Female-specific mutagenic response of mice to hycanthone

Male and female gametogeneses differ markedly in all mammals. While male germ cells are continuously being produced from stem cells throughout the reproductive life span, the number of female germ cells is fixed during prenatal development and, soon after birth, all of the oocytes are arrested in a modified diplotene, or dictyate, stage. Following puberty, dictyate oocytes are hormonally triggered to mature either singly or in groups, resulting in ovulation and the completion of the first meiotic division. It has been hypothesized that female mice are more susceptible to dominant lethal effects of intercalating agents than male mice because oocyte chromosomes, which are arrested in a diffuse state, are generally more accessable to intercalation than are the more condensed chromosomes present within most male germ cell stages. This hypothesis was further tested using the intercalating agent hycanthone methane-sulfonate. Effects of hycanthone were studied in maturing and primordial oocytes and in male germ cells throughout spermatogenesis. No induction of dominant lethality was observed for treated males while a significant increase in embryonic death, expressed around the time of implantation, was observed in females that mated within 4.5 days after treatment. These effects were the result of dominant lethal mutations induced in maturing oocytes and not of maternal toxicity as indicated by the presence of chromosomal aberrations observed at first-cleavage metaphase of zygotes obtained from treated females. These results add support to the hypothesis that certain intercalating chemicals, which are not mutagenic to male mice, may be mutagenic to females and point to a need for more in-depth studies of female-specific mutagenesis.     

The rodent carcinogens 1,4-dioxane and thiourea induce meiotic non-disjunction in Drosophila melanogaster females

The ability of the rodent carcinogens 1,4-dioxane (DX) and thiourea (TU) to induce meiotic non-disjunction (ND) was assessed in 3- and 6-day-old Drosophila melanogaster females. The chemicals were administered orally and three 24 h and one 48 h broods were obtained after mating, to sample oocytes treated in increasingly earlier stages of development. The broods represent mainly mature oocytes (brood I), nearly mature oocytes (brood II), early oocytes (brood III) and very early oocytes (brood IV). The toxicity of DX increased with dose (1% (not toxic), 1.5, 2, 3, 3.5%) as well as a reduction in fecundity which was moderate. Induction of ND in mature oocytes was positive with 2, 3 and 3.5% concentrations and was not related to dose. In immature oocytes it was also positive though already at the lowest concentration tested (1%), suggesting a sensitivity higher than that of mature oocytes. TU at 0.10-10%, did not affect viability, but since fecundity was seriously impaired at high doses, ND was not assessed beyond the 1.5% concentration. TU also induced ND in mature and in immature oocytes; neither a threshold nor a dose effect was detected. The response of mature oocytes was lower than that of immature oocytes. TU induced increases of ND in the earliest cells tested in a more consistent fashion than DX. The data clearly show that both chemicals induced ND in mature oocytes and in the three subsets in which immature oocytes were fractionated. Though toxicity may play a significant unspecific role in the induction of chromosome malsegregation by DX and TU, the induction of ND at low doses, moderately toxic to the oocytes, suggests that the interaction with specific targets contributed to the results obtained.     

Dose-response relationships and R.B.E. values of dominant lethals induced by X-rays and 1.5 MeV neutrons in prophase-1 oocytes and in mature sperm of the two-spotted spider mite Tetranychus urticae Koch (acari, tetranychidae)

Mature sperm and prophase-1 oocytes of Tetranychus urticae Koch were irradiated with 250-kVp X-rays or 1.5 MeV fast neutrons. The X-ray doses ranged from 0.5 to 24.0 krad, and those of the fast neutrons from 0.1 to 16.0 krad. The genetic endpoint measured was lethality, expressed in the stages from egg to adulthood in the F1 progeny. The frequency of recessive lethals in female germ cells was estimated by comparing survival of fertilized versus unfertilized F1 eggs, after irradiation with the same dosage. X-Rays induce dominant lethals in prophase-1 oocytes by the action of both single hits on single targets and multiple hits on multiple targets. 1.5-MeV neutrons induce these effects predominantly by the action of multiple tracks on multiple targets. Dominant lethals were induced in mature sperm by X-rays and by fast neutrons by the action of both single hits on single targets and multiple hits on multiple targets. Both for prophase-1 oocytes and for mature sperm the low R.B.E. value corresponded with the relatively large multiple-target component of induction of dominant lethals by fast neutrons. The nature of dominant lethality in relation to the kinetochore organization of the chromosome is discussed. A non-linear trend in the dose--effect relationship was observed for both X-rays and fast neutrons for the estimated frequency of recessive lethals induced in prophase-1 oocytes. X-Rays were more effective than neutrons in inducing recessive lethals in prophase-1 oocytes at doses lower than 3 krad.     

