Lethal Dose Of X-rays Research Articles

Life-Prolonging Effects of Adipose Tissue-Derived Stem Cell Transplantation into Mice Exposed to a Lethal Dose of X-Rays.

In the event of a high-dose radiation exposure accident, adipose-derived stem cell (ADSC) transplantation might be used as an emergency medical treatment to compensate for bone marrow failure. To investigate the possible course of that treatment, we examined whether transplantation of ADSCs into whole-body X-ray irradiated mice would provide resistance to radiation damage. ADSCs were obtained from a primary culture of adipocytes from adipose tissue of syngeneic mice. The ADSCs were transplanted via an intravenous (i.v.) route after whole-body irradiation (6 Gy, X-rays) of the ICR mice. Fifty days after transplantation, the survival rate of the transplanted group was 40% higher than the control group, and the difference in survival rates was maintained in the following 200 days. After 400 days, however, the difference in survival rates became smaller and disappeared after 650 days. The results indicate that ADSC transplantation may reduce lethality from acute radiation bone marrow injury for several hundred days.

Combined Effects of Radiofrequency Electromagnetic Fields and X-Ray in Renal Tissue and Function.

PurposeBiochemical and histopathological properties of renal tissues were reported to be affected by both radiofrequency electromagnetic fields (RF-EMF) and ionizing radiation. The radiation-induced changes in the kidney, including the serum levels of blood urea nitrogen (BUN) and creatinine (Cr), could lead to adverse health outcomes such as chronic kidney disease. These complications signify the importance of the research in this field. Thus, in this study, the effects of ionizing and non-ionizing radiations, as well as their combination, were assessed by evaluating the alteration in BUN, Cr, and histopathological changes in kidney tissue.Materials and MethodsNinety-six male Wistar rats were randomly divided into six groups and were exposed to either 900/1800MHz (mobile phone) or 2.4 GHz RF-EMF (Wi-Fi) radiation for 14 days, 8Gy x-ray, or their combination. Sera were collected from 2 mL of rat blood, then BUN and Cr levels were determined. Also, renal samples were stained with hematoxylin and eosin and evaluated histopathologically.ResultsBoth BUN and Cr levels raised non-significantly after exposure to 8 Gy x-rays. Moreover, all measurements in the samples of x-ray groups were in borderline or higher than normal values. The BUN levels of control, Wi-Fi, x-ray, and Wi-Fi+x-ray groups were not significantly different. However, Cr levels in the Wi-Fi group were significantly higher than those of the controls, and BUN to Cr ratio levels were significantly lower than those of the controls. Also, tubular atrophy and vessel wall thickening were associated with these exposures.ConclusionExposure to 900/1800MHz, 2400 MHz EMF can alter the kidney function. However, pre-exposure to 900/1800MHz EMF could modulate the acute adverse effects of lethal x-ray dose, which addresses the adaptive response in the kidney.

Boosting the Radiosensitizing and Photothermal Performance of Cu2- xSe Nanocrystals for Synergetic Radiophotothermal Therapy of Orthotopic Breast Cancer.

The small difference between tumor and normal tissues in their responses to ionizing radiation has been a significant issue for radiotherapy of tumors. Herein, we report that dumbbell-shaped heterogeneous copper selenide-gold nanocrystals can serve as an efficient radiosensitizer for enhanced radiotherapy. The mean lethal dose of X-rays to 4T1 tumor cells can be drastically decreased about 40%, that is, decreasing from 1.81 to 1.10 Gy after culture with heterostructures. Due to the synergetic effect of heterostructures, the dose of X-rays is also much lower than those obtained from mixture of Cu2- xSe + Au nanoparticles (1.78 Gy), Cu2- xSe nanoparticles (1.72 Gy) and Au nanoparticles (1.50 Gy), respectively. We demonstrate that the sensitivity enhancement ratio of Cu2- xSe nanoparticles was significantly improved 45% ( i. e., from 1.1 to 1.6) after the formation of heterostructures with gold. We also show that the heteronanocrystals exhibit an enhanced photothermal conversion efficiency, due to the synergetic interactions of localized surface plasmon resonance. These properties highly feature them as a multimodal imaging contrast agent (particularly for photoacoustic imaging, computed tomography imaging, and single photon emission computed tomography after labeled with radioisotopes) and as a radiosensitizer for imaging guided synergetic radiophotothermal treatment of cancer. The research provides insights for engineering low- Z nanomaterials with high- Z elements to form heteronanostructures with enhanced synergetic performance for tumor theranostics.

