Lack of spontaneous sister chromatid exchanges in somatic cells of Drosophila melanogaster

REV1 Coordinates a Multi-Faceted Tolerance Response to DNA Alkylation Damage and Prevents Chromosome Shattering in Drosophila melanogaster

When replication forks encounter damaged DNA, cells utilize DNA damage tolerance mechanisms to allow replication to proceed. These include translesion synthesis at the fork, postreplication gap filling, and template switching via fork reversal or homologous recombination. The extent to which these different damage tolerance mechanisms are utilized depends on cell, tissue, and developmental context-specific cues, the last two of which are poorly understood. To address this gap, we have investigated damage tolerance responses following alkylation damage in Drosophila melanogaster. We report that translesion synthesis, rather than template switching, is the preferred response to alkylation-induced damage in diploid larval tissues. Furthermore, we show that the REV1 protein plays a multi-faceted role in damage tolerance in Drosophila. Drosophila larvae lacking REV1 are hypersensitive to methyl methanesulfonate (MMS) and have highly elevated levels of γ-H2Av foci and chromosome aberrations in MMS-treated tissues. Loss of the REV1 C-terminal domain (CTD), which recruits multiple translesion polymerases to damage sites, sensitizes flies to MMS. In the absence of the REV1 CTD, DNA polymerases eta and zeta become critical for MMS tolerance. In addition, flies lacking REV3, the catalytic subunit of polymerase zeta, require the deoxycytidyl transferase activity of REV1 to tolerate MMS. Together, our results demonstrate that Drosophila prioritize the use of multiple translesion polymerases to tolerate alkylation damage and highlight the critical role of REV1 in the coordination of this response to prevent genome instability.

Dicentric chromosome breakage in Drosophila melanogaster is influenced by pericentric heterochromatin and occurs in nonconserved hotspots

Chromosome breakage plays an important role in the evolution of karyotypes and can produce deleterious effects within a single individual, such as aneuploidy or cancer. Forces that influence how and where chromosomes break are not fully understood. In humans, breakage tends to occur in conserved hotspots called common fragile sites (CFS), especially during replication stress. By following the fate of dicentric chromosomes in Drosophila melanogaster, we find that breakage under tension also tends to occur in specific hotspots. Our experimental approach was to induce sister chromatid exchange in a ring chromosome to generate a dicentric chromosome with a double chromatid bridge. In the following cell division, the dicentric bridges may break. We analyzed the breakage patterns of 3 different ring-X chromosomes. These chromosomes differ by the amount and quality of heterochromatin they carry as well as their genealogical history. For all 3 chromosomes, breakage occurs preferentially in several hotspots. Surprisingly, we found that the hotspot locations are not conserved between the 3 chromosomes: each displays a unique array of breakage hotspots. The lack of hotspot conservation, along with a lack of response to aphidicolin, suggests that these breakage sites are not entirely analogous to CFS and may reveal new mechanisms of chromosome fragility. Additionally, the frequency of dicentric breakage and the durability of each chromosome's spindle attachment vary significantly between the 3 chromosomes and are correlated with the origin of the centromere and the amount of pericentric heterochromatin. We suggest that different centromere strengths could account for this.

Participation of recombination proteins in rescue of arrested replication forks in UV-irradiated Escherichia coli need not involve recombination

Alternative reproductive cycles make use of different strategies to generate different reproductive products. In Escherichia coli, recA and several other rec genes are required for the generation of recombinant genomes during Hfr conjugation. During normal asexual reproduction, many of these same genes are needed to generate clonal products from UV-irradiated cells. However, unlike conjugation, this latter process also requires the function of the nucleotide excision repair genes. Following UV irradiation, the recovery of DNA replication requires uvrA and uvrC, as well as recA, recF, and recR. The rec genes appear to be required to protect and maintain replication forks that are arrested at DNA lesions, based on the extensive degradation of the nascent DNA that occurs in their absence. The products of the recJ and recQ genes process the blocked replication forks before the resumption of replication and may affect the fidelity of the recovery process. We discuss a model in which several rec gene products process replication forks arrested by DNA damage to facilitate the repair of the blocking DNA lesions by nucleotide excision repair, thereby allowing processive replication to resume with no need for strand exchanges or recombination. The poor survival of cellular populations that depend on recombinational pathways (compared with that in their excision repair proficient counterparts) suggests that at least some of the rec genes may be designed to function together with nucleotide excision repair in a common and predominant pathway by which cells faithfully recover replication and survive following UV-induced DNA damage.