Repairs of X-ray induced sublethal damage in class B oocytes of Drosophila melanogaster.

Repair of X-ray-induced sublethal damage (Elkind-type recovery) in class B oocytes of Drosophila melanogaster was studied. Newly hatched females of two different stocks were treated with either acute or fractionated exposures. For the fractionation experiments a constant time interval of one hour between the dose fractions was used. As genetic endpoints dominant lethality, chromosome aberrations (detachments) and non-disjunction were studied. The repair of X-ray-induced sublethal damage in class B oocytes is expressed as a reappearance on the initial shoulder in the fractionation curve. For dominant lethality it could be shown that less sublethal damage is repaired in oocytes of Berlin wild females than in those of attached-X females (on the average 76 per cent and 101 per cent respectively). Complete repair (about 100 per cent) was observed for detachments in occytes of attached-X females. Within the dose ranges used no radiation effects on non-disjunction could be observed. The results are interpreted to show that in class B oocytes (1) sublethal damage is due to chromosome breaks and/or lesions leading to breaks and (2) X-ray-induced dominant lethality is the consequence of chromosome damage (true dominant lethals).     

Strong antimutagenic effects of fluoride on mutation induction by trenimon and 1-phenyl-3,3-dimethyltriazene in Drosophila melanogaster

After fluoride treatment of mature and immature oocytes of Drosophila females, a clear-cut dose-dependent decrease in fertility and fecundity was observed. The hatchability of mature oocytes was reduced by as much as 35%. When immature oocytes were treated, a pronounced dose-dependent reduction in fecundity occurred.

Exposure of mature sperm to NaF resulted in a slight decrease in fertility, comparable to the effect obtained with immature oocytes. Of the criteria used to measure possible mutagenic effects of NaF (sex-linked lethals, partial and total X- and Y-chromosome losses), only the rate of total losses was enhanced significantly.

The slight mutagenic effect of NaF on mature sperm was not related to the strong antimutagenic activity observed, when applied simultaneously with any of the several chemical mutagens. NaF treatment drastically reduced both the Trenimon-induced decrease in fertility and Trenimon-induced increases in recessive lethal mutation frequencies and rates of partial and total chromosome losses. The inhibitory effect of NaF was less pronounced with 1-phenyl-3,3-dimethyltriazene (PDT), a poor chromosome breaker in Drosophila, and absent for A 137, a weak mutagen which so far has failed to induce chromosomal aberrations in Drosophila. Therefore, the data are interpreted as being the result of a specific fluoride inhibition of chemically induced chromosomal breakage.

In mature and immature oocytes, the decreases in fertility and fecundity, and increase in recessive lethal frequency (mature oocytes) produced by Trenimon were also suppressed in the presence of fluoride. However, since Trenimon failed to produce a significant rise in X losses and NDJ in both stages, the effect of NaF on these mutational classes was, of course, not testable.     

A Drosophila simulans mutant strain sensitive to benzo[a]pyrene and 2-acetylaminofluorene.

We have identified a Drosophila simulans mutant, 364 yu, that is sensitive to the toxic effects of the procarcinogens B(a)P and 2-AAF. Heterozygotes obtained by crossing it to the wild resistant Turku strain (female 364 yu x male Turku) were more sensitive than heterozygotes obtained from the reciprocal cross (female Turku x male 364 yu) to both the toxic and the mutagenic effects of B(a)P in Drosophila tests that measured lethality and the induction of somatic mosaicism, respectively. The non-carcinogens pyrene, B(e)P and 4-AAF were only weakly toxic and non-mutagenic. In the Ames test B(a)P activation with S15 fractions prepared from the homogenates of Drosophila larvae and imagoes of the 364 yu strain, as well as of the more resistant D. melanogaster y ++/+ w sn3 heterozygotes, did not significantly increase the number of S. typhimurium TA100 revertants even following pretreatment with inducers of microsomal monooxygenases (B(a)P, PCB, PB). As for 2-AAF, a certain increase was observed following only PB, but not B(a)P pretreatment. Possible mechanisms of B(a)P and 2-AAF sensitivity of the 364 yu strain, and perspectives on using it for monitoring genotoxic environmental pollutants, are discussed.     

