Radiosensitivity of Drosophila lines. VII. The effect of the maternal genotype on the yield of recessive and dominant lethal mutations induced by the gamma irradiation of males

The effect of material repair on induction of paternal mutations was tested with radiosensitive rad(2)201G1 mutant. Basc males were irradiated at doses from 0 to 60 Gy of gamma-rays and mated to the radiosensitive mutant or control females. Frequencies of sex-linked recessive lethals and dominant lethals (induced in the paternal genome) were determined. With control females, the rate of recessive lethals increased linearly from 0 to 60 Gy. With rad(2)201G1 mutant, an increase in spontaneous and induced rates of paternal dominant lethals was observed; the rate of sex-linked recessive lethals increased non-linearly from 0 to 60 Gy.     

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)     

Species comparisons concerning radiation-induced dominant lethals and chromosome aberrations

After X-irradiation of post-meiotic stages, male mice, guinea-pigs, rabbits and golden hamsters differed both in general sensitivity to the induction of dominant lethals, and in the relative sensitivity of the various spermatogenic stages. Guinea-pigs were the least sensitive, and hamsters had a different stage sensitivity pattern, with mature sperm the most sensitive stage.     

The induction of chromosome aberrations in mouse dictyate oocytes by X-rays and chemical mutagens.

Oocytes were collected from female mice and matured in vitro to Metaphase I during the first or third week after treatment with a dose of 400 rad X-rays, 1.6 mg/kg triethylenemelamine (TEM) or 75 mg/kg isopropylmethanesulphonate (IPMS). In week 1 the mean number of oocytes per female was similar for all treatments but in week 3 irradiated females yielded fewer oocytes than the chemically treated or control females. In week 1 the proportion of oocytes maturing was smaller in irradiated females than in the other groups but in week 3 was similar in all groups.Structural chromosome aberrations, scored in the Metaphase I oocytes, were of the chromatid or isochromatid type and involved gaps, breaks, fragments, intrachanges and interchanges. Aberration frequency did not increase with time after either of the chemical mutagens but after irradiation was higher in the third week than in the first week. The aberration yield from IPMS-treated females was similar at both sampling times, while a lower yield was recorded in week 3 following TEM treatment than in week 1.     

X-ray induced dominant lethal mutations in mature and immature oocytes of guinea-pigs and golden hamsters.

The induction of dominant lethal mutations by doses of 100-400 rad X-rays in oocytes of the guinea-pig and golden hamster was studied using criteria of embryonic mortality. For both species higher yields were obtained from mature than from immature oocytes, in contrast to results for the mouse. Data on fertility indicated that in the golden hamster, as in the mouse, immature oocytes were more sensitive to killing by X-rays than mature oocytes but that the converse was true in the guinea-pig. The dose-response relationship for mutation to dominant lethals in pre-ovulatory oocytes of guinea-pig and golden hamsters was linear, both when based on pre- and post-implantation loss and when on post-implantation loss only. The rate per unit dose was higher for the golden hamster, and the old golden hamsters were possibly slightly more sensitive than young ones. The mutation rate data for mature oocytes of the mouse, using post-implantation loss alone, also fitted a linear dose-response relationship, except that the rate per unit dose was lower than for the other two species.     

The interaction of the rad201 gene with mei-9 and mei-41 genes in germline cells of Drosophila female

The effect of Drosophila mutation rad201G1 together with mutations mei-41D5 and mei-9a on the sensitivity of oocytes to induction of dominant lethals (DLs) was studied. To this end, the frequencies of spontaneous and gamma-radiation-induced DLs in consecutive egg batches of females carrying double or single mutations were estimated. Since the effects of the mutations examined are expressed only at the previtellogenetic stages of oogenesis, only newly hatched (0-5-hour-old) females, whose oocytes did not develop farther than stage 7, were irradiated. The results obtained indicated that in intact and irradiated oocytes of double mutants mei-9a rad201G1 and mei-41D5 rad201G1, mutation rad201G1 epistatically suppresses the mutations of the both mei genes.     