Looking at Drosophila mitotic chromosomes

The repertoire of cytological procedures described in the present paper permits full analysis of brain neuroblast chromosomes. Moreover, if brains are cultured for 13 hr in the presence of 5-bromo-2'-deoxy-uridine, our fixation and Hoechst staining protocols allow visualization of sister chromatid differentiation and the scoring of sister chromatid exchanges (Gatti et al., 1979). Finally, we note that our cytological procedures can be successfully employed for preparation and staining of gonial cells of both sexes and male meiotic chromosomes (Ripoll et al., 1985; our unpublished results). Good chromosome preparations of female meiosis are obtained with the procedure described by Davring and Sunner (1977, 1979), Nokkala and Puro (1976), and Puro and Nokkala (1977). In this chapter, we have focused on the organization and behavior of Drosophila mitotic chromosomes, describing a repertoire of cytological techniques for neuroblast chromosome preparations. We have not considered the numerous excellent cytological procedures for embryonic chromosome preparations (for an example, see Foe and Alberts, 1985; Foe, 1989), because these chromosomes are usually less clearly defined than those of larval neuroblasts. In addition, we have not included the whole-mount and squashing techniques that allow chromosome visualization and spindle immunostaining of neuroblast cells (Axton et al., 1990; Gonzalez et al., 1990), male meiotic cells (Casal et al.. 1990; Cenci et al., 1994), and female meiotic cells (Theurkauf and Hawley. 1992), because the fixation methods used in these procedures alter chromosome morphology. Fixation methods for antibody staining result in poorly defined chromosomes, whereas the methanol/acetic acid fixation techniques, such as those described here, preserve very well chromosome morphology but remove a substantial fraction of chromosomal proteins. Thus, one of the major technical breakthroughs in Drosophila mitotic cytology will be the development of fixation procedures that maximize chromosomal quality with minimal removal of proteins. This will be particularly useful for precise immunolocalization of heterochromatic proteins, including those associated with the centromere.

A duplication including the Y allele of Lcp2 and the TRIM retrotransposon at the Lcp locus on the degenerating neo-Y chromosome of Drosophila miranda: molecular structure and mechanisms by which it may have arisen

Evolutionary changes during the process of sex chromosome differentiation in Drosophila miranda are associated with massive DNA rearrangements. Comparing the DNA structure of the larval cuticle protein (Lcp) region from the X2 and neo-Y chromosome pair, we observed insertions, deletions and a large duplication at the neo-Y chromosomal locus. The duplication encompasses a complete copy of the neo-Y allele of Lcp2, and the ISY3 and the ISY4 insertion sequences. The latter was identified as a retrotransposon, termed TRIM. ISY3 shows DNA sequence similarity to P element homologs identified in the Drosophila obscura species group. We were interested in mechanistic aspects generating the duplication. We cannot exclude unequivocally that unequal sister-chromatid exchange could give rise to the observed duplication; however, recombination is a rare event in Drosophila males. Location and sequence of the retrotransposon TRIM served as molecular markers allowing us to reconstruct two intrachromosomal transposition events that could lead to the observed duplication.