Dominant lethal mutation rate after γ-irradiation of the fish, Oryzias latipes

When laying females or males of the small fish Oryzias latipes were irradiated with γ-rays and then mated with a non-irradiated partner, the fertility and hatchability of the embryos were reduced as the doses increased. In respect to hatchability (the induction of dominant lethality), the male was more sensitive than the female, and mature sperm were most sensitive among the various stages of spermatogenetic cells. The dose-rate effects on the production of the dominant lethality were observed in spermatogonia and spermatocytes. The inbred strain of the fish, HB-1, was sensitive to γ-rays. Since the relationship between dose and the decrease in hatchability was almost linear, at least within a limited range, we think that this system would be useful for monitoring mutagenic factors in an aquatic environment.     

Frequency and nature of specific-locus mutations induced in female mice by radiations and chemicals: a review.

The inducibility of heritable mutations in female mammals has been measured in the mouse specific-locus test (SLT). For radiation-induced mutations, a large body of data has been accumulated that includes information about biological and physical factors that influence mutation yields. However, relatively few SLT studies in females have been conducted with chemicals to date. A single estimate of the spontaneous mutation rate in oocytes, 6/536,207, has been derived as the most appropriate one to subtract from experimental rates. This rate is highly significantly below the spontaneous mutation rate in males. Mutations recovered from females mutagenized at any time after about the 12th day post-conception are induced in non-dividing cells. In adult females, most oocytes are arrested in small follicles; maturation from this stage to ovulation takes several weeks. High-dose-rate radiations are more mutagenic in mature and maturing oocytes than in spermatogonia of the male; on the other hand, no clearly induced mutations have been recovered from irradiated arrested oocytes. Efficient repair processes have been invoked to explain the latter finding as well as the upward-curving dose-effect relation for acute irradiation, and the fact that dose protraction drastically reduces mutation yield from mature and maturing oocytes. The dose-protraction effect is much greater than that found in spermatogonia. Radiation-induced mutation rates in embryonic, fetal, and newborn females are overall lower than those in the mature and maturing oocytes of adults. A dose-protraction effect has also been demonstrated at an early developmental stage when the nuclear morphology of mouse oocytes most resembles that of the human. Of only 5 chemicals so far explored for their effect in oocytes, 2 (ethylnitrosourea, ENU, and triethylenemelamine, TEM), and possibly a third (procarbazine hydrochloride, PRC), are mutagenic--with at least one of these (ENU) mutagenic in arrested as well as maturing oocytes. However, the mutation rate is, in each case, lower than for treated male germ cells. By contrast, ENU-induced mutation yield for the maternal genome of the zygote is an order of magnitude higher than that for the zygote's paternal genome or for spermatogonia. A high proportion of mutants derived from chemical treatment of oocytes (including the oocyte genome in zygotes) are mosaics, probably owing to lesions affecting only 1 strand of the DNA. A characteristic of specific-locus mutations induced in oocytes is that they include a considerably higher percentage of large (multi-locus) lesions (LLs) than do mutations induced in spermatogonia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Exposure fractionation effects for X-ray-induced dominant lethals in immature (stage-7) oocytes of Drosophila melanogaster: A re-analysis

Young (0—4-h-old) Drosophila melanogaster females were X-irradiated with single or fractioned exposured over a range up to 6000 R and the induction of dominant lethals in immatuer (stage-7) oocytes was studied. The results show that (i) the frequencies of dominant lethals are higher after single than after fractionated exposures; (ii) at any given exposure level, the higher the number of fractions, the lower is the frequency of dominant lethals; (iii) conserquently, the reduction in dominant lethality relative to single exposures increases with increasing number of fractions; and (iv) this relative reduction in dominant lethality approaches a maximum value when the magnitude of the single X-ray exposure approaches zero (i.e., when tha egg survival after single X-ray exposure approaches 100%); the maximum, however, are different for the different fractionation regimes, being higher with increasing number of fractions.These findings are consistent with the assumed kinetics of X-ray induction of dominant lethality in stage-7 oocytes. It is shown that it is possible to predict the expected relative reduction in dominant lethality after fractionation, from appropriate dominant lethal data from single unfractioned exposures.     

