Induction Of Specific-locus Mutations Research Articles

Quantitative and qualitative comparison of spontaneous and chemical-induced specific-locus mutation in the ad-3 region of heterokaryon 12 of Neurospora crassa

The data from forward-mutation experiments to obtain specific-locus mutations at two closely linked loci in the adenine-3 ( ad-3) region of heterokaryon 12 (H-12) of Neurospora crassa have been tabulated to determine the relative frequencies and mutational spectra of ad-3 mutants occurring spontaneously and those induced by 22 different chemical treatments. Previous studies have demonstrated that specific-locus mutations at these two loci result from 5 major genotypic classes, namely two classes of gene/point mutations ( ad-3A R and ad-3B R), and 3 classes of multilocus deletion mutations ([ ad-3A] IR, [ ad-3B] IR and [ ad-3A ad-3B] IR). In addition, prior studies have demonstrated that some chemical mutagens induced ad-3 mutants exclusively, or almost exclusively, by gene/point mutation and other chemical mutagens by gene/point mutation and multilocus deletion mutation. In the latter cases, there was wide variation in the percentages of ad-3 mutants in these 5 major genotypic classes. Two comparative methods of analysis that also were used to compare spontaneous and chemical-induced ad-3 mutational spectra included χ 2-tests on the numbers of ad-3 mutants resulting in the following two sets of ratios: (1) gene/point mutations and multilocus deletion mutations; and (2) complementing and non-complementing ad-3B R mutants. Combination of the p-values from χ 2-tests for these two methods of comparison demonstrated that all 22 chemicals induce a spectrum of ad-3 mutants that is qualitatively different from that occurring spontaneously. In addition, these same two methods of comparison have been used to compare the mutagenic effects of each of the 22 chemical treatments with each other. Combination of the data from these two methods of comparison has demonstrated that 93.1% (215/231) of the pairwise combinations of these 22 chemicals were different from each other. The implication of these experimental data on the induction of specific-locus mutations in somatic cells of Neurospora for genetic risk assessment exercises is discussed.

Genetic risk assessment and specific-locus mutations in the ad-3 region of Neurospora crassa.

Data from experiments on the induction of specific-locus mutations in model systems are used in genetic risk assessment to estimate potential adverse effects in the human population. In such assessments with radiation or chemical mutagens, the following information is required: a) spontaneous and induced forward-mutation frequencies, b) dose-response curves for the overall induction of specific-locus mutations, c) genetic characterization of spontaneous and induced mutations, and d) dose-response curves for the different genotypic classes. Specific-locus assays in most eukaryote assay systems provide only portions of the information required for such assessments. In recognition of the need for more detailed information for risk assessment, a model system has been developed for specific-locus assays in Neurospora crassa. The adenine-3 (ad-3) specific-locus assay was modeled after the two-gene morphological specific-locus assay in the dilute-short-ear region of the mouse and detects forward-mutations at two closely linked loci: ad-3A and ad-3B. A computerized data management program has made it possible to obtain precise dose-response curves not only for the overall induction of ad-3 mutations, but also for various genotypic subclasses. In addition, computerized statistical programs have been developed to compare dose-response curves. These methods of analysis have shown that the overall dose-response curve for specific-locus mutations in the ad-3 region is a composite of many different genotypic subclasses. In addition, these subclasses may have very different induction kinetics from those of the overall dose-response curve for ad-3 mutations.

Characteristics of spontaneous and induced specific-locus mutation in the ad-3 region of Neurospora crassa: utilization in genetic risk assessment.