Mechanistic and methodological aspects of chemically-induced somatic mutation and recombination in Drosophila melanogaster

This study presents the analysis of chemically-induced somatic mutations and chromosomal damage in the eye imaginal discs of Drosophila larvae, assayed later as twin (TS) and single light (LS) mosaic spots in the adult eyes. Regarding the question as to what kind of DNA alterations contribute to somatic cell mutagenicity, the approach followed here has been to investigate the possible differences in response between male (hemizygous for an X) and female (homozygous) larvae, rod-X/rod-X versus ring-X/rod-X genotypes and inversion-heterozygotes versus genotypes not carrying an inversion. The systems chosen for this analysis were the white-coral/white (wco/w) and the white+/white (w+/w) eye mosaic system. The principle findings with 12 mutagens of different modes of action are as follows: (1) At least 98% of all TS and LS induced by cisplatin (DDP) in wco/w female larvae and about 95% of those by formaldehyde (FA) appear as the result of recombinogenic activity between the two homologous X-chromosomes. The corresponding estimates for MMS, EMS and ENU are 81%, 73% and 61%, respectively. (2) The long scS1L sc8R inversion, which also contains In(1)dl-49, suppresses induction of TS to 83-93%. There was also a sharp decline in the frequency of LS in inversion heterozygotes for DDP (91%), FA (86%), MMS (52%) and EMS (47%). (3) Ethylnitrosourea (ENU) was the mutagen for which introduction of the inverted chromosome reduced only slightly (23%) the frequency of LS, indicating that the majority of them were somatic mutations (and deletions) at the white locus. (4) In w/RX females heterozygous for a ring-X chromosome, the frequency of LS was only approximately one tenth of that of the control (w+/w) group, after exposure to MMS or DDP. The explanation is that exchange processes involving the ring frequently lead to genetic imbalance with subsequent cell killing.

Sister chromatid exchange points in the heterochromatin and euchromatin regions of Chinese hedgehog chromosomes

The Chinese hedgehog has a diploid chromosome number of 48 in which there are eleven pairs of telo- or subtelocentric autosomes, twelve pairs of meta- or submetacentric autosomes, a metacentric X chromosome and a telocentric Y chromosome. The heterochromatin is almost completely distributed in five large distal segments of chromosomes nos. 9 to 12 and no. 18. There is no positive C-band in the centromeres of the chromosomes except for the X chromosome which has a small, weakly stained C-band in the centromere. In Chinese hedgehog cells 52.1% of SCEs are found at the junction between the euchromatin and the heterochromatin, 39.5% in the heterochromatin and 8.4% in the auchromatin. The SCE number per unit C-band is double the SCE number per unit euchromatin. The SCE rate in the heterochromatin or euchromatin regions is not proportional to their chromosome length and can be quite different between different pairs of the chromosomes. Our results indicate that there is a non-uniform distribution of the SCEs in the Chinese hedgehog cells.

Sister chromatid exchange and the evolution of rDNA spacer length

The structures of rDNA spacers from several species have been characterized and virtually all have internally repeated sequences. Different numbers of these internal repeats are responsible for most spacer length variation. Because unequal recombination between these internal repeats will cause new length variation, while unequal exchange between rDNA copies will homogenize the variants, we modeled the interaction of these two processes. Two models were used to simulate both types of unequal exchange at the sister chromatid level. Both models indicate that a narrow range of relative recombination frequencies is required to produce levels of variability comparable to those published. One model puts a lower limit on the number of internal repeats, and the other puts both a lower and upper limit on the number of repeats. The model with both maximum and minimum constraints produces a distribution closer to actual spacer distributions. These results imply that small changes in recombination rates can generate the differences in numbers of length variants observed in different species.

Analysis of hexamethylphosphoramide (HMPA)-induced genetic alterations in relation to DNA damage and DNA repair in Drosophila melanogaster