Incompatibility between X chromosome factor and pericentric heterochromatic region causes lethality in hybrids between Drosophila melanogaster and its sibling species

The Dobzhansky-Muller model posits that postzygotic reproductive isolation results from the evolution of incompatible epistatic interactions between species: alleles that function in the genetic background of one species can cause sterility or lethality in the genetic background of another species. Progress in identifying and characterizing factors involved in postzygotic isolation in Drosophila has remained slow, mainly because Drosophila melanogaster, with all of its genetic tools, forms dead or sterile hybrids when crossed to its sister species, D. simulans, D. sechellia, and D. mauritiana. To circumvent this problem, we used chromosome deletions and duplications from D. melanogaster to map two hybrid incompatibility loci in F(1) hybrids with its sister species. We mapped a recessive factor to the pericentromeric heterochromatin of the X chromosome in D. simulans and D. mauritiana, which we call heterochromatin hybrid lethal (hhl), which causes lethality in F(1) hybrid females with D. melanogaster. As F(1) hybrid males hemizygous for a D. mauritiana (or D. simulans) X chromosome are viable, the lethality of deficiency hybrid females implies that a dominant incompatible partner locus exists on the D. melanogaster X. Using small segments of the D. melanogaster X chromosome duplicated onto the Y chromosome, we mapped a dominant factor that causes hybrid lethality to a small 24-gene region of the D. melanogaster X. We provide evidence suggesting that it interacts with hhl(mau). The location of hhl is consistent with the emerging theme that hybrid incompatibilities in Drosophila involve heterochromatic regions and factors that interact with the heterochromatin.     

Effect of glyoxal pretreatment on radiation-induced genetic damage in Drosophila melanogaster

The effects of glyoxal and of glyoxal pretreatments on radiation-induced genetic damage were investigated in Drosophila melanogaster mature sperm, by means of sex-linked recessive and dominant lethality, reciprocal translocation and chromosome loss tests. In addition, the possible mutagenic effect of glyoxal was assessed in postmeiotic cells up to 7 days after treatment. The results obtained show: (1) the frequencies of recessive lethals after glyoxal treatment were within control values, (2) no clastogenic effect of glyoxal was observed, (3) glyoxal pretreatment did not modify the frequency of recessive lethals induced by X-rays, (4) after pretreatment with glyoxal a consistent, though not significant, increase was seen in the frequency of reciprocal translocations in 3 replicate experiments, (5) the yield of dominant lethals and of complete and partial chromosome loss induced by radiation was significantly increased by pretreatments with glyoxal. It is suggested that the increase of the frequency of genetic endpoints resulting from chromosome breakage, when glyoxal was administered prior to irradiation, could be ascribed to: (a) a sensitizing action of glyoxal to the clastogenic effect of ionizing radiation; (b) the formation of reactive species by the interaction of glyoxal with radiation; and/or (c) interference of glyoxal with the normal handling of radiation-induced lesions in mature postmeiotic male cells.     

Genetic determination of spontaneous and induced dominant lethality in Drosophila

Estimation of heritability ha2 index in experiments on spontaneous and induced dominant lethality gives the possibility to evaluate with sufficient accuracy genetic determination of the effect and its dependence on casual reasons. Besides, the individual breeding method gave quite stable data both on late embryonic lethality index and on heritability value. Directed selection of different test-objects (species or stocks of Drosophila) should be carried out using heritability index, since selection efficiency mostly depends on this value degree. Making stocks resistant and sensitive to certain mutagen classes, their comparison from the point of view of their pharmacogenetic features, on the basis of the indices used, may be taken as the start of studies aimed at further identification of certain biochemical factors or system taking part in manifestation of mutagenic effect. The data obtained permit us to suggest an idea of advisability of studying the genetic control of mutagenesis in humans using the method described.     