Evaluation of the radioprotective properties of recombinant flagellin when used alone or in combination with interleukin-1 beta

Research objective. To evaluate the radioprotective effectiveness of recombinant flagellin when used alone or in combination with interleukin-1 beta for prophylactic or therapeutic effect on animals. Materials and methods. The effect of hybrid flagellin FliC Salmonella typhimurium and human interleukin-1β (Institute of Highly Pure Biopreparations, Saint Petersburg, Russia) on the 30-day survival of male mice exposed to lethal doses of X-ray radiation was studied. Survival of irradiated animals was analyzed by Kaplan-Meier method. Results. The preventive use of flagellin had a protective effect on the 30-day survival of mice irradiated with lethal doses of X-rays. Compared with the irradiated control, the administration of flagellin (1 mg/kg or 2 mg/kg) prior to X-ray exposure (7.5 Gy, 8.0 Gy or 8.5 Gy) increased the animal survival rate to 67-87%. Complex preventive administration of flagellin (1 mg/kg) and interleukin-1 beta (50 μg/kg) provided a 100% survival rate of the irradiated mice. Separate use of drugs was also effective; 92.8% of mice survived when flagellin was administered prior to irradiation and interleukin after. Conclusion. Recombinant flagellin is a promising candidate product for designing new generation of domestic radiation countermeasures, including combinations with interleukin-1 beta.

Effects of fullerenol nanoparticles and amifostine on radiation-induced tissue damages: Histopathological analysis

Fullerenol C60(OH)24 nanoparticles (FNP) are promising radioprotectors in prevention of early and late ionizing radiation injury. The aim of this study was to compare the efficacy of FNP and amifostine (AMI) in protection of rats exposed to whole-body X-ray irradiation (7 or 8Gy). Both compounds (FNP, 100mg/kg ip; AMI, 300mg/kg ip) were given 30min before irradiation throughout the study. The general radioprotective efficacy of FNP and AMI were evaluated in rats irradiated with an absolutely lethal dose of X-rays (8Gy) and their survival were monitored during the period of 30days after irradiation. Both compounds were of comparable efficacy. Tissue-protective effects of tested compounds were assessed in rats irradiated with an sublethal dose of X-rays (7Gy). For this purpose, the animals were sacrificed on the 7th and 28th day after irradiation. Their lung, heart, liver, kidney, small intestine and spleen were taken for histopathological and semiquantitative analysis. Careful examination of established tissue and vascular alteration revealed better radioprotective effects of FNP compared to those of AMI on the small intestine, lung and spleen, while AMI had better radioprotective effects than FNP in protection of the heart, liver and kidney. Results of this study confirmed high radioprotective efficacy of FNP in irradiated rats that was comparable to that of AMI, a well-known radioprotector.

Long-term hematopoietic reconstitution by transplantation of kidney hematopoietic stem cells in lethally irradiated clonal ginbuna crucian carp ( Carassius auratus langsdorfii)