This paper reports the results of a study on the mutagenic profile of HMPA in Drosophila melanogaster. HMPA produced all types of genetic damage tested for in post-meiotic cells of treated males; at the concentrations used, recessive lethals and ring-X losses were induced at significant rates while 2-3 translocations, entire and partial Y-chromosome losses only occurred at low rates. From a comparison with alkylation-induced mutational spectra, we note a number of peculiarities of HMPA mutagenesis: there is no storage effect on HMPA-induced translocations; the ratio of F2-lethals: F3-lethals varies from 6:1 to 9:1, indicating a low capacity of HMPA for delayed mutations; the use of the DNA-repair-deficient mei-9L1 females instead of an excision-proficient control strain has no influence on the recovery of mutations (recessive lethals) induced in males; the high frequencies of chromosome loss (CL) induced by HMPA, which are mostly due to ring-X loss, leads us to speculate that one (or more) of its metabolites acts as a DNA-crosslinking agent. In experiments on maternal effects with mei-9LI females, there is a 20-40% reduction in the rates of induced CL. Conversely, with mei-41D5 females, there is a weak increase in CL frequencies. HPLC analysis of DNA reacted with [14C]HMPA exhibits no methylation at the O6 or the N-7 of guanine. This finding, together with the observed inactivity of hexaethylphosphoramide (HEPA) in the recessive lethal assay, suggests that the formation of DNA-bound forms from HMPA may not be the result of simple methylation reactions. This conclusion is supported by the genetic data, i.e., the lack of a storage effect on HMPA-induced chromosome rearrangements. Consistent with a hypothesis by Brodberg et al. (1983) to explain the action of cisplatin in Drosophila, comparisons of the spectra of genetic alterations produced by HMPA, A 139 (bifunctional) and Thio-TEPA (trifunctional) in the assay for chromosome loss suggest the involvement of two distinct mechanisms in the formation of ring-X loss by crosslinking agents. One pathway concerns induction of chromosome loss as a consequence of sister-chromatid exchanges (SCEs). The second mechanism may be due to DNA adducts or a single adduct responsible for both a fraction of CL and for induced partial Y-loss (PL). Inactivation of the mei-9+ function has two consequences: SCE-mediated ring-X loss frequency is lowered in mei-9 females in comparison to the repair-proficient control strain, while the opposite effect is indicated for that fraction of ring-X loss generated by the second mutational pathway.(ABSTRACT TRUNCATED AT 400 WORDS)

SCE and DNA methylation

The interrelationship between sister chromatid exchange (SCE) formation and DNA methylation was studied in Chinese hamster V79 and Indian muntjac cells. A DNA methylation inhibitor, 5-azacytidine (5AzaC), induced SCEs only when it had been present in cells for at least 2 rounds of DNA replication. This result suggests that SCEs are formed during replication of hemimethylated or demethylated DNA possessing 5AzaC, and that hypomethylated sites may become fully methylated after they pass 1 cell division. It also appears that hypomethylated DNA is not more sensitive to ultraviolet light (UV) or 3-aminobenzamide (3AMB) than normal chromosomes, but sensitized to mitomycin C (MMC) for the induction of SCEs. An analysis of sites of SCEs induced by 5AzaC within Indian muntjac chromosomes showed that the SCE frequency was enhanced at the 5 methylcytosine-rich regions where spontaneous SCEs were intensively suppressed. The SCE mechanism at the junction between contiguous replicons with different replication timing was discussed.

Sister chromatid exchange levels and cell cycle time in human bone marrow cells and lymphocytes

The frequency of sister chromatid exchange (SCE) and the cell-cycle-specific pattern of mitoses were analyzed at the same time in normal bone marrow cells and lymphocytes of six healthy donors. The SCE frequency was found to be significantly higher in lymphocytes. The cell-cycle-specific pattern revealed significantly shorter cell cycle times for normal bone marrow cells as compared with those of phytohemagglutinin (PHA) stimulated lymphocytes. Chromosomes of bone marrow metaphases displayed a more contracted morphology.

