X-ray-induced specific-locus mutation rate in newborn male mice

Muller's genetic load/species extinction hypothesis

The genetic load hypothesis of Hermann Muller raised the profound question of possible species extinction, even for humans, following a prolonged accumulation of recessive genes due to ionizing radiation exposure within the population. Two major mouse radiation research teams in the United States provided the most extensive tests of Muller's hypothesis. One group continued its study for more than two decades, over 82 consecutive generations, approximating 2500 human years. Even though Muller had stressed for decades his fear of species-threatening effects, no significant effects were observed for related factors such as reproductive fitness and longevity. Yet, the paper presenting the data of the 82-generation negative study has only been cited five times in 45 years. Altogether numerous laboratories worldwide collected vast amounts of data on mice, rats, and swine in an unsuccessful attempt to see if there was convincing evidence to support the genetic load theory and claims that species might deteriorate or be rendered extinct. This paper re-examines Muller's genetic load hypothesis with a new evaluation of how that hypothesis was tested and the significance of the findings, with most of those studies being completed before the BEIR I Committee Report in 1972. That committee briefly discussed the available evidence, mostly ignoring those results as they proceeded to make hereditary risk estimates both for (1) the first generation after a radiation exposure and (2) for the time, in the distant future, when a hypothetical genetic equilibrium would be reached. Their estimates assumed accumulation of harmful mutations and a linear no-threshold dose response extending all of the way down to a single ionization. More recent data on induction by ionizing radiation of dominant mutations that affect the skeletons of mice provide further robust supporting evidence that the generationally cumulative and LNT-based assumptions underpinning Muller's genetic load hypothesis are not correct.

Preservation of chromosomal integrity in murine spermatozoa derived from gonocytes and spermatogonial stem cells surviving prenatal and postnatal exposure to γ-rays in mice

Pre- and postnatal male mice were acutely (659-690 mGy/min) and continuously (0.303 mGy/min) exposed to 2 Gy γ-rays to evaluate spermatogenic potential and chromosome damage in their germ cells as adults. Acute irradiation on Days 15.5, 16.5, and 17.5 post-coitus affected testicular development, as a result of massive quiescent gonocyte loss; the majority of the seminiferous tubules in these testes were devoid of germ cells. Acute irradiation on Days 18.5 and 19.5 post-coitus had less effect on testicular development and spermatogenesis, even though germ cells were quiescent gonocytes on these days. Adverse effects on testicular development and spermatogenesis were observed following continuous irradiation between Days 14.5 and 19.5 post-coitus. Exposure to acute and continuous postnatal irradiation after the differentiation of spermatogonial stem cells and spermatogonia resulted in nearly all of the seminiferous tubules exhibiting spermatogenesis. Neither acute nor continuous irradiation was responsible for the increased number of multivalent chromosomes in primary-spermatocyte descendents of the exposed gonocytes. In contrast, a significant increase in cells with multivalent chromosomes was observed following acute irradiation on Days 4 and 11 post-partum. No significant increases in unstable structural chromosomal aberrations or aneuploidy in spermatozoa were observed, regardless of cell stage at irradiation or the radiation dose-rate. Thus, murine germ cells that survive prenatal and postnatal irradiation can restore spermatogenesis and produce viable spermatozoa without chromosome damage. These findings may provide a better understanding of reproductive potential following accidental, environmental, or therapeutic irradiation during the prenatal and postnatal periods in humans.

A critical role of PUMA in maintenance of genomic integrity of murine spermatogonial stem cell precursors after genotoxic stress

Neonatal gonocytes are precursors of spermatogonial stem cells. Preserving their integrity by elimination of damaged germ cells may be crucial to avoid the transmission of genetic alterations to progeny. Using gamma-irradiation, we investigated by immunohistochemistry, flow cytometry and real-time PCR components of the death machinery in neonatal gonocytes. Their death was correlated with caspase 3 activation but not with AIF translocation into the nucleus. The in vivo contribution of both the extrinsic and the intrinsic pathways was then investigated. We focused on the roles of TRAIL/Death Receptor 5 (DR5) and PUMA. Our results were validated using knockout mice. Whereas DR5 expression was upregulated at the cell surface after radiation, caspase 8 was not activated. However, we detected caspase 9 cleavage associated with cytochrome c release. In mice deficient for PUMA, radiation-induced gonocyte apoptosis was reduced, whereas invalidation of TRAIL had no effect. Overall, our results show that genotoxic stress-induced apoptosis of gonocytes is caspase-dependent and involves almost exclusively the intrinsic pathway. Furthermore, PUMA plays a critical role in the maintenance of genomic integrity of spermatogonial stem cell precursors.