The study of hematopoietic stem cells (HSCs) has been facilitated by the transplantation of bone marrow cell populations into lethally irradiated mice. It is widely known that HSCs have the capacity for long-term and multilineage hematopoietic reconstitution in lethally irradiated hosts. Here, we developed a transplantation model system using clonal ginbuna crucian carp (Carassius auratus langsdorfii) that were exposed to a lethal dose of X-rays. The minimum lethal dose (MLD) of ginbuna was approximately 25Gy, which is lethal due to hematopoietic failure. The transplantation of kidney hematopoietic cells into lethally irradiated ginbuna resulted in the rescue of recipient fish for more than 180 days. We examined the reconstitution activity of head kidney (HK), body kidney (BK), and spleen cells. Transplantation experiments showed that only HK and BK cells had the long-term and multilineage reconstitution activity. These results indicate that teleost HSCs have the ability to fully reconstitute the hematopoietic system in lethally irradiated hosts, and that they are present in HK and BK, but not in spleen. This transplantation model system using clonal ginbuna is useful for studies of in vivo kinetics and functions of HSCs in teleosts.

Tissue-protective effects of fullerenol C 60(OH) 24 and amifostine in irradiated rats

Polyhydroxylated fullerenes, named fullerenols (C 60(OH) n ; n = 12–26) are excellent antioxidants. Harmful effects of ionizing radiation on living organism are mainly mediated by free radical species and fullerenols attract an attention as a potential radioprotectors. Our preliminary investigations on mice and rats subjected to radiation injury show that fullerenol C 60(OH) 24 provides high survival rate of irradiated small rodents. Radioprotective effect was comparable to that of the standard radioprotector amifostine. The aim of this study was to compare the efficacy of fullerenol C 60(OH) 24 (10 and 100 mg/kg i.p.) and amifostine (300 mg/kg i.p.) in protection of rats against harmful effects of ionizing radiation. The animals were whole-body irradiated by X-rays (8 MV). Both compounds were given 30 min before irradiation. In order to evaluate the general radioprotective efficacy of fullerenol and amifostine rats were irradiated with an absolutely lethal dose of X-rays (8 Gy) and their survival and body mass gain were monitored during the period of 30 days after irradiation. The aim of the second part of the study is to investigate the tissue-protective effects of tested compounds (100 mg/kg i.p. of fullerenol and 300 mg/kg i.p. of amifostine, 30 min before irradiation). It was carried out on rats irradiated with a sublethal dose of X-rays (7 Gy). Influence of ionizing radiation on hematopoesis as well as the radioprotective efficiency of the compounds given were evaluated by determining blood cell count during 28 days after irradiation. For this purpose the blood was taken from tail vein before irradiation and on the 3rd, 7th, 14th, 21st and 28th day after irradiation. In order to estimate the radioprotective effects of fullerenol and amifostine on other rat tissue, the animals were sacrificed on the 7th and 28th day after irradiation and their main organs (lung, heart, liver, kidney, small intestine and spleen) were taken for histopathological analysis. In the experiment in which the general radioprotective efficacy of fullerenol and amifostine was examined, fullerenol given in a dose of 100 mg/kg produced better protection than given in a dose of 10 mg/kg. This effect was comparable to that of amifostine. The results of hematological investigations showed that fullerenol better than amifostine prevented radiation-induced reduction in the white cell count (granulocytes and lymphocytes), particularly in the first 7 days after irradiation. Pathohistology examinations revealed better radioprotective effects of fullerenol compared to those of amifostine on the spleen, small intestine and lung, while amifostine had better radioprotective effects than fullerenol in protection of the heart, liver and kidney. These results confirm satisfactory radioprotective efficacy of fullerenol and encourage further investigations as a potential radioprotector.

Photodynamic therapy-generated vaccine for cancer therapy.