Analysis of baseline and BrdU-dependent SCEs at different BrdU concentrations

In the present paper we have used a rationale based on the development of theoretical equations that define sister-chromatid exchange (SCE) frequencies as a function of two variables, namely the baseline (BrdU-independent) and the BrdU-dependent SCE frequencies. The experimental design includes the estimation of SCE frequencies in second division chromosomes when both cycles occurred in the presence of BrdU and when BrdU incubation took place only during the first cycle in a wide range of BrdU concentrations. The final SCE yields in second division chromosomes could be separated into three different components: (1) The BrdU-independent, 'spontaneous' or baseline SCEs, whose low but biologically significant frequency was calculated to be about 0.06 SCEs per pg of DNA; this figure could be similar for most of the cell types; (2) the BrdU-dependent SCEs whose frequency increases with BrdU dose, probably as a result of BrdU substitution for thymidine; (3) the BrdU-dependent SCEs as a consequence of other cellular factors such as disturbance of nucleotide pool sizes. At high BrdU concentrations (300 microM upward) the three components appear to have a significant value in the final SCE yield, whereas at lower BrdU doses the third component seems to be negligible.

The effects of incorporated tritium and bromodeoxyuridine on the frequency of sister chromatid exchanges

Cells in third mitosis treated during the first cell cycle with 3H-TdR and during the next two cycles with BrdU (without 3H-TdR) show a typical pattern of chromosome differentiation which allows identification of sister chromatid exchanges occurring during the first (SCE1), second (SCE2) and third cycles (SCE3). Chromosomes labeled only with 3H-TdR had the most SCEs; those labeled only with BrdU, the second highest number; and those labeled with 3H-TdR plus BrdU, the fewest. Since BrdU and 3H-TdR are well known inducers of SCEs, the relatively low frequency of exchanges produced by the combined action of these two compounds is paradoxical. It is assumed that SCEs are generated by the abnormal recombination of double-strand DNA breaks occurring at the junctions between completely and partially duplicated replicon clusters. Thus, agents that induce absolute blocks to DNA fork displacement will favor the appearance of SCEs because double-strand breaks have more time to occur at junctions. Conversely, agents that inhibit the initiation of replication will decrease the probability of SCEs. Ionizing radiation delays the onset of cluster replication. Therefore, in 3H-TdR plus BrdU-substituted chromosomes the radiation from tritium may inhibit the appearance of BrdU-induced SCEs. Since the inhibition does not exist in chromosomes substituted only with BrdU, the frequency of SCEs in these elements is higher than in double-substituted chromosomes. During the first cell cycle the onset of cluster replication is normal. However, the incorporation of 3H-TdR in the replication fork may enhance the appearance of double-strand breaks, thus inducing a high frequency of SCEs.

Influence of age on the frequency of sister-chromatid exchanges and X-ray-induced chromosome aberrations in muntjac

The BrdU-differential staining technique was used in a study of the frequency of sister-chromatid exchanges (SCEs) and X-ray-induced chromosome aberrations in peripheral blood lymphocytes of the same individual muntjacs. Blood was collected periodically from immediately after birth (1 day old) to the adult stage (1 year). The results showed that both the frequency of base-line SCEs and induced chromosome aberrations changed as a function of age. At a young age, the frequency of SCEs was significantly low, whereas a high frequency of chromosome aberrations was observed. But with increase in age of the individuals, an enhanced frequency of SCEs and a decreased frequency of induced chromosome aberrations were observed; and as the age advanced further, the frequencies of both SCEs and chromosomal aberrations came to a steady level.

The effect of age and cell proliferation on the frequency of sister chromatid exchange in human lymphocytes cultured in vitro

The impact of ageing on the frequencies of sister chromatid exchange of humans was determined in lymphocytes of 43 healthy male non-smokers. The frequencies of sister chromatid exchange in individuals under 70 years were the same, but were significantly lower than in men of 70 years and older. Since we did not see a difference in the frequencies of sister chromatid exchange between fast- and slow-cycling lymphocytes, this difference probably would still hold if the lymphocyte subpopulations of these people were all examined.

