomes of cultured, early-passage Xenopus cells among variable numbers of micronuclei could be induced by extended colcemid treatment. A large, obvious nucleolus occurred in a maximum of 4 micronuclei per colcemid-induced tetraploid cell. The large, deeply-stained nucleoli incorporated [³H]uridine and appeared by electron microscopy to have typical nucleolar morphology with fibrillar and granular areas disposed in nucleolonema. In situ hybridization to radioactive ribosomal RNA (rRNA) resulted in heavy labelling of nucleoli in no more than 4 micronuclei per cell. The other micronuclei generally contained small bodies (blobs) which stained for RNA and protein as well as with ammoniacal silver. In the electron microscope, these appeared as round, dense bodies resembling nucleoli segregated by actinomycin D treatment. Nucleoplasmic RNA synthesis occurred in all micronuclei regardless of whether they contained definitive nucleoli. These observations suggest that micronuclei which formed large, typical, RNA-synthesizing nucleoli contained nucleolar organizer chromosomes, while the other micronuclei, which contained nucleolus-like “blobs” probably lacked nucleolar organizer loci. It is possible that the nucleolus-like bodies may have been aggregates of previously synthesized nucleolar RNA and protein trapped in micronuclei after mitosis.     

Qualitative and Quantitative Studies on DNA and RNA Synthesis in Loxodes striatus Nuclei*

SYNOPSIS. In this study the characteristics of the synthesis of DNA and RNA in the nuclei of Loxodes were investigated. Loxodes striatus is a primitive ciliate with 2 pairs of structurally differentiated diploid nuclei, the macro- and micronuclei. The macronuclei are differentiated morphologically into a clearly recognizable central core and an outer zone. To determine DNA and RNA synthesis, individual organisms were analyzed by autoradiography after incubating groups of cells with a ³H-labeled precursor ([³H]thymidine for DNA and [³H]uridine for RNA). The following observations were made: (A) All portions of macro- and micronuclei appeared to contain DNA as judged by the localizations of incorporated [³H]thymidine. (B) The macro- and micronuclei did not synthesize DNA at the same time; moreover, the duration of DNA synthesis in the former was much longer than of the latter nucleus. (C) Replication of DNA in the inner core and outer zone of the macronucleus occurred at separate times with little if any overlap. (D) All of the detectable [³H]uridine incorporation was found in the macronucleus and none in the micronucleus. Within the macro-nucleus the central core was more heavily labeled. (E) The quantitative differences in the label of the different components of the nuclear complex were investigated. (F) Contrary to the previously reported information our results suggest that DNA synthesis can occur in adult macronuclei. The possible explanation of these results is discussed in the context of the nuclear evolution of ciliates and of recent information on nuclear differentiation.     

Effect of various concentrations of acyclovir on cell survival and micronuclei induction on cultured HeLa cells

The HeLa cells were treated with 0, 0.01, 0.1, 1, 10 and 100 μM acyclovir (ACV) for 8 h duration and the growth kinetics, cell survival and micronuclei induction were determined. Treatment of HeLa cells with various concentrations of ACV resulted in a concentration-dependent decline in growth kinetics, cell proliferation indices and cell survival. ACV, 100 μM, completely inhibited cell division, where no appreciable changes in cell number were observed from 1 to 5 days post-treatment. This is reflected in cell survival, where the surviving fraction of cells was reduced to 1/2 at 100 μM ACV. Conversely, the frequency of micronuclei showed a concentration-dependent elevation at 20, 30 and 40 h post-treatment. ACV not only induced one micronuclei-bearing binucleate cell but also binucleate cells bearing two and multiple micronuclei in a concentration-dependent manner. The micronuclei frequency increased with time up to 30 h post-treatment and declined thereafter. The relationship between micronuclei induction and cell survival was determined by plotting the former on Y- and the latter on X-axes, respectively. The surviving fraction of cells declined with the elevation in micronuclei frequency and a best fit was observed for linear quadratic formalism.     

Replicating Units (Replicons) of DNA in Cultured Mammalian Cells

Exponentially growing L5178Y mouse leukemic cells were incubated in the presence of 5′-bromodeoxyuridine (BUdR) for about 4 hr, transferred to the nonBUdR-containing medium for a certain period (t hours), and then pulse-labeled with TdR-³H for 10 min. When DNA isolated from these cells was subjected to CsCl gradient centrifugation, the ³H-activity was found to shift gradually from the heavy BUdR-containing peak to the light nonBUdR-containing peak with increasing time t. The average time required for the complete shift of ³H-activity from the heavy to the light DNA fraction was 2.76 hr. Taking this as the average replicating time and the size of DNA fragments in the present preparation as 1.3 × 10⁷ daltons, the rate of replication was found to be 2.1 nucleotides per strand per replicon per sec. By taking the upper limit of the average replicating time as the S period (7.3 hr), various characteristics of the replicating units, such as the lower and upper limits of average size, the average replicating time, the average number of replicating units, etc., were calculated (see Table I).     

