A haploid and a diploid cell coexist in an in vitro immortalized spermatogenic cell line.

We have recently established a conditionally immortalized germ cell line [GC-2spd(ts)] that, at the permissive temperatures of 37 degrees C and 32 degrees C, is able to undergo meiosis in vitro and form round spermatids [Hofmann et al., (1994): Proc Natl Acad Sci USA 91:5533-5537]. In this report, we provide data that indicate that the GC-2spd(ts) cell line consists of two cell populations undergoing a haploid (n/2n) cell cycle and a diploid (2n/4n) cell cycle, respectively. The cells containing 2n DNA, when sorted by fluorescence-activated cell sorting, are able to reconstitute the full population of n/2n/4n DNA cells, indicating that they are able to commit to the reductive meiotic division and form haploid spermatids or to continue self-replication through a diploid cell cycle. This GC-2spd(ts) cell line provides a valuable tool to study the molecular mechanisms involved in the cellular decision between self-renewal by mitosis and commitment to meiosis.

[The effect of sodium butyrate and luminol on reciprocal exchanges and gene conversion during extrachromosomal DNA recombination in cultured animal cells]. / Issledovanie vliianiia butirata natriia i liuminola na retsiproknye obmeny i gennuiu konversiiu pri ékstrakhromosomnoi rekombinatsii DNK v kul'tiviruemykh kletkakh zhivotnykh.

For determination of the extrachromosomal homologous DNA recombination efficiency, somatic cells of various lines have been transformed with plasmid DNAs which contain copies of neo-gene with non-overlapping deletions. Reconstruction of the neo-gene functional activity, which imparts a geneticin-resistant phenotype to cells, indicates that recombination has occurred. If dP1 and dR copies of the neo-gene are used, a single (reciprocal) exchange is necessary for reconstruction of the neo-gene by homologous DNA recombination, but a double exchange (gene conversion) is needed in the case of dP1 and dS copies. It is shown that in human cells of line HeLa and in mouse cells of line LMtk-, in contrast to the Chinese hamster cells of line A238, the frequency of double exchanges is comparable to that of the single DNA exchanges which is an evidence of participation in DNA recombination of gene conversion in addition to a single exchange mechanism. The treatment of cells with sodium butyrate and luminol exerts different influences on the rate of the single DNA exchanges and on that of gene conversion (double exchanges) in cells of lines LMtk- and HeLa, respectively. Essential distinctions in correlation of the single DNA exchange frequency and the gene conversion frequency in cells of the studied lines, and the possibility to distinguish between these mechanisms of recombination, under the treatment by sodium butyrate and luminol, may suggest the existence of two mechanisms of homologous DNA recombination in cultured animal cells, which function independently of one another, to a considerable extent.

[Agents that act on chromatin structure affect the rate of intrachromosomal homologous DNA recombination in cultured cells]. / Agenty, vozdeistvuiushchie na strukturu khromatina, vliiaiut na skorost' vnutrikhromosomnoi gomologichnoi rekombinatsii DNK v kul'tiviruemykh kletkakh.

A study was made of the influence of sodium butyrate and luminol on the rate of homologous recombination of aminoglycosidephosphotransferase gene copies, which contain non-overlapping deletions and are integrated in chromosomes of the Chinese hamster cells of line A238. It is shown that treatment of cells of transformed clones with sodium butirate, which is known to stimulate DNA reparation and to loosen chromatin structure, inhibits intrachromosomal homologous recombination. Dexametazone, which is also capable of stimulating DNA reparation, displayed similar effect. In contrast, the treatment of cells with luminol, inhibiting DNA reparation and condensing chromatin structure, increased the efficiency of intrachromosomal homologous DNA recombination.

[The characteristics of homologous DNA recombination in somatic cells. II. The characteristics of the extrachromosomal recombination of plasmid DNA in cultured cells differing in their tumor potential]. / Izuchenie osobennostei gomologichnoi rekombinatsii DNK v somaticheskikh kletkakh. II. Osobennosti ékstrakhromosomnoi rekombinatsii plazmidnykh DNK v kul'tiviruemykh kletkakh, razlichaiushchikhsia opukholevym potentsialom.