The induction of chromosome aberrations by cis-platinum(II) diamminodichloride in Drosophila melanogaster

We have determined the in vivo effects of cis-platinum(II)diamminodichloride (cis-PDD) treatment on the induction of chromosome aberrations in Drosophila melanogaster germ cells. cis-PDD treatment induces significant increases in chromosome breakage in all stages of spermatogenesis in a battery of test systems using ring or rod-X males and repair-proficient or deficient females. Since no increase in nondisjunction was induced by cis-PDD in either male or female germ cells, any aneuploidy inducing effects of this compound should result from its clastogenic action. We also find that mei-9 excision repair function is involved in the repair of cis-PDD-induced DNA lesions in a manner that provides additional evidence that partial and ring chromosome losses are not completely homologous.

Meiosis in Drosophila melanogaster, III. The effect of orientation disruptor (ord) on gonial mitotic and the meiotic divisions in males

Orientation disruptor (ord), a meiotic mutant that is recombination defective in females and disjunction defective in males and females, has been analyzed using serial section electron and light microscopy. From analysis of primary spermatocytes we have confirmed that ord males are defective in some aspect of the mechanism(s) that holds sister chromatids together during meiosis. In addition, we have determined that ord causes high frequencies of nondisjunction during spermatogonial mitotic divisions, as well as during the meiotic divisions. Mitotic nondisjunction involves the large autosomes more frequently than the sex chromosomes or chromosome 4 and results in high frequencies of primary spermatocytes that are either monosomic or trisomic for chromosome 2 or 3. Abnormalities in spermatocyte cyst formation are also observed in males homozygous for ord. These abnormalities include loss of regulation of meiotic synchrony and the number of gonial cell divisions.

Region-specific effects on chromosome integrity of mutations at essential loci in Drosophila melanogaster

Two mutagen-sensitive loci of Drosophila melanogaster, mus-105 and mus-109, previously identified by viable alleles, are shown to specify essential functions. Lethal alleles at the loci produce larvae that have degenerate imaginal discs and die at the larva-pupa boundary. Our data suggest that the causes of lethality are intolerable levels of cell death produced by high frequencies of chromosome aberrations (in excess of 0.5 aberration per cell per cycle). The pattern of aberrations is a locus-specific character. In mus-105 mutants the most common aberrations are breaks and exchanges in euchromatic portions of the genome whereas in mus-109 mutants the most common aberrations are breaks at heterochromatin-euchromatin junctions. The sensitivity of these junctions to breakage in mus-109 mutants is a property of all such junctions whether natural or produced by a rearrangement that juxtaposes heterochromatin and euchromatin. Larvae carrying the combination of two viable mutants, mus-105(A1) mus-109(D1), die at the larva-pupa boundary and display a high frequency of aberrations (0.7 per cell vs. 0.075 for either mutant alone) clustered at euchromatin-heterochromatin junctions. This synergistic interaction suggests there is a class of lesions that can be repaired by both mus-105(+) and mus-109(+). Thus, the apparent euchromatic specificity of mus-105(+), which was inferred from the pattern of predominantly euchromatic breakage observed in mus-105 mus-109(+) flies, is in fact generated by the wild-type function of mus-109(+) masking an effect of mus-105 in the heterochromatin. The fact that lethal mutants at the mus-105 and mus-109 loci have small imaginal discs coupled with the observation of a maternal effect of mus-105 suggests a paradigm for the control of cell division during the life cycle of Drosophila.

Selective differential staining of sister chromatids of the facultative heterochromatic X chromosome in the female mouse

Selective differential staining of sister chromatids for the facultative heterochromatic X chromosome in the female mouse has been achieved by the combination of two differential staining techniques; one for the heterochromatic X chromosome and the other for sister chromatids. Thermal hypotonic treatment moderately destroyed the chromosome structure except for the heterochromatic X in BrdU labelled metaphase cells, resulting in the selective sister chromatid differentiation of this X with Giemsa stain. This technique enables us to know the exact frequency of th spontaneous sister chromatid exchanges in the heterochromatic X without using 3H-TdR labelling for detecting the late DNA replication. The results indicate that the sister chromatid exchange frequency of the heterochromatic X chromosome is not affected by its late DNA replication during S phase, or by the genetic inactivation and the resulting heterochromatinization.