Data from experiments on the induction of specific-locus mutations in model systems are utilized in genetic risk assessment to estimate potential adverse effects in the human population. In such assessments with radiation or chemical mutagens, the following information is required: (1) spontaneous and induced forward-mutation frequencies, (2) dose-response curves for the overall induction of specific-locus mutations, (3) genetic characterization of spontaneous and induced mutations, and (4) dose-response curves for the different genotypic classes. Specific-locus assays in most eukaryote assay systems provide only portions of the information required for genetic risk assessment. In recognition of the need for a more comprehensive data base, a model system was developed for specific-locus studies in Neurospora crassa. The adenine-3 (ad-3) specific-locus assay was modeled after the 2 gene, morphological specific-locus assay in the dilute-short-ear region of the mouse, and it detects forward-mutations at two closely linked loci: ad-3A and ad-3B. The ad-3 assay system has provided precise dose-response curves not only for inactivation, but also the overall induction of ad-3 mutations. The utilization of this assay in experiments with radiation or chemical mutagens has provided a data base on the induction and genetic characterization of specific-locus mutations that is unique among eukaryotic organisms. In this assay, gene/point mutations, multilocus deletion mutations, and 3 different classes of multiple-locus mutations can be identified. The latter consist of specific-locus mutations associated with recessive lethal mutations located either closely linked to the ad-3 region or elsewhere in the genome. The overall data base on the heterozygous effects of X-ray-induced ad-3 mutations demonstrates that such effects are allele specific, genotype specific, and locus specific. There are probably a variety of mechanisms by which the heterozygous effects of individual allelic mutations at different genetic loci can be affected. In conclusion, unless the frequencies of all of the different classes of induced specific-locus mutations are determined, and utilized in genetic risk assessment exercises, the risk of human exposure to environmental mutagens may be grossly underestimated.

Radiation-induced dominant cataract mutations in golden hamsters and mice

The induction of dominant cataract mutations by procarbazine was studied concomitantly with the induction of specific-locus mutations in treated male mice. The most effective dose in the specific-locus test, 600 mg/kg of procarbazine, and a fractionated dose of 5 × 200 mg/kg were used. The frequencies of dominant cataract mutations were higher, but not significantly different from the historical control. The ratio between the number of recovered specific-locus and dominant cataract mutations was in accordance with that found in our experiments with γ-rays (Ehling et al., 1982; Kratochvilova, 1981) or in experiments with ethylnitrosourea (Favor, 1986).A total of 3 dominant cataract mutations were recovered in the offspring of procarbazine-treated spermatogonial stem cells. Two mutations had complete penetrance while the third exhibited a reduced penetrance of approximately 70%. The viability and fertility of the heterozygotes of all 3 mutations were not affected. Only 1 mutation was shown to be viable as a homozygote.

The induction of specific-locus mutations with N-propyl- N-nitrosourea in stem-cell spermatogonia of mice

A specific-locus test to determine the effect of N-propyl- N-nitrosourea (PNU) on the stem-cell spermatogonia of mice has been performed. Male wild-type mice (C3H/He) were treated with an intraperitoneal injection of 2000 mg/kg of PNU. Eight weeks after the injections, the males were mated with tester stock females (PW), homozygous for 6 visible recessive genes. Twelve mutants among 8605 offspring were observed. The mutation frequency with PNU was calculated to be 23.2 × 10 − 5/locus/gamete, showing a significant difference from that of the non-treated control. The mutations were all heritable and half of them were viable in homozygous condition. The mutation frequency with PNU was about one-third of that with Nethyl- N-nitrosourea, a highly potent mutagen for mouse stem-cell spermatogonia.

Comments on Commentary by D. Billen

Billen (1) proposes that the spontaneous to DNA due primarily to the thermodynamic instability arising from storage in the living genome at 37?C can be used to determine a negligible for ionizing radiations. The concept of DNA instability is, I believe, of great relevance to radiobiology, but logically it leads to a rather different conclusion, namely that however small the dose and however low the dose rate there is a finite chance that a biological effect will be caused. The concept of negligible does not belong in the domain of the physicochemical properties of DNA but in the domain of the acceptability of risk, a sociopolitical issue. My argument is as follows. I agree with Billen (1) that about 100 events occur in the genome of the diploid mammalian cell per minute and that cells have evolved efficient repair processes to enable them to cope with this rate of induction. The nature of these sites in double-stranded DNA, the repository of the genetic information, is base loss, base damage, and single-strand breaks, i.e., single-strand damage. While this is being continually produced it must be equally continually being repaired; i.e., there is a dynamic equilibrium between degradation and repair in the living cell. Even at this rate of formation the distances between the transitory lesions are enormous, and so the generation of double-strand due to the coincidence of single-strand sites in opposing strands is indistinguishable from zero. Cells do generate double-strand breaks; however, this is not primarily but rather part of their normal functioning and a controlled process. Radiation induces both singleand double-strand and DNA-protein crosslinks (2). I think, as does Billen, that provided the single-strand is not induced at a rate comparable to the rate of induction of spontaneous single-strand then the continually active repair processes will satisfactorily repair both the radiation-induced and the spontaneous damage. This is then irrelevant to biological effect. However, the double-strand (mainly breaks) and the DNA-protein crosslinks caused by radiation will not necessarily be repaired effectively, and it is these lesions that can constitute relevant damage and lead to biological effects at low dose rates. Thus I would predict that at low dose rates, radiation-induced biological effects would be directly proportional to dose and invariant with dose rate. At dose rates where the ate of induced single-strand was comparable with that ue to spontaneous I would predict a transition to a dependence of effect on dose rate due to the disruption of the dynamic equilibrium between DNA degradation and repair and the consequent build-up of unrepaired of both spontaneous and radiolytic origin. Indeed, such a transition is exactly what is observed for the induction of specific locus mutations in mice when males are irradiated at the spermatogonial stage (to be published elsewhere). At dose rates of 0.015 Gy/min (a singlestrand induction rate of about one-third that due to spontaneous damage) a transition is observed; below this rate mutation frequency is invariant with dose rate, and above this rate it increases sharply with increasing dose rate. Viewed in this way I believe the concept of DNA instability is of considerable value to radiobiology and radiological protection in the following ways:

Induction of specific-locus mutations in male germ cells of the mouse by acrylamide monomer

Acrylamide monomer (AA), injected into male mice at the maximum tolerated dose of 5× 50 mg/kg (24-h intervals), significantly increased the specific-locus mutation rate in certain poststem-cell stages of spermatogenesis, but not in spermatogonial stem cells. Germ-cell stages in which the treatment induced dominant lethals — namely, exposed spermatozoa and late spermatids (number of surviving offspring only 3% and 27%, respectively, of those in concurrent controls) — jointly yielded the highest frequency of specific-locus mutations. AA thus conforms to Pattern 1 in our earlier classification of chemicals according to the spermatogenic stage at which they elicit maximum response (Russell et al., 1990). No specific-locus mutations were observed among 17,112 offspring derived from exposed spermatogonial stem cells, a result which rules out (at the 5% significance level) an induced mutation rate greater than 2.3 times the historical control rate. A sustained high productivity in matings made for severalmonths following week 3 indicates that there is no significant spermatogonial killing and that cell selection is presumably not the explanation for the negative result. On the basis of genetic and/or cytogenetic evidence, the mutations induced postmeiotically by AA were ‘large lesions’ (multi-locus), while one of 2 recovered from exposure of differentiating spermatogonia is probably a small lesion. An earlier survery of mammalian mutagenesis results led us to conclude that, regardless of the classification of a chemical according to the stage at which it elicits its maximum response, the nature of mutations is determined by the germ-cell stage in which they are induced (Russell et al., 1990). The AA results on lesion size and on distribution of mutations among the loci fit the general pattern.

Induction of specific-locus mutations in male mice by ethyl methanesulfonate (EMS)

Using a sequential mating procedure, the induction of specific-locus mutations by ethyl methanesulfonate (EMS) was reinvestigated in male mice. Doses of 175 mg/kg b.w. and 250 mg/kg b.w. of EMS induce gene mutations in the mating intervals 5–8 and 9–12 days post treatment. However, only the frequency of dominant lethal mutations increases with the dose, not the frequency of specific-locus mutations. This observation implies that with a higher dose of EMS a larger fraction of mutagenized spermatozoa and spermatids are selectively eliminated, leading to underestimation of the specific-locus mutation yield at high doses. EMS does not induce specific-locus mutations in spermatogonia.

X-ray-induced specific-locus mutations in the ad-3 region of two-component heterokaryons of Neurospora crassa: IV. Irreparable mutants of genotype ad-3A and ad-3B result from multilocus deletion and an unexpectedly high frequency of multiple-locus mutations