The frequencies of homologous extrachromosomal recombination, following plasmid transfection into cell lines differing in their tumour potential were measured. A 10-fold increase in extrachromosomal plasmid recombination has been reported in tumour cells (Ra-2, Jf-1) relative to immortal (Rat-2) and normal (REF). The differences between the normal and tumour cells are preserved in the in vitro recombination experiments (the recombination in the cell nuclei extracts). However, the recombination activity of nuclear extracts from the immortality transformed cells (Rat-2) is similar to that of nuclear extracts from the tumour cells.

[The characteristics of homologous DNA recombination in somatic cells. I. The construction of a DNA plasmid containing deletion variants of the aminoglycoside phosphoribosyltransferase gene]. / Izuchenie osobennostei gomologichnoi rekombinatsii DNK v somaticheskikh kletkakh. I. Konstruirovanie plazmidnykh DNK, soderzhashchikh deletsionnye varianty gena aminoglikozidfosforiboziltransferazy.

A plasmid DNA family containing deletion variants of Neo-gene has been constructed for investigation of somatic cell homologous recombination. Experimental data are presented provided by studies of extrachromosomal recombination, recombination following plasmid transfection into somatic cells, and recombination in the nuclear extracts. The opportunities of application of the plasmid constructions in the different pattern homologous recombination experiments performed on mammalian cells have been discussed.

[Genetic transformation of somatic cells. VII. The loss of the transformant phenotype is accompanied by both stable and unstable changes in DNA]. / Geneticheskaia transformatsiia somaticheskikh kletok. VII. Poteria transformantnogo fenotipa soprovozhdaetsia kak stabil'nymi tak i nestabil'nymi izmeneniiami v DNK.

From 6 clones of Chinese hamster cells varying in the rate of the loss of transformant phenotype and containing a thymidine kinase gene (tk-gene) of Herpes simplex virus type 1 (HSV1), 25 subclones negative in thymidine kinase (TK-) were isolated on a medium with 50 micrograms/ml 5-bromodeoxyuridine (BrdU). A study was made of the frequency of spontaneous reversions to the TK+ phenotype in cell populations of BrdU-resistant subclones, and of the transforming activity (upon transformation of TK- cells of A238 clone to the TK+ phenotype) of DNA preparations from a row BrdU-resistant subclones. In 7 of 11 BrdU-resistant subclones the TK- phenotype is associated with changes reducing significantly the transforming activity of DNA. Some of these alterations are stable and undergo no spontaneous reversion, while the other ones are unstable, being reversed or suppressed at a high frequency. BrdU-resistant subclones produced from clones more stable in transformant phenotype are on the whole more stable in the TK- phenotype than BrdU-resistant subclones from the clones with the high rate of the loss of the TK+ phenotype.

[Genetic transformation of somatic cells. IX. The loss of the transformant phenotype is accompanied by rearrangements in the plasmid DNA containing the thymidine kinase gene of the herpes virus]. / Geneticheskaia transformatsiia somaticheskikh kletok. IX. Poteria transformantnogo fenotipa sopriazhena s perestroikami plazmidnoi DNK, soderzhashchei gen timidinkinazy virusa gerpesa.

As demonstrated by dot-hybridization, the cells of HT-subclones isolated from the cells of transformant clones cultured on a non-selective medium differ significantly in the number of copies of thymidine kinase gene (tk-gene) of Herpes simplex virus (HSV1). Since the cells of transformant clones lose thymidine kinase-positive (TK+) phenotype during cultivation, this data are indicative of high frequence rearrangements in the region of transforming DNA as responsible for the transformant phenotype nonstability. These rearrangements, among other things, induce alterations in the number of copies of tk gene of HSV1. The analysis of cells of subclones isolated on a medium containing 5-bromodeoxyuridine (BrdU) shows that the number of copies of tk gene of HSV1 decreases as compared to the cells of parental clones. The decrease in the number of copies of tk gene of HSV1 in a row of BrdU-resistant subclones is accompanied by simultaneous increase in the number of sequences of pBR325 plasmide DNA to which tk gene of HSV1 is linked covalently in the pST826 plasmide introduced into cells of transformant clones. This evidence implies a most complex nature of transforming DNA rearrangements reducing the number of copies of tk gene of HSV1 due possibly to a genetic correction. The analysis of results permits a hypothesis that instability of cells in transformant phenotype may be determined by the genetic instability of insertion type. The rate of the loss of transformant phenotype depends on the frequency of rearrangements in the transforming DNA locus.