Approximation of baseline and BrdU-induced SCE frequencies

In the present paper we have developed a new rationale and an experimental schedule to approximate the frequency of SCEs which occur independently of BrdU incorporation, namely, the baseline frequency of SCEs. The method used includes the analysis of SCE yields in second and third division chromosomes after BrdU-substitution for 1, 2, and/or 3 successive replication rounds in the presence of this thymidine analogue, leading to a set of ten different experimental results. As a result of formulating various mathematical equations and applying them to the data, an accurate estimation of the frequency of baseline (BrdU-independent) and BrdU-induced SCEs, can be made, thus avoiding the difficulties inherent in the current extrapolation methods. The conclusions are that 1) SCEs seem to be formed after DNA synthesis (by exchanging post-replicative DNA portions), but, obviously, very near to the replication fork and 2) that under our experimental conditions about 0.065 SCEs per picogram of DNA per cell cycle occur as a consequence of chromosome replication, this frequency being increased by BrdU-substitution. The methodology seems to be reliable enough to be used in other species and systems in order to compare baseline SCE frequencies.

Sister-chromatid exchanges: a report of the GENE-TOX program

The effects of a number of chemicals on sister-chromatid exchange (SCE) frequencies in in vivo and in vitro systems are reviewed. Standardized protocols for future SCE testing in important systems, as well as for evaluation of test results, are presented. Data reported thus far suggest that SCE analysis may prove useful, especially at a secondary level, as a test of mutagenic carcinogens. Strengths and limitations of SCE analysis are summarized as a guide for future evaluation and use of this procedure.

Spontaneous sister-chromatid exchanges in Chinese hamster cells in vivo and in vitro

The incidence of sister-chromatid exchanges (SCEs) was studied in Chinese hamster cells. The cells used were an established cell line (Don), an aneuploid secondary culture still exhibiting contact inhibition of growth, a primary culture, bone-marrow cells in vivo, a Don-derived clone having ring chromosomes, and endoreduplicated Don cells. The frequency of SCEs in the presence of bromodeoxyuridine (BUdR) increased in the following order: bone-marrow cells less than Don less than secondary culture cells less than primary culture cells. Marked increases in BUdR concentration induced only slight increases of SCEs. Some ring chromosomes showed Moebius strip and concatenated ring structures, indicating spontaneous occurrence of SCEs in the absence of BUdR. The incidence of spontaneous SCEs observed in ring chromosomes approximates that of SCEs observed in ordinary rod chromosomes in the presence of low dose levels of BUdR. SCEs occurring in the first and second cell cycles were separately counted in endoreduplicated cells. The ratio of single SCEs, which occurred in the second cell cycle, to twin SCEs, which occurred in the first cell cycle, was about 2, when the count of single SCEs was corrected for the induction effect. This implies that uninemy chromatids retain the polarity of DNA when SCEs occur, that bifilarly and trifilarly BUdR-substituted DNA strands give equal numbers of SCEs and that incidence of SCEs is independent of the length of S in the Don cells used here.

Relationship between sister chromatid exchanges and DNA replication in somatic cells of Drosophila melanogaster

In Drosophila melanogaster cell lines and larval neuroblast cells, two aspects of the phenomenon of sister chromatid exchanges were analyzed: (1) the frequency of SCEs in relation to the ploidy level (comparing diploid and tetraploid cells) and in relation to the cell type (comparing embryonic and larval cells) (2) the localization of the sites of exchange with reference to eu- and heterochromatin. A good correlation between SCE frequency and genome size in the same cell type (in distant species also), but a significant difference in the SCE rate between different cell types within the same species, were found. The results confirmed also the non-random distribution of SCEs in the different portions of the genome since a preferential localization in the euchromatin was clearly demonstrated. Moreover, a direct proportionality between SCE frequency and the length of the S phase was supposed, favouring the hypothesis of a relationship between the phenomenon of sister chromatid exchanges and DNA replication.