A target tumor-derived whole cancer cell therapeutic vaccine was developed based on an in vitro pre-treatment by photodynamic therapy (PDT) and was investigated using a poorly immunogenic tumor model. The vaccine was produced by incubating in vitro expanded mouse squamous cell carcinoma SCCVII cells for 1 h with photosensitizer benzoporphyrin derivative (BPD), then exposing to light (690 nm, 1 J/cm2) and finally to a lethal X-ray dose. Treatment of established subcutaneous SCCVII tumors growing in syngeneic C3H/HeN mice with 2 x 10(7) PDT-vaccine cells per mouse by a peritumoral injection produced a significant therapeutic effect, including growth retardation, regression and cures. Tumor specificity of this PDT-generated vaccine was demonstrated by its ineffectiveness when prepared from a mismatched tumor cell line. Vaccine cells retrieved from the treatment site at 1 h postinjection were intermixed with dendritic cells (DC), exhibited heat shock protein 70 on their surface, and were opsonized by complement C3. Tumor-draining lymph nodes treated by the PDT-vaccine contained dramatically increased numbers of DC as well as B and T lymphocytes (with enlarged memory phenotype fraction in the latter), while high levels of surface-bound C3 were detectable on DC and to a lesser extent on B cells. The PDT-vaccine produced no therapeutic benefit against tumors growing in C3-deficient hosts. It is suggested that surface expression of heat shock proteins and complement opsonization are the two unique features of PDT-treated cells securing avid immune recognition of vaccinated tumor and the development of a strong and effective antitumor adaptive immune response.

Radioprotective Efficiency of Fullerenol in Irradiated Mice

In vitro studies have demonstrated that fullerenol, a polyhydroxylated derivative of fullerene (C60(OH)n n = 12-26), has a high antioxidative potential. Since any radiation injury is mainly a consequence of the action of free radical species, the aim of this study was to examine radioprotective efficiency of fullerenol in whole-body irradiated mice. The experiment was performed on male, adult, white mice, whole-body irradiated with Xrays doses of 6 to 8 Gy (X-ray energy of 8 MV). Fullerenol C60(OH)24 was given in doses of 10 and 100 mg/kg i.p. 30 minutes before irradiation. The experimental groups consisted of 25-30 animals each. The survival rate and body mass gain of irradiated animals were monitored for 30 days after irradiation. The mean lethal times (LT50) of irradiated mice and mean lethal dose of X-rays were calculated and compared. The results showed that fullerenol C60(OH)24, in a dose of 100 mg/kg i.p., prolonged LT50 of irradiated mice. This effect was especially pronounced in mice irradiated with 7 and 8 Gy of X-rays. It seems that radioprotective efficiency of fullerenol C60(OH)24 is more marked in mice irradiated by higher doses of X-rays.

Effects of low-level 50 Hz magnetic fields on the level of host defense and on spleen colony formation.

The results of 3 sets of experiments on the effects of 22 microT sinusoidal 50 Hz magnetic fields (MF), applied for 1 h on 5 successive days (1 h/5 days), on the level of host defense and on spleen colony formation are reported. The first set of experiments shows the effects on the number of colony-forming units (CFUs) on the spleen and on the cellularity of the thymus in mice. The MF exposures resulted in an increase in CFUs which was statistically significant with respect to the controls, but not with respect to the shams. Statistically significant changes in the thymic weight and thymic index with respect to both the controls and the shams were measured 1 h after the last MF exposure. In the second set of experiments, the mice were given a sublethal dose of X-rays (6 Gy), which was followed by exposure 2 h later to the MF. The MF exposure was repeated at the same time of day for 5 days. The number of colonies per spleen showed a consistent, statistically significant increase with MF exposure and the number of CFUs per femur was decreased. In the third set of experiments, bone marrow was taken from mice which had been exposed to 22 microT fields and injected into mice which had been exposed to a lethal dose of X-rays (9 Gy). The number of CFUs per femur in the recipient mice was shown to be reduced by a statistically significant amount at 1 and 4 days after injection.