The induction of specific-locus mutations in the ad-3 region of Neurospora crassa after X-irradiation was studied in a two-component heterokaryon to determine: (1) the ratio of reparable ad-3 mutants (presumed gene/point mutations, designed ad-3 R) to irreparable ad-3 mutants (presumed multilocus deletions, designated ad-3 IR), and (2) the induction kinetics of each class (Webber and de Serres, 1965). More extensive genetic tests made subsequently (de Serres, 1989a) on the 832 X-ray-induced specific-locus mutations recovered in those experiments showed that unexpected high frequencies of reparable and irreparable ad-3 mutants are actually multiple-locus mutants that have additional, but separate, sites of recessive lethal ( RL CL) damage in the immediately adjacent genetic regions (designated ad-3 R + RL CL or ad-3 IR + RL CL). The frequencies of these X-ray-induced multiple-locus mutants in the ad-3 region are orders of magnitude higher than expected on the basis of target theory (where the frequency of the double mutant is expected to be the product of the frequencies of each single mutant) and classical models of chromosone structure during interphase (de Serres, 1989a). In the present paper, a random sample of 832 X-ray-induced ad-3 mutants of genotype ad-3A or ad-3B that are irreparable have been subjected to more extensive genetic fine-structure analysis. These experiments were designed to determine the extent of the functional inactivation in individual mutants in the ad-3 and immediately adjacent genetic regions in mutants classified as presumptive multilocus deletions or multiple-locus mutations. These experiments have shown that in Neurospora crassa most X-ray-induced irreparable mutants of genotype ad-3A or ad-3B map as a series of overlapping multilocus deletions. Among the 29 irreparable mutants of genotype ad-3A, there are 16 different subgroups of complementation patterns; and among the 63 irreparable mutants of genotype ad-3B, there are also 16 different subgroups. In addition, mutants classified as presumptive multiple-locus mutants result from a variety of separate, but closely linked, sites of genetic damage. Combining data from the present experiments with previously published data (de Serres, 1989a), the frequency of multiple-locus mutations of genotype ad-3 R + RL CL among X-ray-induced gene/point mutations ( ad3A R or ad-3B R) in the ad-3 region is 3.4% ( 22 644 ), and the frequency of multiple-locus mutations of genotype ad-3 IR + RL CL (or ad-3 IR + RL CL + RL CL) among X-ray-induced multilocus deletions ([ ad-3A ad-3B] IR, [ ad-3A ad-3B nic-2] IR) in the ad-3 region is 6.7% ( 13 195 ).

Induction of specific locus mutations in mouse spermatogonial stem cells by combined chemical X-ray treatments

Data that demonstrate how the biology of spermatogenesis plays an important role in determining the yield of genetic damage from ionizing radiation are briefly reviewed. It is suggested that for valid extrapolations of data from mouse mutation experiments to man detailed knowledge of the spermatogonial stem cell systems in the two speicies is required. Two new sets of mouse specific mutation data are presented. (1)When a 2 mg/kg dose of triethylenemelamine (TEM) was used as a conditioning dose and followed 24 h later by 6 Gy X-rays, the mutation yield from spermatogonial stem cells was over twice as high (30.20 × 10 −5/locus/gamete) as that when the X-ray dose was given alone (13.75 × 10 −5/locus/gamete). No such effect was found when the TEM was given only 3 h prior to the X-irradiation. Since TEM at the dose used is inefficient at inducing specific-locus mutations, an augmentation of the X-ray response is indicated. It has therefore been concluded that the augmented mutation responses obtained with equal 24 h X-ray fractionations at high dose are attributable to mutation induction by the second dose. The responsive cells would be the formerly resistant component of the stem cell population that had survived the TEM treatment and that had been ‘triggered’ into a radiosensitive phase by the population depletion. (2) When 2 doses of 500 mg/kg hydroxyurea (HU) were given 3 h apart 3 h prior to 6 Gy X-rays to reduce the numbers of stem cells in the S and G2 phases of the cell exposed to the radiation, the mutation responses was greatly enhanced to a level that is the highest yet recorded per unit X-ray dose (7.10 × 10 −5/locus/gamete/Gy). No such effect was obtained when the intervals between the HU and X-ray treatments were either shorter (<0.5 h) or longer (24 h). It was concluded that X-ray-induced specific-locus mutations derive principally from stem cells in the G1 phase of the cell cycle. The reasons why the X-ray-induced mutation-yields from repopulating stem cells (with a short cell cycle and, hence, short G1 phase) are similar to those from undamaged stem cell populations, in contrast to translocation yields, therefore remains unresolved.

Induction of specific-locus mutations in female mice by 1-ethyl-1-nitrosourea and procarbazine

Using the specific-locus method, the ability of 1-ethyl-1-nitrosourea (ENU) and procarbazine hydrochloride (procarbazine) to induce gene mutations in mouse oocytes was tested and confirmed. The sensitive stage for the induction of mutations in oocytes with 160 mg/kg of ENU is 2–4 weeks post treatment. The induced mutation frequency in this mating interval was 5.1 × 10 −7 mutations/locus/ gamete/mg/kg. The induction of mutations by procarbazine occured 8–33 weeks after treatment. The induced mutation frequency in this mating interval for the 400 mg/kg group was 0.4 × 10 −7 mutations/ locus/gamete/mg/kg. One third of the induced mutations was lethal in homozygous condition in both experiments. ENU and procarbazine have a lower mutational response in oocytes than in stem-cell spermatogonia.