Paternal transmission of genetic damage: findings in animals and humans

The concept that mutations can be induced in the male germ-line and result in adverse effects in the offspring has achieved only limited acceptance despite considerable theoretical appeal. This is partly because fetal malformations are generally perceived to be induced solely as a result of maternally mediated events during gestation and partly because the low incidence of the end-points concerned make experimental approaches costly and time-consuming. Nonetheless, a substantial body of work relating to the hypothesis has accumulated in the last 20 years, which has never been reviewed in its entirety. A consideration of the available evidence indicates that preconceptional paternal exposure to mutagens (particularly radiation, cyclophosphamide and ethylnitrosourea) can indeed, under certain conditions, have adverse effects on offspring. The results suggest two principal mechanisms by which such effects may be induced: the induction of germ-line genomic instability or the suppression of germ cell apoptosis.

Mechanisms of mutation induction in germ cells of the mouse as assessed by the specific locus test

Mouse germ cell specific locus mutagenesis data and a molecular characterization of mutant alleles have been reviewed to arrive at an understanding of the mechanism of mutation induction in mammals. (a) The spermatogenic stage specificity for the sensitivity to mutation induction by 20 chemical mutagens is considered. (b) The effects of a saturable repair process and its recovery over time are examined for the mutagenic efficiency of ethylnitrosourea. (c) The mutagenic events following methylnitrosourea and chlorambucil are shown to be mainly deletions. In contrast the mutations recovered after ethylnitrosourea treatment are almost exclusively base pair substitutions. (d) It is emphasized that to date very few specific locus experiments have been designed to test for mutagenic events outside the interval stem cell spermatogonia-mature spermatozoa. A specific locus mutation has recently been shown to be due to loss of heterozygosity via mitotic recombination in an early zygote stage and suggests a broader range of possible mechanisms of mutation when these stages are considered. (e) With the cloning of all 7 marker loci mutation analysis at the molecular level will allow a more direct assessment of the mutation process in future studies.

Life cycle of the mammalian germ cell: implication for spontaneous mutation frequencies

A brief history of the developmental life cycle of the mammalian germ cell, from fertilization to gametogenesis in the mature gonad, is presented. The differences between gametogenesis in the mature gonad of males and females are also described with regard to properties that may affect their susceptibilities to mutation. It is emphasized that any historical control background rate of necessity will include mutations that occur in germinal tissue at all stages of development and differentiation, although it is not always possible to determine at what stage of germline development a spontaneous mutation has occurred. Studies of induced mutations suggest that the impact on the molecular level and the distribution of mutations among the F1 and F2 progeny may be partly determined by the stage and sex of germ cells in which spontaneous mutations occur. In summary, historical control rates should only be considered the sum total of mutations that occur during the entire life of the individual and cannot represent the control values of any individual germ cell stage. Nonetheless, it is certainly important and valid to use historical control data for calculating human risk, because the primary use of the estimation of mutant frequencies is to access the potential impact of agents in increasing the genetic load in the human population.

Spontaneous mutations recovered as mosaics in the mouse specific-locus test

The specific-locus test (SLT) detects new mutants among mice heterozygous for seven recessive visible markers. Spontaneous mutations can be manifested not only as singleton whole-body mutants in controls (for which we report new data), but as mosaics-either visible (manifesting mottled coat color) in the scored generation (G2) or masked, among the wild-type parental generation (G1). Masked G1 mosaics reveal themselves by producing clusters of whole-body mutants in G2. We provide evidence that most, if not all, mosaics detected in the SLT (both radiation and control progenies) result from a single-strand spontaneous mutation subsequent to the last premeiotic mitosis and before the first postmeiotic one of a parental genome-the "perigametic interval." Such events in the genomes of the G1 and Gzero results, respectively, in visible and masked 50:50 mosaics. Per cell cycle, the spontaneous mutation rate in the perigametic interval is much higher than that in pregamete mitotic divisions. A clearly different locus spectrum further supports the hypothesis of different origin, and casts further doubt on the validity of the doubling-dose risk-estimation method. Because mosaics cannot have arisen in mitotic germ cells, and are not induced by radiation exposure in the perigametic interval, they should not be included in calculations of radiation-induced germ-line mutation rates. For per-generation calculations, inclusion of mosaics yields a spontaneous frequency 1.7 times that calculated from singletons alone for mutations contributed by males; including both sexes, the multiple is 2.2.