Characterization of chromosome instability in interspecific somatic hybrids obtained by X-ray fusion between potato (Solanum tuberosum L.) and S. brevidens Phil

Asymmetric somatic hybrids between Solanum tuberosum L. and S. brevidens Phil. have been obtained via the fusion of protoplasts from potato leaves and from cell suspension culture of S. brevidens. The wild Solanum species served as donor after irradiation of its protoplasts with a lethal X-ray dose (200 Gy). Selection of the putative hybrids was based on the kanamycin-resistance marker gene previously introduced into the genome of Solanum brevidens by Agrobacterium-mediated gene transfer. Thirteen out of the 45 selected clones exhibited reduced morphogenic potential. The morphological abnormalities of the regenerated plantlets were gradually eliminated during the extended in vitro culture period. Cytological investigations revealed that the number of chromosomes in the cultured S. brevidens cells used as protoplast source ranged between 28-40 instead of the basic 2n=24 value. There was a high degree of aneuploidy in all of the investigated hybrid clones, and at least 12 extra chromosomes were observed in addition to the potato chromosomes (2n=48). Interand intraclonal variation and segregation during vegetative propagation indicated the genetic instability of the hybrids, which can be ascribed to the pre-existing and X-ray irradiation-induced chromosomal abnormalities in the donor S. brevidens cells. The detection of centromeric chromosome fragments and long, poly-constrictional chromosomes in cytological preparations as well as non-parental bands in Southern hybridizations with restriction fragment length polymorphism (RFLP) markers revealed extensive chromosome rearrangements in most of the regenerated clones. On the basis of the limited number of RFLP probes used, preferential loss of S. brevidens specific markers with a non-random elimination pattern could be detected in hybrid regenerants.

An in vitro 125IUdR-release assay for measuring the kinetics of cell death.

A radionuclide release assay for measuring the in vitro kinetics of cell death has been developed. CHO cells were labelled for 24 h with 3.0 hBq/ml of [125I] iododeoxyuridine (125IUdR) and the fate of the labelled cells and their progeny was monitored at daily intervals by measuring the rate of 125I release. Prelabelling with 125IUdR did not alter the plating efficiency, the doubling time or the selection of mitotic cells. The rate of 125I release from labelled (but otherwise untreated) CHO cells was approximately equal to 4% day. Treatment with a lethal dose of X-rays (30 Gy), heat (46 degrees C, 1 h), cold (-90 degrees C, 1 h) or the antibiotic Geneticin (300 micrograms/ml, continuously) resulted in the release of greater than 99% the 125I activity associated with the cells. Cell death was rapid after heating or freezing, and delayed after treatment with X-rays or Geneticin. The results illustrate the efficacy of the 125I release assay for measuring the kinetics of cell death in mammalian tissue culture cells.

Changes in the relative proportion of transformation-sensitive polypeptides in giant HeLa cells produced by irradiation with lethal doses of x-rays.

Irradiation of HeLa cells with 1,100 rads (1 rad = 0.01 J/kg = 0.01 Gy) of x-rays yielded a pure population of giant cells 5-7 days after irradiation. These cells do not divide but go through an intermittent DNA synthetic phase. The population of giant cells in S phase (8%) is considerably lower than that of control asynchronous HeLa cells (30%), but 80% of the giant cells go through S phase as determined by 48-hr labeling with [3H]thymidine. Previous studies with high-resolution two-dimensional gel electrophoresis identified 58 [35S]methionine-labeled polypeptides common to human epithelial amnion cells and lung fibroblasts, whose rate of synthesis is sensitive to neoplastic transformation [Bravo, R. & Celis, J. (1982) Clin. Chem. (Winston-Salem, NC) 28, 949-955]. These polypeptides also have been identified in HeLa cells and other transformed human cells such as Detroit 98, Chang liver, Fl-amnion, and WISH-amnion [Bravo, R. & Celis, J. (1982) Clin. Chem. (Winston-Salem, NC) 28, 949-955]. After irradiation of HeLa cells and giant cell formation, the relative proportions of most of the transformation-sensitive polypeptides (43 of 47) reverted to levels similar to those observed in non-tumorigenic cells. This suggests that their relative proportions are dependent on the growth properties of the cells. In particular, the relative proportions of three polypeptides (designated 12g and 60d1 in isoelectric focusing and 27b in nonequilibrium pH gradient electrophoresis) were not affected, indicating that their reduced amounts in transformed cells could reflect a fundamental change that develops during transformation.