The induction by X-rays of chromosome aberrations in germ cells of male guinea-pigs, golden hamsters and rabbits. I. Dose response in post-meiotic stages.

The induction by X-rays of translocations in post-meiotic germ cells of the guinea-pig, golden hamster and rabbit was studied by cytological analysis of male offspring of the irradiated animals. As reported previously for the mouse, the pattern of sensitivity to dominant lethal induction, as indicated by litter-size, was similar to that for translocation induction in both the guinea-pig and golden hamster. In both speciesspermatids were more sensitive than spermatozoa, and in the golden hamster spermatocytes gave a lower yield than spermatids. The translocation frequency among post-meiotic germ cells treated with 600 rad was higher in the rabbit than the guinea-pig, and both were above that for the golden hamster. However, for spermatozoa, species differences with respect to the recovered translocation yield appeared to depend on dose. In the hamster, the translocation frequency after 600 rad, as measured in the female offspring, was similar to that obtained in the male offspring. A small amount of data on the induction of sex-chromosome aneuploidy by 200 rad in golden hamsters suggested that the hamster might be as sensitive as the mouse.     

EP-1不育剂对黑线毛足鼠种群繁殖的影响

为检验EP-1 不育剂对野外鼠类的施用效果,2004 年在内蒙古锡林郭勒盟阿巴嘎旗白音图嘎800 hm² 草场进行了大田实验,并随后进行了逐月的夹线跟踪调查,分析了不育剂(EP-1) 对黑线毛足鼠种群繁殖的效应。结果表明,EP-1 不育剂对黑线毛足鼠种群繁殖的抑制效果良好,投药区与对照区相比,投药区黑线毛足鼠的子宫损伤率达到80% ,平均胎仔数下降到对照区的2 /3 水平,妊娠率也下降到对照区的20% 。但EP-1 不育剂对雄鼠睾丸下降率的作用不明显。一次性投放EP-1 不育剂,对黑线毛足鼠种群的繁殖作用时间可维持4 个月以上,基本可实现对整个繁殖期的控制成效,这可能与黑线毛足鼠具有储藏种子的习性,储藏药饵多次进食有关。     

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.     

Discrimination between the effects of X-ray irradiation of the mouse oocyte and uterus on the induction of dominant lethals and congenital anomalies

In order to test whether irradiation of the postimplantation maternal environment had any effect on the apparent induction of dominant lethals or congenital anomalies by radiation, preimplantation embryos were surgically transferred between females which had been irradiated before conception or left untreated. A high proportion of preimplantation embryos, collected from females that had been irradiated 15-21 days prior to conception with 3.6 Gy X-rays, were either arrested or developmentally retarded compared with those collected from untreated females. The transfer experiments indicate that irradiation of the uterus has no significant effect on the frequency of subsequent postimplantation mortality or on mean fetal weight. However, it remains unclear whether irradiation of the uterus contributes to the induction of congenital anomalies.     

Social influences on reproductive development and fertility in female Djungarian hamsters (Phodopus campbelli).

Social influences on the sexual maturation of female Djungarian hamsters were investigated in two experiments. In the first experiment females were housed from weaning with an adult male, by themselves, or with a weanling sister. Maturation was accelerated in females housed with males as indicated by younger age at first ovulation, increased rates of ovarian and uterine growth, and lower LH levels at some ages. Maturation was delayed in females housed with sisters compared to those housed alone as measured by time of first ovulation and by lower estradiol levels at some ages. The most marked differences between groups occurred 8 to 12 days after weaning, suggesting that events during this period are particularly important in the social mediation of sexual maturation. In the second experiment the effects of reproductive suppression (caused by living with a sister) on the subsequent fertility of females housed with males were examined. If male-female pairs were housed in clean cages, no effects were observed; however, pairs housed in cages previously soiled by the female and her sister had fewer young surviving until 1 week of age despite no differences in the age of pregnancy onset or in the initial litter size. Thus, even cues present in unrenewed soiled bedding may have subtle but long lasting effects on reproductive function.     