Dominant cataract and recessive specific-locus mutations detected in offspring of procarbazine-treated male mice

The induction of dominant cataract mutations by procarbazine was studied concomitantly with the induction of specific-locus mutations in treated male mice. The most effective dose in the specific-locus test, 600 mg/kg of procarbazine, and a fractionated dose of 5 × 200 mg/kg were used. The frequencies of dominant cataract mutations were higher, but not significantly different from the historical control. The ratio between the number of recovered specific-locus and dominant cataract mutations was in accordance with that found in our experiments with γ-rays (Ehling et al., 1982; Kratochvilova, 1981) or in experiments with ethylnitrosourea (Favor, 1986). A total of 3 dominant cataract mutations were recovered in the offspring of procarbazine-treated spermatogonial stem cells. Two mutations had complete penetrance while the third exhibited a reduced penetrance of approximately 70%. The viability and fertility of the heterozygotes of all 3 mutations were not affected. Only 1 mutation was shown to be viable as a homozygote.

Germ-cell sensitivity in mammals

The differences in mammalian germ-cell sensitivity have been studied with a number of mutagenic agents using both chromosomal aberrations and point mutations as genetic endpoints. I have tried to give a survey on the present knowledge. Several conclusions can be drawn at present: 1. In spermatogenesis the sensitivity to mutation induction differs between various post-spermatogonial stages and spermatogonia. 2. Heritable structural chromosomal changes are primarily recovered from post-spermatogonial germ cells. The sensitivity pattern is similar in dominant lethal and heritable translocation experiments. The effect recovered from spermatogonia is near zero. 3. Structurally related chemicals show a difference in their spermatogenic response. The most obvious example gave the induction of specific locus mutations by ENU and MNU. 4. Since too little information is available it seems prudent to assume that oocytes and spermatogonia may show similar sensitivity so that in risk estimates effects on female germ cells cannot be neglected. The final result from the previous discussion must be that the limitations of knowledge are obvious, they lie in the limited number of chemicals tested in mammalian germ-cell assays. A larger data base of mammalian germ-cell response to chemical mutagens is urgently needed.

Induction of specific-locus mutations in female mice by triethylenemelamine (TEM)

Induction of specific-locus mutations in female mice by triethylenemelamine (TEM)

The induction of specific-locus mutations and sister-chromatid exchanges by 5-bromo- and 5-chloro-deoxyuridine

The induction of 6-thioguanine-resistant (TG r) mutants and sister-chromatid exchanges (SCE) by 5-bromo- and 5-chloro-deoxyuridine (BrdU and CldU) was determined with Chinese hamster ovary cells in culture. Over a concentration range of 0.5–200 μM, CldU treatment caused 3–5 times the number of SCEs found with BrdU; for example, with 10μM CldU, 60.0±14.2 SCEs/metaphase (mean±S.D.) were induced, while with 10μM BrdU, only 14.6 ± 4.1 SCEs/metaphase were found. Despite this difference in SCE induction, both compounds were mutagenic only at concentrations greater than 50 μM, and BrdU was more mutagenic than CldU. These results indicate that SCEs and gene mutations are induced by these thymidine analogs through 2 different mechanisms.

The possible use of bacteria for quantifying the mutagenic potency of chemicals

The induction of dominant cataract mutations by procarbazine was studied concomitantly with the induction of specific-locus mutations in treated male mice. The most effective dose in the specific-locus test, 600 mg/kg of procarbazine, and a fractionated dose of 5 × 200 mg/kg were used. The frequencies of dominant cataract mutations were higher, but not significantly different from the historical control. The ratio between the number of recovered specific-locus and dominant cataract mutations was in accordance with that found in our experiments with γ-rays (Ehling et al., 1982; Kratochvilova, 1981) or in experiments with ethylnitrosourea (Favor, 1986).A total of 3 dominant cataract mutations were recovered in the offspring of procarbazine-treated spermatogonial stem cells. Two mutations had complete penetrance while the third exhibited a reduced penetrance of approximately 70%. The viability and fertility of the heterozygotes of all 3 mutations were not affected. Only 1 mutation was shown to be viable as a homozygote.