Studies on mutagen-sensitive strains of Drosophila melanogaster I. The enhanced X-ray sensitivity of a mutant strain (ebony) which is UV-sensitive and also deficient in photorepair

Yegorova and colleagues (1978) showed that a mutant strain of Drosophila melanogaster (ebony) was more sensitive to UV-induced killing of embryos and also less proficient in photoreactivating (PR) ability than a wild-type (Canton-S) strain and that the genes governing UV sensitivity and PR ability were different and presumably located on the autosomes. The experiments reported in the present paper were designed to compare the patterns of sensitivity of these 2 strains and their hybrids to X-irradiation. The sensitivity of the larvae to the killing effects of X-irradiation, and of male and female germ-cell stages to the X-ray induction of genetic damage was studied.It was found that the larvae of the ebony strain are more sensitive to X-ray-induced killing than those of the Canton-S strain. The frequencies of radiation-induced dominant lethals and sex-linked recessive lethals are higher in spermatozoa sampled from ebony males than in those of Canton-S males. In spermatozoa sampled from hybrid males, the yields of dominant lethals are no higher than in those sampled from Canton-S males and do not seem to depend on the origin of the X-chromosome. There are no statistically significant differences between the ebony and Canton-S strains in the sensitivity of their spermatozoa to the induction of autosomal translocations.Stage-7 oocytes sampled from ebony females are more sensitive to the X-ray induction of dominant lethality than are those from Canton-S females; oocytes sampled from hybrid females manifest a level of sensitivity that is significantly lower than that in either parental strain. The frequencies of X-chromosome losses induced in in this germ-cell stage are significantly lower in ebony than in Canton-S females at least at the exposure level of 3000 R at which 3 experiments were carried out. There are no measurable differences in the amount of dominant lethality induced in stage-14 oocytes of ebony, Canton-S and hybrid females.When X-irradiated Berlin-K males are mated to ebony or Canton-S females, the yields of dominant lethals are higher when ebony females are used, showing that there is a “maternal effect” for this kind of damage. Such a maternal effect is also found for sex-linked recessive lethals (irradiated Muller-5 males mated to ebony or Canton-S females). However, when irradiated ring-X-chromosome-carrying males are mated to ebony or Canton-S females, the frequencies of paternal sex-chromosome losses (scored as XO males) are lower when ebony females are used.These results have been interpreted on the assumption that the ebony strain is homozygous for recessive, autosomal genes that confer increased radiosensitivity and that the Canton-S strain carries the normal, wild-type alleles for these genes. The higher yields of dominant and recessive lethals in mature spermatozoa and of dominant lethals in stage-7 oocytes are a consequence of an enhanced sensitivity to the mutagenic (in particular, to the chromosome-breaking) effects of X-irradiation and/or of defective repair of radiation-induced genetic damage. The lower yield of XO males from irradiated stage-7 oocytes of ebony females is probably a consequence of a defect in the repair of chromosome-breakage effects, resulting in the conversion of potential X losses in females into dominant lethals. The “maternal effects” for dominant lethals, sex-linked recessive lethals and for the loss of ring-X chromosomes are assumed to have a common causal basis, namely, a defective repair of chromosome-breakage events in the females of the ebony strain.     

X-chromosomal nondisjunction induced by aging oocytes of Drosophila melanogaster: the special susceptibility of mature eggs

In Drosophila melanogaster suppression of oviposition results in an aging of both mature and immature oocytes. When oviposition was suppressed for four days, the incidence of X-chromosomal nondisjunction (XXY exceptions) in mature oocytes was more than doubled, whereas in immature oocytes the nondisjunction frequency was not increased. It is shown that this special susceptibility of mature oocytes to aging-induced nondisjunction has to be considered in experiments on the induction of nondisjunction by chemical agents.     