[Genetic transformation of somatic cells. VIII. The effect of the DNA methylation inhibitor 5-azacytidine on the stability of thymidine kinase-negative and -positive phenotypes of transformant clone cells]. / Geneticheskaia transformatsiia somaticheskikh kletok. VIII. Vliianie ingibitora metilirovaniia DNK 5-azatsitidina na stabil'nost' negativnogo i pozitivnogo po timidinkinaze fenotipa kletok transformantnykh klonov.

A study was made of the effect of an DNA methylation inhibitor 5-azacytidine (azaC) on the frequency of reversion to a thymidine kinase-positive (TK+) phenotype in 5-bromodeoxy-uridine (BrdU)-resistant subclones obtained from clones of Chinese hamster cells transformed by thymidine kinase gene (tk-gene) of Herpes simplex virus type 1 (HSV1). It is shown that in 8 of 15 BrdU-resistant subclones azaC increases 2-1000-fold the frequency of reversion to TK+ phenotype. Variations in the inducibility of reversions to TK+ phenotype indicate that the DNA methylation associated with TK- phenotype affects but differently tk gene of HSV1. Cultivation of TK+ cells of transformant clones in the presence of azaC may lead to stabilization (or decrease in the rate of the loss) of TK+ phenotype, or may not influence the stability of transformant phenotype. The reaction of TK+ cells of transformant clones depends both on genetically determined rate of the loss of TK+ phenotype, and on the structure of transforming DNA introduced to cells. A conclusion is drawn that the TK- phenotype of transformant clone cells arises due to processes which are not associated with methylation of tk gene of HSV1 in spite of the fact that such a methylation may later stabilize significantly the TK- phenotype.

[Expression in Chinese hamster transformant TK+-cells of the bacterial gene for beta-lactamase]. / Ekspressiia v transformantnykh TK-kletkakh kitaiskogo khomiachka bakterial'nogo gena beta-laktamazy.

Using monospecific immune antiserum the expression of beta-lactamase bacterial gene (from the plasmid pBR322) in transformed TK+ Chinese hamster cells was studied. It is shown that in some TK+-clones the beta-lactamase gene is expressed and this expression is stimulated by an inhibitor of DNA methylation-5-azacytidine. Two physically linked genes (the Herpes simplex thymidine kinase gene, and the beta-lactamase gene) introduced into the Chinese hamster cells on the same plasmid are found to be expressed independently.

[Genetic transformation of somatic cells. V. Inheritability of the rate of loss of the trait and the stabilization of the transformant phenotype]. / Geneticheskaia transformatsiia somaticheskikh kletok. V. Nasleduemost' skorosti poteri priznaka i stabilizatsiia transformantnogo fenotipa.

Subclones were isolated both on selective and nonselective medium from the Chinese hamster cells transformed by thymidine kinase gene (TK-gene) of Herpes simplex virus (HSV-1) and varying in the rate of the loss of transformant phenotype. The study of the stability of thymidine kinase-positive (TK+) phenotype in cell populations the subclones shows that the nonstability and the rate of the loss of transformant phenotype are the characters that are inherited in the cell generations. Durable cultivation on a HAT-selective medium may lead to a complete or partial (expressed as a reduced rate of the loss of the character) stabilization of TK+-phenotype of the cells of transformant clones. The rate of stabilization of TK+-phenotype may differ depending on the structure of transforming DNA introduced into cells of transformant clones.

[Genetic transformation of somatic cells. VI. Transfer of the trait of the rate of loss of transformant phenotype by means of DNA from cells containing the thymidine kinase gene of the herpes simplex virus]. / Geneticheskaia transformatsiia somaticheskikh kletok. VI. Peredacha priznaka "skorost' poteri transformantnogo fenotipa" s pomoshch'iu DNK iz kletok, soderzhashchikh gen timidinkinazy virusa prostogo gerpesa.

Using dot-hybridization with thymidine kinase gene (tk gene) of Herpes simplex virus type 1 (HSV 1) of DNA preparations obtained from isolated metaphase chromosomes and lysate fractions of metaphase cells, which presumably contain smaller particles compared to metaphase chromosomes, it has been shown that the tk gene of HSV 1 is localized in chromosomes of cells of transformant clones unstable in TK+-phenotype. The DNA isolated from the metaphase chromosomes from cells of transformant clones is 1.5- or 2-fold more efficient in transforming TK-Chinese hamster cells than is the total high molecular weight DNA from the same cells. Upon transformation of TK- cells by the high molecular weight DNA from the tk gene of HSV 1-containing clones, varying in the rate of the loss of TK+-phenotypes, the character "rate of the loss of transformant phenotype" is transferred together with the tk gene of HSV 1 in 22% of cases. Cells of rerevertant clones, produced from TK- subclones of transformant clones, display the rate of the loss of transformant phenotype characteristic of cells of parental TK+-clones. A comparison of the results allows a conclusion that DNA sequences, determining the character "rate of the loss of transformant phenotype", are linked tightly with the transforming DNA proper containing the tk gene of HSV 1, but are not localized inside such a DNA.