Sister chromatid exchanges and heterochromatin

The inter- and intrachromosomal distribution patterns of SCEs obtained with or without mutagen treatment are reviewed and compared, with each other as to their relation to heterochromatin and with the distribution patterns of chromatid aberrations that occurred either "spontaneously" in chromosomes of repair-defective human syndromes or after treatment with the mutagens (BrdU, ethylalcohol, DMBA, TMBA, maleic hydrazide, MMS, MMC). The conclusions are: No general rule is detectable for nonrandom involvement of heterochromatin in spontaneous SCEs. Mutagen-induced SCEs show the same or very similar distribution patterns as the spontaneous ones and are in no case as preferentially located as chromatid aberrations (which involve mainly the junctions between eu- and heterochromatin or other special regions). Therefore, a specific mutagen sensitivity of heterochromatin-aberrations does not exist (or is less pronounced) for SCEs. This supports the inference that different mechanisms underlie the origins of the two phenomena.

Sister chromatid exchange analysis

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Effects of recombination-deficient and repair-deficient loci on meiotic and mitotic chromosome behavior in Drosophila melanogaster

The results of recent genetic and cytological studies on recombination-defective and repair-defective mutants of Drosophila melanogaster are summarized. These studies show that there is substantial overlap between the functions used in various aspects of DNA metabolism in Drosophila. Most loci first identified by either recombination-defective or mutagen-sensitive mutants have been shown also to function in nonmutagenized mitotic cells where their action is necessary to maintain the integrity of the genome: mutants at particular loci produce elevated frequencies of chromosome breakage, mitotic exchange, mutation, and/or chromosome loss. Genetic studies of meiotic recombination show that many of the loci identified by recombination-defective mutants restrict where along the chromosome arms exchange may occur. Recent EM studies suggest that the products of at least some of these loci are components of recombination nodules. Region-specific control of DNA metabolism is also indicated by the finding of nonrandom patterns of chromosome breakage in some mutagen-sensitive mutants. Recombination-defective mutants at two loci have been studied for their effects on sister chromatid exchanges (SCEs) and x-ray induced aberrations. Mutants at both loci are defective in steps necessary for the production of symmetrical chromatid interchanges but have little effect on SCEs.

Mutation induction in repair-deficient strains of Drosophila

Experimental evidence indicates a polygenic control of mutagenesis in Drosophila melanogaster. In oocytes chromosome aberrations detected as half-translocations or dominant lethals depend on a repair system which in a number of genetically nonrelated strains shows different repair capacities. Sister chromatic exchanges (SCE) are easily studied as ring chromosome losses. They develop through a genotype controlled mechanism from, premutational lesions. Stocks with particular pairs of third chromosomes were discovered in which increased sensitivity of larvae to the toxic effects of a monofunctional alkylating agent (MMS) correlates with high frequencies of x-ray induced SCE's. Sex-linked mutagen-sensitive mutants could be shown to control mutation fixation: Pronounced maternal effects were found when sperm carrying particular types of premutational lesions were introduced into different types of mutant oocytes. The mutant mus(1)101D1 was found to be unable to process lesions induced by the crosslinking agent nitrogen mustard (HN2) into point mutations (measured as sex-linked recessive lethals). Alkylation damage leads to increased point mutation frequencies in the excision repair deficient mutant mei-9L1, but to reduced frequencies in the post-replication repair deficient mutant mei-41D5. It became clear that the study of maternal effects on mutagenized sperm represents an efficient tool to analyze the genetic control of mutagenesis in the eukaryotic genome of Drosophila melanogaster.

Sister chromatid exchange in Ph1-positive chronic myelocytic leukemia

The sister chromatid exchange (SCE) frequency in bone-marrow cells of 12 untreated patients with chronic myelocytic leukemia (CML) was analysed and compared with the SCE frequency in bone-marrow cells of nine healthy persons. In normal persons the SCE ranged from 3.64 to 5.15 per cell. In CML patients the SCE was significantly lower, ranging from 2.32 to 3.44 per cell. The differences found were unrelated to patients' age and contraction state of the chromosomes. It is suggested that the leukemic process could account for the low SCE rate.