Mutagenicity of 7,12-dimethyl[a]anthracene and some other aromatic mutagens in Drosophila melanogaster

The optimal conditions for mutagenesis studies with DMBA and some other aromatic carcinogens in Drosophila were investigated in detail. The results presented in this paper indicate the following.The mutagenic effectiveness of DMBA is dependent on the route of administration, injection being far more effective when compared with feeding.The choice of the solvent is a crucial experimental condition. DMBA, when dissolved in oil/DMF, is ineffective whereas a special fat emulsion of DMBA gives high mutation frequencies.There appears to be an extreme strain dependence in the mutagenicity of DMBA. Mutagenic effectiveness in strain Berlin-K was rather low, whereas Oregon-K and Karsnäs-60 proved to be very susceptible to DMBA.Under the conditions of test, DMBA did not induce loss of a ring-X chromosome and did not produce recessive lethal mutations in such a chromosome.DMBA did not produce 2–3 translocations to any significant extent.An increase in DMBA-induced recessive lethal mutations was found upon storage of treated sperm with an optimal storage time of 4–10 days.DMBA is efficient in the production of delayed recessive lethal mutations in strain Berlin-K. Twice as many lethals were recovered with the F3 generation as compared with those in F2. In strain Oregon-K, where the frequency of F2 lethals was much higher than in strain Berlin-K, the ratio of F3/F2 lethals was clearly lower.Enzyme induction with phenobarbital reduces the mutagenic effectiveness of DMBAWith TMBA, similar strain differences in sensitivity were observed as those found for DMBA. Whereas TMBA was not mutagenic in Berlin-K, considerable mutagenicity was observed in Oregon-K and Karsnäs-60.Injection of carcinogenic polycyclic aromatic hydrocarbons and aromatic amines, when dissolved in special fat emulsions, enhances the mutagenic effectiveness of some compounds (DMBA, TMBA, DA and AcO-AAF), but this procedure does not always solve the problems-pertinent to these classes of promutagens in Drosophila.     

Investigations on radiosensitive and radioresistant populations of Drosophila melanogaster: XII. The resistance factor rar-3: stage specificity

In earlier studies the recessive genetic factor rar-3 (3-49.8) of Drosophila melanogaster had been found to reduce the sensitivity of immature oocytes to the mutagenic action of X-rays. The present work was devoted to an extension of these studies to other germ-cell stages in both male and female and also somatic cells. The results show that, in the female, the effects of rar-3 are manifest in all germ-cell stages including gonia and nurse cells but not in mature oocytes. In the male germ-cell stages, rar-3 was without any measurable effect; maternal-effect studies were likewise negative. Somatic tissues were also unaffected. Futhermore, rar-3 was apparently not active in larval oogania. It is therefore concluded that the activity of rar-3 is switched on in oogonia during puparium formation or metamorphis and persists until before the formation of the mature oocyte.     

Induction of congenital anomalies in offspring of female mice exposed to varying doses of X-rays

Female mice were exposed to varying absorbed doses (108–504 rad) of X-rays and mated at different intervals after irradiation (1–7, 8–14, 15–21 and 22–28 days). Uterine contents were examined at late pregnancy in order to detect early fetal deaths (dominant lethality) and malformations in the live fetuses.Two trends were apparent from data on abnormal fetuses. At each weekly interval, the incidence of abnormalities tended to rise with increase in dose, and, at any given dose, the incidence tended to increase with time after irradiation. Dwarfism and exencephaly were the two most common malformations found.The changes in incidence of dominant lethality and of abnormal fetuses with time and with dose follow each other closely, the highest incidence for both being reached in week 3 (59±4.7% for dominant lethals and 12.5±3.1% for abnormal fetuses, after 504 rad) indicating increased radiosensitivity of less mature oocytes. These results parallel those obtained from known genetic effects reported by other workers and suggest that testing for incidence of congenital malformations among offspring of treated animals may prove a useful means of assessing genetic hazards of radiation of chemicals.     

Mammalian genes expressed in Drosophila: a transgenic model for the study of mechanisms of chemical mutagenesis and metabolism.