[Genetic transformation of somatic cells. II. An analysis of the status of the plasmid nucleotide sequences in chromosomal DNA and the thymidine kinase activity in transformant clone cells]. / Geneticheskaia transformatsiia somaticheskikh keltok. II. Analiz sostoianiia plazmidnykh nukleotidnykh posledovatel'nostei v khromosomnoi DNK i aktivnosti timidinkinazy v kletkakh transformantnykh klonov.

Chinese hamster A238 TK- -cells were transformed with plasmids (derivatives of pBR325) containing thymidine kinase (TK) gene of Herpes simplex virus type 1 (HSV1). The results of dot- and blot-hybridization indicate the presence of pBR325 sequences in the chromosomal fractions of DNA in the transformant clones. These sequences are probably tandemly arranged, and each cluster contains 25--50 copies. SV40 sequences cloned in pBR325 were introduced into the Chinese hamster cells by co-transformation with TK-gene of HSV1-containing plasmid DNA, and all the co-transformant clones selected for TK+-phenotype were shown by hydridization to contain 3V40 DNA fragments. Isoelectrofocusing in polyacrylamide gel shows that thymidine kinase from TK+-transformant clones is of viral type (isoelectric point 7), in contrast to the cellular enzyme (coded by chromosomal gene) having alkaline isoelectric point (pH 9). The results suggest that the true TK+-transformant cells are selected by the procedure used in this study.

[Genetic transformation of somatic cells. IV. The rate of loss of transformant phenotype depends on the structure of the transforming DNA and changes when the cells are treated with the tumor promotor 12-o-tetradecanoyl-phorbol-13-acetate]. / Geneticheskaia transformatsiia somaticheskikh kletok. IV. Skorost' poteri transformantnogo fenotipa zavisit ot struktury transformiruiushchei DNK i izmeniaetsia pri obrabotke kletok opukholevym promotorom 12-o-tetradekanoil-forbol-13-atsetatom.

Analysis was made of the phenotype stability of some clones of thymidine kinase deficient (TK-) Chinese hamster cells transformed by thymidine kinase gene (TK-gene) of Herpes simplex virus type (HSV 1). The presence of a fragment of human satellite DNA III in the plasmid DNA carrying the TK-gene of HSV 1 reduced notably the rate of the loss of TK+-phenotype, and the treatment of the cells with a tumour promoter--12-o-tetradecanoyl-phorbol-13-acetate--immediately after transformation destabilizes TK+-phenotype of transformant clones. Removal of the eukaryotic carrier DNA for the plasmid DNA without the TK-gene of HSV 1 destabilizes the clone transformant phenotype. Changes in the structure of the plasmid DNA containing no TK-gene of HSV 1 and introduced into cells simultaneously with TK-gene containing plasmids affects the rate of the loss of TK+-phenotype transformed cells.

[Genetic transformation of somatic cells. I. Clone of Chinese hamster cells defective in thymidine kinase and characterized by high transformation efficiency]. / Geneticheskaia transformatsiia somaticheskikh kletok. I. Klon kletok kitaiskogo khomiachka, defektnykh po timidinkinaze, otlichaiushchiisia vysokoi éffektivnost'iu transformatsii.

A characteristics is given of clone A238 of the Chinese hamster cells deficient in thymidine kinase (TK). The isolation procedure is described. Upon transformation with the aid of DNA of plasmids, containing thymidine kinase gene (tk-gene) of Herpes simplex virus type 1 (HSV1) clone A238 cells show frequency (7.10(-5) and efficiency (130 TK+ colonies per 1 microgram of plasmid DNA) compatible with those of mouse line LMtk- cells. Modified transformation and selection conditions of clone A238 cells expressing TK-gene of HSV1 are demonstrated. A simple method is described for discriminating somatic cells, expressing either their proper or a virus TK-gene according to the cloning efficiency of cells on the HAT medium containing thymidine in concentration 100 micrograms/ml. It is shown that at the fixed total DNA concentrations a complete replacement of the eukaryotic carrier DNA for the plasmid DNA, containing no tg gene of HSV1, decreases but only insignificantly the frequency and efficiency of transformation.