Dose-dependent induction of recessive mutations with N-ethyl-N-nitrosourea in primordial germ cells of male mice

Using a specific locus test, we previously found that N-ethyl-N-nitrosourea (ENU) induces recessive mutations at a relatively high rate in male mouse primordial germ cells (PGC) at 8.5, 10.5 and 13.5 days of development (G8.5, G10.5 and G13.5). A large difference was observed on the induced mutation rate between 30 and 50 mg/kg ENU in 10.5-day PGC. We therefore carried out specific locus tests to ascertain whether ENU induces recessive mutations in a dose-dependent manner in G8.5 and G10.5 PGC. We also gave multiple doses of 25 mg/kg ENU using an 18-h interval, the approximate doubling time of PGC at these developmental stages, to test for an additive effect on the induced mutations rate. A dose-dependent induction of recessive mutations by ENU was observed in both G8.5 and G10.5 PGC, and multiple dosing of 25 mg/kg ENU showed an additive effect. Comparing these results to data on spermatogonial stem cells, we conclude the capacity to repair ENU-induced premutagenic damages is less effective in male mouse PGC at these developmental stages than in spermatogonial stem cells.

Biological basis of germline mutation: comparisons of spontaneous germline mutation rates among drosophila, mouse, and human

Spontaneous mutation rates per generation are similar among the three species considered here--Drosophila, mouse, and human--and are not related to time, as is often assumed. Spontaneous germline mutation rates per generation averaged among loci are less variable among species than they are among loci and tests and between gender. Mutation rates are highly variable over time in diverse lineages. Recent estimates of the number of germ cell divisions per generation are: for humans, 401 (30-year generation) in males and 31 in females; for mice, 62 (9-month generation) in males and 25 in females; and for Drosophila melanogaster, 35.5 (18-day generation) in males and 36.5 (25-day generation) in females. The relationships between germ cell division estimates of the two sexes in the three species closely reflect those between mutation rates in the sexes, although mutation rates per cell division vary among species. Whereas the overall rate per generation is constant among species, this consistency must be achieved by diverse mechanisms. Modifiers of mutation rates, on which selection might act, include germline characteristics that contribute disproportionately to the total mutation rates. The germline mutation rates between the sexes within a species are largely influenced by germ cell divisions per generation. Also, a large portion of the total mutations occur during the interval between the beginning of meiosis and differentiation of the soma from the germline. Significant genetic events contributing to mutations during this time may include meiosis, lack of DNA repair in sperm cells, methylation of CpG dinucleotides in mammalian sperm and early embryo, gonomeric fertilization, and rapid cleavage divisions.

Specific-locus experiments show that female mice exposed near the time of birth to low-LET ionizing radiation exhibit both a low mutational response and a dose-rate effect

Female mice were exposed to 300 R of 73-93 R/min X-radiation either as fetuses at 18.5 d post conception (p.c.) or within 9 h after birth. Combining the similar results from these two groups yielded a specific-locus mutation frequency of 9.4 X 10(-8) mutation/locus/R, which is statistically significantly higher than the historical-control mutation frequency, but much lower than the rate obtained by irradiating mature and maturing oocytes in adults. Other females, exposed at 18.5 days p.c. to 300 R of 0.79 R/min gamma-radiation, yielded a mutation frequency that was statistically significantly lower than the frequency at high dose rates. The low-dose-rate group also had markedly higher fertility. It appears that the dose-rate effect for mutations induced near the time of birth may be more pronounced than that reported for mature and maturing oocytes of adults. A hypothesis sometimes advanced to explain low mutation frequencies recovered from cell populations that experience considerable radiation-induced cell killing is that there is selection against mutant cells. The reason for the relatively low mutational response following acute irradiation in our experiments is unknown; however, the finding of a dose-rate effect in these oocytes in the presence of only minor radiation-induced cell killing (as judged from fertility) makes it seem unlikely that selection was responsible for the low mutational response following acute exposure. Had selection been an important factor, the mutation frequency should have increased when oocyte killing was markedly reduced.