Female-specific dominant lethal effects in mice

For some chemicals, induction of presumed dominant lethal mutations has been observed only in female mice and not in males. In those cases, questions arise as to (1) whether the increased embryonic mortality is due to genetic effects of the chemicals in the oocyte or, (2) is caused indirectly through maternal toxicity, and, if genetic, (3) the basis for the sex difference. These questions were studied using the compounds adriamycin and platinol. Neither compound induces dominant lethals in male germ cells, but both increased early embryonic mortality when females were treated prior to mating (treatment of maturing oocytes). Reciprocal zygote transfer experiments rules out, either entirely or for the large part, maternal toxicit as the cause, and cytogenetic analysis of first-cleavage metaphases revealed high incidences of chromosomal aberrations. The results of both of these experiments thus provide evidence that the early embryonic mortality resulted from genetic effects induced in oocytes. Most interestingly, each compound produced unexpected types of chromosomal aberrations. Adriamycin produced deletions, rings, and presumed chromosome-type rearrangements. Platinol, on the other hand, produced a few chromatid-type aberrations, but the bulk of aberrations were characterized by disorganization of the chromatin, minute fragments, and thread-lik chromatin bridges between fragments and chromosomes or between two or more chromosomes. The latter type of cytogenetic damage was observed primarily in the compounds are associated with the diffused state of the maturing oocyte chromosomes.     

The induction by X-rays of chromosome aberrations in male Guinea-pigs and golden hamsters. IV. Dose response for spermatogonia treated with fractionated doses.

The effect of dose fractionation on the induction of translocations by 400 and 600 rad X-rays in spermatogonia of guinea-pigs and hamsters was investigated cytologically. Three types of fractionation were used, dividing the dose into (a) two equal fractions 24 h apart, (b) two equal fractions 8 weeks apart, and (c) eight or twelve equal fractions of 50 rad, at intervals of one week. The two species responded similarly throughout, but gave lower translocation yields than the mouse. The effects of the first and third types of fractionation were similar to those described previously in the mouse, and suggested that a first radiation dose modifies the spermatogonial population so that its sensitivity to a dose 24 h later is altered, and that repeated radiation doses result in development of resistance to translocation induction. After 8-week fractionation the results suggested that in guinea-pigs and hamsters the spermatogonial population had not returned to normal by 8 weeks after the first dose, whereas in the mouse normal sensitivity had returned by this time. The results, reported previously, of single doses of X-rays suggest that the spermatogonial population consists of fractionated doses in the mouse suggest that the sensitive and resistant types represent different phases of the same cell type rather than two distinct types of cell. In the guinea-pig and hamster this question remains open.     

Post-natal patterns of plasma androgen-binding activity in Djungarian (Phodopus sungorus) and golden (Mesocricetus auratus) hamsters

The binding of sex steroids to plasma proteins was examined in post-natal Djungarian and golden hamsters. With dihydrotestosterone or testosterone as ligands, steady-state polyacrylamide gel electrophoresis revealed 2 androgen binding components in the plasma of young Djungarian hamsters of both sexes. The fast-moving component exhibited a low affinity and high capacity for androgens and corresponded to albumin in stained gels. In contrast, the slow moving component was a beta-globulin with high affinity (Ka = 10(9) M-1) and low capacity for androgens, and was identified as a specific sex steroid-binding protein (SBP; also SHBG). This SBP did not bind oestrogens or corticosteroids and was electrophoretically distinct from corticosteroid-binding globulin (CBG). In addition, this protein did not appear to be of testicular origin because it was present in immature females and in immature males that had been castrated for 8 days. Plasma concentrations of SBP in males as measured by a diethyl-aminoethyl-cellulose binding assay were low at birth, became significantly elevated shortly thereafter when plasma androgen values were also elevated, and subsequently fell to low levels during puberty. These changes follow the same general pattern that has been described for other mammals, including humans, during this period of reproductive development. Although the significance of elevated SBP concentrations during prepuberty has yet to be determined, it appears that the increased concentrations of high-affinity androgen binding in the plasma of Djungarian hamsters plays a role in the asynchronous pubertal development of the testes and accessory organs which occurs in this species. The post-natal SBP pattern in females was similar to that observed in males. Plasma SBP levels were low or undetectable in adults of both sexes. As previously described for adult golden hamsters, the plasma of post-natal male and female golden hamsters lacked a specific SBP: androgen binding in this species is apparently limited to albumin and CBG.     