Mammalian cytochrome P450s provide our first line of defence against the toxic effects of environmental chemicals. Ironically these enzymes also convert some compounds to their ultimate toxic or mutagenic species. Our knowledge of these mammalian enzymes and the role they play in chemical toxicity and mutagenesis has stemmed mostly from in vitro studies. In order to establish the role of specific enzymes in the toxicological response in vivo we have generated transgenic Drosophila which express mammalian cytochrome CYP2B1, which is a member of a large gene family encoding several important drug metabolising enzymes. The gene was fused to a Drosophila promoter which confers expression in the larval fat body. Using the Somatic Mutation And Recombination Test (SMART) we have demonstrated that transgenic larvae expressing the P450 are hypersensitive to the anticancer drug cyclophosphamide, a procarcinogenic substrate which is activated by the enzyme. This work demonstrates the potential of such transgenic Drosophila strains as an in vivo model for studying the role of specific mammalian drug metabolising enzymes in the pathways and metabolic cascades associated with the action of cytotoxic and carcinogenic chemicals, and also the chemical properties of specific classes of mutagen to be determined.     

Radiation- and chemically-induced chromosome aberrations in mouse oocytes: a comparison with effects in males.

Data from studies on radiation- and chemically-induced chromosome aberrations in mouse oocytes have been summarized. An attempt has been made to assess the relative sensitivity to mutagenic agents of female and male germ cells through comparison of observations from mutation studies of female and male mice. No unequivocal evidence of a mutagenic effect limited to a single sex could be found in the cytogenetic data, although differences in relative germ cell sensitivity could be inferred for ionizing radiation and some chemicals. However, the pattern of inter-sex variations was not consistent: for example, irradiation of dictyate oocytes yielded a lower rate of heritable chromosome translocations than the same dose to spermatogonia; in contrast, some chemicals, such as mitomycin C, yielded a larger incidence of chromosome anomalies after treatment of dictyate oocytes than spermatogonia. Overall, the limitations in quality and quantity of cytogenetic data, and the uncertainties associated with comparing information obtained in disparate assays, place severe constraints on the use of observations on induced chromosome aberrations to assess the relative sensitivities of female and male germ cells to environmental mutagens.     

Mutagenic activity of ethylene oxide and propylene oxide under XPG proficient and deficient conditions in relation to N-7-(2-hydroxyalkyl)guanine levels in Drosophila

Ethylene oxide (EO) and propylene oxide (PO) are direct acting mutagens with high Swain-Scott s-values, which indicate that they react preferentially with ring nitrogens in the DNA. We have previously described that in the X-linked recessive lethal (RL) assay in Drosophila postmeiotic male germ cells EO is, per unit exposure dose, 5-10 times more mutagenic than PO. Furthermore, at the higher dose range of EO tested, 62.5-1000 ppm, up to 20-fold enhanced mutation rates were measured in the absence of maternal nucleotide excision repair (NER) compared to repair proficient conditions. The lower dose range of EO tested, 2-7.8 ppm, still produced a small increased mutation rate but without a significant elevated effect when the NER system is being suppressed. The lowest dose of PO tested, 15.6 ppm, produced only in NER- condition an increased mutation rate. The aim of the present study was to compare the mutagenic effect of EO and PO in the RL assay under XPG proficient and deficient conditions with the formation of N-7-(2-hydroxyethyl)guanine (7-HEG) and N-7-(2-hydroxypropyl)guanine (7-HPG), respectively, the major DNA adducts formed. The formation of 7-HEG and 7-HPG was investigated in Drosophila males exposed to EO and PO as a measure of internal dose for exposures ranging from 2 to 1000 or 2000 ppm, respectively, for 24h. Analysis of 7-HEG and 7-HPG, using a highly sensitive 32P-postlabelling assay, showed a linear increase of adduct levels over the entire dose range. The non-linear dose-response relationship for mutations could therefore not be explained by a reduced inhalation or increased detoxification at higher exposure levels. In analogy with the four times higher reactivity of EO the level of N-7-guanine alkylation per ppm was for EO 3.5-fold higher than that for PO. Per unit N-7-guanine alkylation EO was found to be slightly more mutagenic than PO, whereas PO was the more potent clastogenic agent. While this research has not identified the DNA lesions that cause the increase in repair deficient flies, it supports the hypothesis that efficient error-free repair of some N-alkylation products can explain why these agents tend to be weakly genotoxic or even inactive in repair-competent (premeiotic) germ cells of the mouse and the Drosophila fly.