ENU-induced allele of brachyury (Tkt1) exhibits a developmental lethal phenotype similar to the original brachyury (T) mutation

New alleles of brachyury (Tkt1, Tkt4) were induced in the mouse complete tw5 haplotype by ethylnitrosourea (ENU). Like the original brachyury (T) mutation, the new alleles cause a short-tailed phenotype in heterozygotes, and interact with the t complex tail interaction factor (tct) in trans to cause phenotypically tailless mice. Because ENU is mainly a point mutagen, it is important to determine that the new alleles are homozygous embryonic lethal mutations like the original T allele, and to characterize their embryonic lethal phenotype. Moreover, the Tkt1 mutation maps to an inverted position relative to quaking (qk) in t haplotypes as compared with its position on normal chromosome 17. The Tkt1 allele was separated from the resident tw5 lethal gene, tclw5, by recombination, allowing embryology studies to be performed. Embryological analyses show that the Tkt1 allele is nearly identical to the classic T allele. At 9 and 10 days of development, homozygous Tkt1/Tkt1 embryos are grossly abnormal with properties including 1) irregular, disorganized somite pairs, 2) a shortened posterior end of the embryo, 3) an irregular neural tube, and 4) an abnormal notochord. In addition, 10 day-old abnormal embryos have anterior limb buds that point dorsally rather than ventrally, and are smaller than normal littermates. We conclude that the Tkt1 mutation is a valuable allele for both mapping and molecular characterization of the brachyury locus.

The effects of ionizing radiation, microwaves, and ultrasound on the developing embryo: clinical interpretations and applications of the data

The term "radiation" evokes emotional responses both from lay individuals and from professionals. Many spokespersons are unfamiliar with radiation biology or the quantitative nature of the risks. Frequently, microwave, ultrasound, and ionizing radiation risks are confused. Although it is impossible to prove no risk for any environmental hazard, it appears that exposure to microwave radiation below the maximal permissible levels present no measurable risk to the embryo. Ultrasound exposure from diagnostic ultrasonographic imaging equipment also is quite innocuous. It is true that continued surveillance and research into potential risks of these low-level exposures should continue, but at present ultrasound not only improves obstetric care but also reduces the necessity of diagnostic x-ray procedure. In the field of ionizing radiation, we have as good a comprehension of the biologic effects and the quantitative maximum risks as of any other environmental hazard. Although the animal and human data support the conclusion that no increases in the incidence of gross congenital malformations, intrauterine growth retardation, or abortion will occur with exposures below 5 rad, that does not mean that there are definitely no risks to the embryo exposed to lower doses of radiation. Whether there exists a linear or exponential dose-response relationship or a threshold exposure for genetic, carcinogenic, cell-depleting, and life-shortening effects has not been determined. In establishing maximum permissible levels for the embryo at low exposures, we use the information in Tables 2, 3, 4, 7, 10, and 14. It is obvious that the risks of 1-rad or 5-rad acute exposure are far below the spontaneous risks of the developing embryo, since 15% of human embryos abort, 2.7%-3.0% of human embryos have major malformations, 4% have intrauterine growth retardation, and 8%-10% have early- or late-onset genetic disease. The maximum risk attributed to a 1-rad exposure, approximately 0.003%, is thousands of times smaller than the spontaneous risks of malformations, abortion, or genetic disease (see Table 10). Thus, the present maximum permissible occupational exposures of 0.5 rem for pregnant women and 5 rem for medical exposure are extremely conservative. Medically indicated diagnostic roentgenograms are appropriate for pregnant women, and there is no medical justification for terminating a pregnancy in women exposed to 5 rad or less because of a radiation exposure.(ABSTRACT TRUNCATED AT 400 WORDS)

Reproductive and genetic effects of continuous prenatal irradiation in the pig

The stem germ cells of the prenatal pig are highly vulnerable to the cytotoxic effects of ionizing irradiation. This study was conducted to determine whether sensitivity to killing was also marked by a sensitivity to mutation and how prenatal depletion of the germ-cell population affects reproductive performance. Germ-cell populations were reduced by continuously irradiating sows at dose rates of either 0.25 or 1.0 rad/day for the first 108 days of gestation. The prenatally irradiated boars were tested for sperm-producing ability, sperm abnormalities, dominant lethality, reciprocal translocations, and fertility. Prenatally irradiated females were allowed to bear and nurture one litter, then tested for dominant lethality in a second litter; germ cell survival and follicular development were assessed in their serially sectioned ovaries. Sperm production was not significantly affected in the 0.25-rad boars, but boars irradiated with 1.0 rad per day produced sperm at only 17% of the control level. Incidence of defective sperm was 4.9% and 11.1% in the 0.25 and 1.0 groups, respectively. Four of the 1.0-rad boars were infertile, but prenatal irradiation apparently caused neither dominant lethality nor reciprocal translocations in fertile males. Number of oocytes was reduced to 66 +/- 7% of control in the 0.25-rad gilts, but reproductive performance was unaffected and no dominant lethality was observed. Only 7 +/- 1% of the oocytes survived in the 1.0-rad group. Reproductive performance was normal for the first litter, but four of the 23 sows tested were infertile at the second litter and a significant incidence of dominant lethality was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Deletion mapping of the T/t complex: evidence for a second region of critical embryonic genes