Breeding system can affect the offspring sex ratio in Syrian hamsters

Nulliparous female Syrian hamsters were used to investigate the effect of two different breeding systems on the fertility of the female Syrian hamster. We hypothesized that females submitted to a harem system (HS) would deliver smaller and more female-biased litters than in a monogamic system. Ten female and 10 adult male hamsters housed individually (G1) were kept in a monogamic temporary breeding system, while 10 females and five males (G2) were submitted to HS with two females and a male permanently housed together since female weaning. Females from G1 and G2 delivered, respectively, 47 and 50 litters, and produced 364 (G1) and 383 (G2) weaned pups without any difference in litter size, mean weight of weaned pups and body condition of dams. Interparturition intervals were shorter and the percentage of male pups per litter was higher in the HS possibly as a result of different endocrine conditions provided by different breeding systems. Besides providing evidence that housing conditions can influence the sex of hamster offspring, our findings suggest a mechanism for the non-random distribution of male and female pups in hamster litters.     

Reproductive Decisions in Female European Ground Squirrels: Factors Affecting Reproductive Output and Maternal Investment

Physiological and behavioural parameters associated with reproductive effort and success were investigated in female European ground squirrels Spermophilus citellus . The proportion of reproductive (lactating) females in the study population was over 90% and was not related to age. Timing of oestrus and ovulation was found to be affected by the female's emergence date and condition. Females with low emergence mass showed delayed oestrus. Differences in ovulation dates were shown to affect reproductive output in terms of litter size and sex ratio. Early litters were larger and male biased. X-ray techniques were used to determine intrauterine litter size in individual females. The results indicated that litter size and sex ratio were fixed prenatally. Lactation costs were reflected in the intensity of mass loss and duration of lactation. Mass loss varied with litter size, in that females with large litters showed a more rapid loss than others. The second parental investment parameter, lactation duration, varied among individual females and was dependent on the timing of reproduction and litter size (except yearlings). Early born litters, which were, in most cases, larger than later ones, were nursed longer. Prolonged lactation periods affected female condition in that they started prehibernation fattening later and entered hibernation with a lower mass than individuals that had shorter lactation periods. Yearling females probably could not afford the energetic costs of long lactation, independent of their offspring number. These results indicated that females with higher reproductive output and higher investment were unable to compensate these costs before hibernation. Consequences for these individuals could therefore be lower over-winter survival or a delayed oestrus in the following season.     

The relationship between radiation-induced and transposon-induced genetic damage during Drosophila spermatogenesis