[Nature of the phenotypic variability of somatic cells in culture: unstable phenotypic changes]. / Priroda fenotipicheskoi izmenchivosti somaticheskikh kletok v kul'ture: nestabil'nye fenotipicheskie izmeneniia.

It was stated elsewhere ( Glebov , Abramyan , 1983) that the appearance of a number of phenotypic variants detected in somatic cell populations with high frequency should be provided by genetical unstable alterations. The properties of somatic cell variants that reproduce unstably a changable phenotype in the course of cell generations are analysed. These variants: (1) appear accidentally and independently on selectivity agent; (2) as a rule, the frequency of the variant arising does not increase under the action of mutagens; (3) the phenotypic reversion of unstable variants is a stochastic process; (4) such variants are characterized by intraclonal heterogeneity and by the segregation of stable alternative variants. The number of properties of phenotypically unstable variants isolated by one-step selection is similar to those for somatic cell variants isolated in the course of multistep selection. The latter are characterized by phenotypic reversion too. The appearance of unstable phenotypic variants is concluded to be associated with the genetical unstable alterations. It is argued that at least part of above alterations should be induced by the insertion of mobile genetic elements. The features of karyotypical variation in somatic cell population allow to conclude that the karyotype of cultured somatic cells is a genetically unstable attribute. The features mentioned above are: a high frequency of karyotypical alterations which is inherited by the cells with difference in the frequency of arising of karyotypical alteratons . The unstability of karyotype is restricted to the genetic unstability that is seen from non-random karyotypic variation, and interclonal difference in the chromosome stability. The site-specificity of karyotype alterations that proceed with high frequency allow to put forward a hypothesis that the process of mobile genetic element transposition is induced on the early stages of the history of constant cultured cell lines.

[Nature of the phenotypic variability of somatic cells in culture: stable and high-frequency phenotypic changes]. / Priroda fenotipicheskoi izmenchivosti somaticheskikh kletok v kul'ture: stabil'nye i voznikaiushchie s vysokoi chastotoi chastotoí fenotipicheskie izmeneniia.

The properties of stable phenotypic variants that appear with low frequency are considered. The data are presented that these variants are associated with both mutational and epigenetic changes. The properties of high frequency phenotypic variants that really appear and/or are detected in cell populations are analyzed more in detail. These variants: 1) arise accidentally and independently on the selectivity agent; 2) reproduce the phenotype that was changed in the course of cell generations constantly; 3) the frequency of the arising of such variants does not, as a rule, increase under the mutagen pressure, and 4) site-specificity of high frequency of changes is typical for those variants. The high frequency of phenotypic alterations causes intraclonal heterogeneity of somatic cell populations. This heterogeneity cannot be elicited by manifold subcloning. The properties of genetic unstable variations and accessible expression of these variations in populations of somatic cells in culture are discussed. The arising of a number of variants detected with high frequency is concluded to be generally provided by unstable genetic alterations. These alterations are suggested to be a result of the insertion of mobile genetic elements.

[Hereditary changes in gene activity as 1 of the causes of phenotypic heterogeneity in 8-azaguanine-resistant Chinese hamster cells (CHO-K1)]. / Nasleduemye izmeneniia gennoi aktivnosti kak odna iz prichin fenotipicheskoi geterogennosti ustoichivykh k 8-azaguaninu kletok kitaiskogo khomiachka (CHO-K1).

Eight Chinese hamster clones (CHO-K1) growing at the 30 mg/ml concentration of 8-azaguanine (AG) were studied. Clones were differentiated by their resistance to AG and to 6-thioguanine, by their plating efficiency on HAT medium, and by the level of hypoxantine incorporation in cells. The differences in phenotypic properties were shown to be associated with variability in hypoxanthine-guanine phosphoribosyltransferase (HPRT) activity. HPRT Michaelis constant (KM) for hypoxanthine and phosphoribosylpyrophosphate, and maximal reaction rate (Vm) offered considerable differences between all the resistant clones and sensitive cells. The only possible reason of these differences is a change in the HPRT coding locus. According to the results of the analysis of B15-4b-4 subclones, phenotypic and HPRT activity differences are also connected with each other; however, all subclones have the same KM of HPRT as that of the parental clone. So, differences in HPRT activity (and in Vm) may reflect changes in the HPRT content in cells of subclones. Hence, phenotypic heterogeneity of AG-resistant clones is determined by the interaction of mutational changes in the HPRT locus, and hereditable changes of genetic activity, responsible for variation of HPRT quantity in cells.