The developmental effects of three different deletion mutations of the T/t complex of the mouse have been studied. The three mutations, TOak Ridge (OR), TOrleans (TOrl), and THair pin (THp), each produce a unique homozygous lethal phenotype: THp homozygotes fail to develop normally past the morula stage, TOrl homozygotes past the blastocyst stage, and TOR homozygotes past the egg cylinder stage. In compound embryos (TX/TY), the lethal phenotype observed corresponds to the shared length of deleted chromosome. This interaction allows the regions of chromosome 17, containing genetic information critical to early mammalian development, to be mapped.

Cytogenetic effects of protracted gamma exposures from conception of male mice

In order to gain an overall picture of the genetic effects of an increased level of background radiation it is necessary to study the results of protracted exposures to embryonic and immature germ-cell stages as well as to stages found in the mature organism. For this purpose, litters produced by female mice, kept in a 10 or 20 rad/day 60 Co gamma-irradiation field, were kept in the same fields from conception until about 60 days later, having absorbed doses of 526 and 1078 rad respectively. Tests on exposed female offspring showed them to be sterile. 8 weeks after removal from the gamma field, mean testis masses of males in the 20 rad/day series were only half normal but those receiving 10 rad/day were little affected. Frequencies of translocations in spermatocytes at diakinesis/metaphase I were only slightly increased in the exposed series, differences not being significant. Estimated rates of translocation induction were around 5 x 10(-6) per rad, about one-third of those found after protracted gamma-irradiation of stem-cell spermatogonia in the adult. Embryonic lethality in progeny of other similarly irradiated males(absorbed doses of 560 and 1040 rad), mated 2 months after removal from the radiation fields, was also increased slightly, but not significantly. Results are compared with others on the induction of chromosome aberrations and gene mutations, mainly by acute irradiation, in prenatal and neonatal male mice. It is concluded that early male germ-cell stages generally show a reduced genetic radiosensitivity after both acute and chronic exposures.

Mutation frequencies in male mice and the estimation of genetic hazards of radiation in men

Estimation of the genetic hazards of ionizing radiation in men is based largely on the frequency of transmitted specific-locus mutations induced in mouse spermatogonial stem cells at low radiation dose rates. The publication of new data on this subject has permitted a fresh review of all the information available. The data continue to show no discrepancy from the interpretation that, although mutation frequency decreases markedly as dose rate is decreased from 90 to 0.8 R/min (1 R = 2.6 x 10(-4) coulombs/kg) there seems to be no further change below 0.8 R/min over the range from that dose rate of 0.0007 R/min. Simple mathematical models are used to compute: (a) a maximum likelihood estimate of the induced mutation frequency at the low dose rates, and (b) a maximum likelihood estimate of the ratio of this to the mutation frequency at high dose rates in the range of 72 to 90 R/min. In the application of these results to the estimation of genetic hazards of radiation in man, the former value can be used to calculate a doubling dose--i.e, the dose of radiation that induces a mutation frequency equal to the spontaneous frequency. The doubling dose based on the low-dose-rate data compiled here is 110 R. The ratio of the mutation frequency at low dose rate to that at high dose rate is useful when it becomes necessary to extrapolate from experimental determinations, or from human data, at high dose rates to the expected risk at low dose rates. The ratio derived from the present analysis is 0.33.

The mouse specific-locus test with agents other than radiations: interpretation of data and recommendations for future work