The combined effect of transposon mobility and X-rays on X-linked recessive lethals and dominant lethals was measured in the germ line of F₁ male hybrids in the P-M system of hybrid dysgenesis. X-linked lethal mutation rate was measured in the chromosome derived from the P-strain father of the M × P cross. Mutations induced in irradiated dysgenic males were compared to those of unirradiated males, as well as to irradiated nondysgenic males derived from M × M crosses. Three four-day broods of sperm were tested for both X-linked lethals and dominant lethals. X-linked lethal mutation rate in dysgenic control males was 6.38%, 6.36% and 4.55% in broods 1, 2 and 3 respectively, thus showing a decrease in older males. The mutation rate in the same broods of irradiated, nondysgenic control males was 3.66%, 4.46% and 6.38%, respectively. The rate obtained in dysgenic irradiated males was 10.33, 11.16 and 7.97 in the same 3 broods. These results demonstrate that when X-rays and P element mobility were and 7.97 in the same 3 broods. These results demonstrate that when X-rays and P element mobility were combined as a source of mutagenesis, a strickly additive effect on genetic damage was observed in the first two broods of sperm which represent primarily mature sperm and spermatids respectively. The third brood, representing mostly spermatocytes showed a less than additive effect, probably due to germinal selection. In contrast, the induction of dominant lethals showed a clearly synergistic effect in the last two Broods of sperm tested, when X-rays and transposon mobility were combined. The X-ray component of dominant lethlity in brood 1, representing mostly mature spermatozoa, was negative, indicating a lower than expected lethality induced by X-irradiation in the presence of P element mobility. The X-ray-induced component of dominant lethality, was expressed as the per cent of embryo lethality after adjusting the results obtained with each brood of sperm from nondysgenic and dysgenic males to their respective unirradiated controls. These values were 32.3%, 30.5% and 64.7% for brood 1, 2 and 3 respectively from nondysgenic males, and 14.1%, 56.1% and 71.4% for the same broods from dysgenic males. Thus the differential effect of X-rays in sperm broods 1, 2 and 3 was −18.2, +25.6 and +6.7% respectively. These results suggest that the synergistic effect may be due to the common component of X-ray and P element-induced genetic damage, namely chromosome breaks, and that the interaction of these lesions resulted in a greater than additive number of of unrestitude chromosome breaks and nonviable chromosomal rearrangements.     

Food-restricting first generation juvenile female hamsters (Mesocricetus auratus) affects sex ratio and growth of third generation offspring

Female hamsters (Generation 1) were fed ad libitum or were food-restricted to 65-75% of the amount consumed by controls during their first 50 days of life. Subjects were mated at 91 days of age. Their offspring (Generation 2) were fed ad libitum throughout the experiment, and female offspring were also mated at 91 days of age. Generation 3 litters were monitored every fifth day from birth until Day 25 post partum for litter size, sex ratio, and pup weights. Although there were no significant differences in Generation 3 litter sizes at birth, litters descended from food-restricted Generation 1 females (Group R) were significantly smaller on Days 5-25 than litters descended from control Generation 1 females (Group A). Sex ratios remained significantly greater in Group A than in Group R litters from birth to Day 25 but did not vary over time, suggesting similar post-partum mortality rates for both male and female pups. Weights of Generation 3 male and female pups did not vary significantly within treatments at any time. Group A males weighed significantly more than Group R males from birth through Day 25, but weights of Group A and Group R females were always similar. Food restriction early in life may have long-term consequences on sex ratios of subsequent generations in hamsters.     

Genetic polyandry and sexual conflict in the sandbar shark, Carcharhinus plumbeus, in the western North Atlantic and Gulf of Mexico

To investigate patterns of polyandry in the sandbar shark (Carcharhinus plumbeus), 20 pregnant females were sampled from the western North Atlantic and Gulf of Mexico. Five species-specific microsatellite markers were used to genotype each shark and its litter. Of 20 litters, 17 (85%) were shown to have multiple sires. In multiply sired litters, the estimated minimum number of sires ranged from two to five with an average of 2.3 males per litter. Regression analysis did not demonstrate a significant relationship between female reproductive success and female body size or sire number and female body size. There was a high incidence of reproductive skew noted in litters, and two groups of males with significantly different mean reproductive success were observed. Analyses using Bateman's principles suggest that there is less direct benefit for females that acquire multiple mates than for males who bias paternity within litters. In light of past morphological and behavioural studies, these data suggest that patterns of polyandry in elasmobranchs may be determined by coercive mating, and that breeding behaviour has likely evolved in the context of sexual conflict.     

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.