[Intraclonal heterogeneity and instability of CHO-K1 Chinese hamster cells in sensitivity to ultraviolet light and resistance to 8-azaguanine]. / Vnutriklonal'naia geterogennost' i nestabil'nost' kletok kitaiskogo khomiachka CHO-K1 po chuvstvitel'nosti k ul'trafioletovomu svetu i ustoichivosti k 8-azaguaninu.

The phenotypic instability of a 8-azaguanine (AG)-resistant clone A14--2c-1 was previously reported (Abramyan et al., 1979) to be determined by genetic (replicative) instability. Further, phenotype gene activity changes are characteristic of genetically instable "mutant", which may be "passed" from one locus to another. In the present work, some clones were isolated from clone A14-2c-1 differing in their sensitivity to lethal UV-radiation, lethal dose D37 differences being almost 6 times. During a further cultivation through 90 passages (300 cell generations), two of four clones changed their D37 values: for clone 2c-15 it increased by 3 times, for clone 2c-16 it decreased more than twice. Besides, subclones of 2c-15 and 2c-16 clones had also different D37 values. With respect to AG-resistance, clone 2s-15 was shown to have LD50 to AG, similar to that of the parental one-while in 3 other clones LD50 was 3 times as much. These differences are associated with variations in hypoxanthine-guanine phosphoribosyltransferase (HPRT) activity: in clones A14-2c-1 and 2c-15 this activity is two times higher than in other clones. All the clones have the same value of the Michaelis constant for hypoxanthine and phosphoribosylpyrophosphate. It can be outlined that difference in HPRT activity and quantity in cells are closely related. Thus, phenotypic instability of A14-2c-1 clone offers characteristic features of genetic (replicative) instability: instability in AG-resistance and UV-sensitivity coincides with interclonal heterogeneity according to unstable markers; the unstable property may be transmitted from one locus (responsible for AG-resistance) to be another one (UV-sensitivity); and different level of AG-resistance in clones is probably determined by changes in gene activity, which lead to differences in HPRT quantity in cells.

[Karyotypic structure of Chinese hamster clone cell populations in prolonged culture]. / Kariotipicheskaia struktura klonovykh populiatsii kletok kitaiskogo khomiachka pri dlitel'nom kul'tivirovanii.

The Karyotypic structure of some clones and subclones of Chinese hamster cells has been studied (CHO-KI). 60-80% of all the clonal cells demonstrated the identical karyotype (the main structural variant of karyotype - MSVK). The distribution of cells according to the number of chromosomes does not vary within 90 passages of clone cultivation (about 300 cell generations) and 35 passages of subclone cultivation (more than 100 cell generations). In two of three clones the frequencies of cells with MSVK and with additive karyotype variants do not change, while in one clone a long cultivation leads to lowering the frequency of cells with MSVK and to growing the frequency of cells with other karyotype variants. Recloning does not lead to decreasing the karyotypic heterogeneity of clonal populations. The distribution of clonal cells according to the deviation from the modal number of chromosomes is closely related to the Poisson distribution, while the distributions of cells from two clones with the stable karyotypic structure according to the number of greater structural greater deviations from MSVK are related to a degeneration Poisson distribution. Different chromosomes are involved in karyotypic changes in variable proportions, and the frequencies of cells with simultaneous deviations in various chromosome groups are in most instances higher than frequencies expected in the case of independent deviations. The reasons of stability of karyotypic structure in clonal populations is discussed.

[Variability of Chinese hamster clones in resistance to 8-azaguanine]. / Izmenchivost' klonov kitaiskogo khomiachka po ustoichivosti k 8-azaguaninu.

The stability of the 8-azaguanine resistance marker during the number of cell generations has been analyzed for 63 clones of Chinese hamster cells. The resistant clones analyzed were spontaneous or induced after the treatment of parent cells by 5-BUdR, EMS, or histone H1. The clones have revealed different levels of AG-resistance as well as different ability to maintain the above level, under the cultivation in the drug-free medium. Subclones, isolated from the unstable clone, were found to be different in both the resistance level and the stability of resistant state.