The mouse specific-locus test with visible markers (SLT) has been the only extensively used method for detecting and quantifying the induction of heritable point mutations (intragenic changes and small deficiencies) in mammals. Mutations are detected in first-generation offspring; and scoring is simple, objective, and rapid. Different germ-cell stages can be sampled, including those of greatest pertinence for genetic risk assessment. The differential probability of involving the various loci of the marked set makes the method capable of detecting qualitative (as well as quantitative) differences between the actions of mutagens. Control SLT frequencies for males reported by 4 sets of investigators are in excellent agreement and were summed as a "historical control" (801406 observations) for use in our calculations. Experimental results were classified as positive, negative, or inconclusive based upon a multiple-decision procedure produced by the testing of the following 2 hypotheses: (1) the mutation frequency (induced + spontaneous) of treated mice is not higher than the spontaneous mutation frequency, and (2) the induced mutation frequency of treated mice is no less than 4 times the historical-control mutation frequency. Each hypothesis was tested at the 5% significance level. Because of the low mutation frequency in a very large control, the SLT is capable of yielding positive results in relatively small samples. We reviewed 58 publications, SLT results have been reported for 25 chemical agents, of which 17 (representing 21 chemical classes) gave results that were positive or negative by our criteria. The frequency of positive agents was 6 of 14, 5 of 5, and 0 of 1 conclusively tested, respectively, in spermatogonia, post-spermatogonial stages, and unspecified male germ cells. Depending on the chemical used, post-spermatogonial stages can be of greater, less, or equal sensitivity relative to spermatogonia. The SLT was strongly positive for some chemicals that are not mutagenic (or only weakly so) in lower systems, and there are several examples of the reverse situation. Factors which presumably operated to cause these differences (e.g., metabolism, transport, repair in germ cells) are likely also to operate for transmitted point mutations in man.

Dominant and recessive effects of induced-lethals in female mice by exposure to gamma-irradiation during the 10th to 14th day of intrauterine life

The frequency of (dominant and) recessive lethal mutations induced by 160-rad chronic gamma-irradiation given with a dose rate of 0.03 rad/min during the 10th to 14th day of gestation has been studied in female CBA-mice. An increased rate of recessive lethal equivalents by about 10% has been noted. This increase corresponds to a mutation rate of 6.3 X 10(-4) mutation/rad/genome. There were not found any dominant mutations, nor any dominance effects from the induced recessive lethal equivalents. The hazards after irradiation during foetal development are discussed.

Dose-response data for X-ray induced translocations in spermatogonia of Rhesus monkeys

The yields of translocations in spermatocytes after irradiation of spermatogonia of Rhesus monkeys with doses of 100, 200 or 300 rad X-rays were low, and consistent with a humped dose-response curve with a peak at about 200 rad. Such a curve would agree well with earlier results on the marmoset and man, but the yields at any dose in the Rhesus monkey were lower.

A comparison of chromosomal radiosensitivities of somatic cells of mouse and man

A cell culture technique for quantitative analysis of radiation-induced chromosome aberrations in somatic cells has been developed and used for the comparison of chromosomal sensitivity of skin cells of mouse and man to 60Co-gamma-rays. This includes culture of irradiated tissues or cells in culture in arginine and isoleucine-deficient medium and subsequent refeeding with complete medium (CM). With this technique, radiation-induced chromosome aberrations can be analyzed selectively in the cells exposed in G1 phase and recovered at their first post-irradiation mitosis. When tested on the human embryonic cells, the dicentric yield was essentially the same whether they were skin cells irradiated in silu or cultured cells at various in vitro passages irradiated in vitro. In contrast, when studied in the skin cells irradiated in silu, mouse embryos and newborns were insensitive to the induction of dicentrics. In young mice on day II however, the sensitivity was at a level comparable to that in human embryonic cells and it was intermediate on day 4. Such embryonic insensitivity of the mouse cells was rapidly lost during serial transfer in vitro; and, when tested at 4th or later subculture generations, mouse and human cells were equally sensitive to the induction of dicentrics. These results suggest that the chromosomal radiosensitivity is essentially the same for mouse and human cells but can be modified by some biological factors, possibly DNA repair mechanisms, which differ between species as well as among the states of differentiation of particular cell types. Special attention was paid to the parellelism between the age-dependent changes in the chromosomal, mutational and carcinogenic radiosensitivities in the mouse. If this parallelism can be carried over to man, human pre-natal irradiation will not present any reduced genetic hazards.

Observations on a set of radiation-induced dominant T-like mutations in the mouse

Genetic analysis of seven dominant short tailed mutations independently induced by radiation of male mice showed that six were allelic toT(Brachyury) but not identical to it. Homozygotes for each mutant die at least 2 days earlier thanT/Thomozygotes; two that were studied histologically are indistinguishable from one another. The development of these abnormal embryos is arrested by seven days of gestation, when cells of embryonic ectoderm cease proliferation and become pycnotic. Endoderm and extra-embryonic ectoderm do not seem to be primarily affected, and survive and grow for at least 2 days more. Serological studies of one of these mutations suggest that it is a deletion. A review is presented of these and otherT-like mutations that have been described; from this it appears that five different categories ofT-like mutants are discernible.