Induction of gene mutations in germ cells of the mouse.

Methods available to study the induction of gene mutations in mammalian germ cells are described. Results of specific locus experiments with mice indicate the importance of this test for the validation of short term tests. Out of 5 compounds studied extensively in the Environmental Research Programme of the Commission of the European Communities 2 induced mutations in the Salmonella typhimurium test and in the specific locus test, while 2 others induced mutations only in mammals. In contrast to radiation, chemical mutagens can express mutations in certain gametogenic stages only. The different gametogenic sensitivity of mammalian germ cells to chemical mutagens gives a clue for the evaluation of mutagenicity data. In the linear part of the dose-effect-curve fractionation of a dose of 600 mg/kg procarbazine reduced the yield of specific locus mutations. In the descending part of the dose-effect-curve fractionation of a dose increased the rate of specific locus mutations above the single total dose. Experiments with procarbazine indicate that oocytes are less sensitive than spermatogonia for the induction of specific locus mutations. The genetic risk is defined as the increase in percent of the spontaneous mutation rate. The genetic risk can be calculated with high confidence on the basis of relatively few animals in the experimental group. The ratio between the therapeutic dose and the doubling dose gives the percentage increase of the spontaneous mutation frequency of a patient (individual risk). The ratio between the population dose and the doubling dose gives the percentage increase of the spontaneous mutation frequency of a population (population risk). The quantification of the genetic risk of the first generation is possible on the basis of dominant cataract mutations in mice. All extrapolations are based on the assumption that man and mice are equally sensitive to the induction of gene mutations.

Evaluation and re-evaluation of genetic radiation hazards in man III. Other relevant data and risk assessment

Some of the advances in mammalian radiation genetics, human genetics and cytogenetics that were made during the last 2–3 years and that have either a direct bearing on, or that may be potentially useful in, the evaluation of genetic radiation hazards in man have been examined. Among these are (1) the new data on the incidence of genetic diseases in man; (2) the latest results of the study of mortality rates among children born to survivors of the atomic bombings of Hiroshima and Nagasaki; (3) new data on the radition-induction of reciprocal translocations in human spermatogonia; (4) new results from radiation studies with mice on skeletal mutations, autosomal recessive lethals, sex-chromosome losses, translocation induction and recovery etc., and (5) a re-analysis of the earlier data on dose-rate effects for the induction of specific locus mutations in mouse spermatogonia. Using the pertinent new information as a basis, quantitative estimates are presented employing both a direct method of expressing risks in terms of effects per unit dose of irradiation and the indirect doubling-dose method of expressing these as increments over the load of genetic disorders occuring spontaneously in man.

Mutagenic evaluation of antischistosomal drugs and their derivatives in Neurospora crassa.

The mutagenic activities of lucanthone, hycanthone, niridazole, and the indazole analogs of lucanthone (IA-3 and IA-5) or hycanthone (IA-4 and IA-6) were studied by assaying for the induction of specific locus mutations in the ad-3 region of N. crassa. The results show that lucanthone, hycanthone, and their indazole analogs (IA-3 through IA-6) are all mutagenic in N. crassa when conidia are treated with any of these compounds. On a per mole basis, hycanthone is the least toxic and mutagenic, whereas IA-3 is the most toxic and mutagenic compound among the six closely related agents. In general, compounds with a methyl group at the C-4 position are more mutagenic than compounds with a methanol group; 6-chloroindazole analogs are more mutagenic and more toxic than nonchlorinated analogs. Niridazole is not mutagenic when conidial suspensions are treated. However, the mutation frequency increased more than 50-fold when niridazole was added to the medium used to grow vegetative cultures. Thus, it appears that the mutagenic activity of this latter compound requires metabolic activation.

Differential spermatogenic response of mice to the induction of specific locus mutations by environmental mutagens : Ehling, U. H., Abteilung für Genetik, Gesellschaft für Strahlen- und Umweltforschung, 8042 Neuherberg (Germany)

Differential spermatogenic response of mice to the induction of specific locus mutations by environmental mutagens : Ehling, U. H., Abteilung für Genetik, Gesellschaft für Strahlen- und Umweltforschung, 8042 Neuherberg (Germany)