Radiation induction of micronuclei in human lymphocytes

An examination of the cytokinesis-blocked micronucleus technique confirmed its potential usefulness as a method of biological dosimetry for radiation accidents. Several advantages and disadvantages of the system are discussed. It has been demonstrated that under these conditions of these experiments, the blocking agent, cytochalasin B does not induce micronuclei or unstable chromosome aberrations. The induction of sister-chromatid exchanges proved just significant.Analysis of the dose response for 250 kVp X-rays indicates that although the Y = αD + βD² model fits the data, the relationship does not correspond to that for total aberration induction as might have been expected. The background frequency of micronuclei and the value of the α coefficient are higher than for total aberrations and the β term is lower. This indicates that simple incorporation of acentric chromosome fragments into micronuclei may not wholly account for the phenomenon.     

Mitotic dynamics of micronuclei induced by amiprophos-methyl and prospects for chromosome-mediated gene transfer in plants

Summary Mitotic dynamics and the kinetics of mass induction of micronuclei after treatment of Nicotiana plumbaginifolia cell suspensions with the spindle toxin amiprophos-methyl (APM) are reported. The addition of APM to suspension cells resulted in the accumulation of a large number of metaphases. The course of mitosis was strikingly different from normal. Metaphase chromosomes showed neither centromere division nor separation of chromatids. Single chromosomes and groups of 2 or more chromosomes were scattered over the cytoplasm. After 5–6 h of APM treatment, chromosomes decondensed and formed micronuclei. When treatment duration was increased, the frequency of cells with micronuclei as well as those showing lobed micronuclei increased. Similarly, with an increase in APM concentration the frequency of cells with micronuclei increased. After removal of APM, chromosome grouping disappeared, cells showing lobed micronuclei further increased and mitoses with doubled chromosome numbers appeared in the next cell division. Cytological observations and DNA measurements revealed that several sub-diploid micronuclei containing 1 or a few chromosomes can be obtained, and that flow cytometry can detect and sort out these micronuclei. The applications of micronuclei for genetic manipulation of specific chromosomes and gene mapping are indicated.     

Inactivation of a macronuclear intra-S-phase checkpoint in Tetrahymena thermophila with caffeine affects the integrity of the micronuclear genome

Aphidicolin (APH), an inhibitor of DNA polymerase α, arrested cell divisions in Tetrahymena thermophila. Surprisingly, low concentrations of APH induced an increase of macronuclear DNA content and cell size in non-dividing cells. In spite of the cell size increase, most proliferation of basal bodies, ciliogenesis and development of new oral primordia were prevented by the APH treatment. The division arrest induced by APH was partly overridden by caffeine (CAF) treatment, which caused the fragmentation ("pulverization") of the chromosomes in G2 micronuclei. Somatic progeny of dividers with pulverized micronuclei (APH+CAF strains) contained aneuploid and amicronucleate cells. The amicronucleate cells, after losing their oral structures and most of their cilia, and undergoing progressive disorganization of cortical structures, assumed an irregular shape ("crinkled") and were nonviable. "Crinkled" cells were not formed after APH + CAF treatment of the amicronuclear BI3840 strain, which contains some mic-specific sequences in its macronucleus. Most of the APH +CAF strains had a typical "*"- like conjugation phenotype: they did not produce pronuclei, but received them unilaterally from their mates and retained old macronuclei. However, 4 among 100 APH+CAF clones induced arrest at meiotic metaphase I in their wt mates. It is likely that the origin of such clones was enhanced by chromosome pulverization.     

Micronuclei induced by selenium,mercury, methylmercury and their mixtures in binucleated blocked fish erythrocyte cells

The genotoxic effects of low concentrations of Se(IV), Hg²⁺, and CH₃HgCl added separately or together (Se(IV)/Hg²⁺ and Se(IV)/CH₃HgCl) were studied by determining the induction of micronuclei in the binucleated erythrocytes of Prussian carp. The frequencies of micronuclei were elevated in a dose-dependent manner in all treatments when compared to the relevant controls. Addition of Se(IV) reduced the frequencies of micronuclei in treatments with both forms of mercury. This novel approach to genotoxicity testing using binucleated cytokinesis-blocked erythrocytes in fish appears to be worth further study as a method for monitoring the genotoxicity of waterborne pollutants.