Reaginic antibody formation in the mouse: XI. Participation of long-lived antibody-forming cells in persistent antibody formation

A persistent IgE anti-hapten antibody response was established by immunization of B 6D 2F1 mice with a minute dose of aluminum hydroxide-gel-absorbed dinitrophenyl-keyhole limpet hemocyanin (DNP-KLH) or DNP-ovalbumin (OA). Irradiation of the immunized mice with a lethal dose of X-rays suppressed secondary anti-hapten antibody responses to DNP-homologous carrier conjugate, and depleted hapten-primed B memory cells in their spleen. However, persistent antibody formation of both IgE and IgG isotypes in the immunized mice was not affected by the lethal dose of X-rays. The results indicated that recruitment of B memory cells to antibody-forming cells is not responsible for the maintenance of antibody levels. The hapten-primed B cell population was expanded by adoptive transfer of DNP-KLH-primed spleen cells into irradiated mice followed by challenge with homologous antigen. Culture of T-cell-depleted spleen cells of the recipients, in the absence of exogenous antigen, resulted in the production of both IgE and IgG anti-DNP antibodies. The antibody titer in the culture supernatants continued to increase for 14 days, indicating that long-lasting antibody-forming cells are present in the spleen. The antibody-forming cells are radioresistant. The in vitro production of IgE anti-DNP antibodies by the spleen cells was not affected by the presence of IgG anti-DNP antibody, but was inhibited by exposure of the cells to multivalent DNP-heterologous carrier conjugates, such as DNP-bovine IgG (BGG) or DNP-mouse IgG (MGG). The persistent anti-hapten antibody formation of DNP-OA-primed and irradiated mice was depressed by an injection of DNP-MGG, which induced secondary anti-hapten antibody responses in the antigen-primed and nonirradiated mice. The results support the concept that radioresistant long-lived antibody-forming cells are involved in persistent antibody formation in the irradiated mice.

The effect of X-rays on properties of superoxide dismutase

Cuprozinc superoxide dismutase was isolated from liver of rats irradiated with lethal doses of X-rays. In contrast to the enzyme isolated from normal animals, superoxide dismutase from irradiated rats was found to be in reduced inactive state. Under aerobic conditions the reduced enzyme is oxidized spontaneously during a week resulting in preparations with properties of the normal enzyme. The irradiation by X-rays of superoxide dismutase isolated from normal animals also led to reduction of the enzyme and decrease in its activity. The enzyme reduced by irradiation is again oxidized during storage under aerobic conditions.

Factors Controlling Stem Cell Recirculation. III. Effect of the Thymus on the Migration and Differentiation of Hemopoietic Stem Cells

Experiments were carried out to investigate the effect of thymectomy on the migration and differentiation of hemopoietic stem cells released from shielded parts of bone marrow after irradiation of mice with lethal doses of x-rays. In 2-3 wk after thymectomy, the rate of migration declined and the differentiation of stem cells into granulocytic colonies was inhibited. Transplantation of syngeneic thymus or lymph node cells into thymectomized mice enhanced the migration of stem cells from the bone marrow and restored the usual pathways of their differentiation.

Factors controlling stem cell recirculation. III. Effect of the thymus on the migration and differentiation of hemopoietic stem cells

Experiments were carried out to investigate the effect of thymectomy on the migration and differentiation of hemopoietic stem cells released from shielded parts of bone marrow after irradiation of mice with lethal doses of x-rays. In 2–3 wk after thymectomy, the rate of migration declined and the differentiation of stem cells into granulocytic colonies was inhibited. Transplantation of syngeneic thymus or lymph node cells into thymectomized mice enhanced the migration of stem cells from the bone marrow and restored the usual pathways of their differentiation.

Residual Radiation Injury: Hematopoietic and Gastrointestinal Involvement in Relation to Age

Residual radiation injury (the depression in resistance to 140 R/day, 5 days a week) owed to 500 R given 30 days earlier has been measured in C57BL/6J female mice between the ages of 3 and 23 months. Control mice (previously unirradiated) demonstrate a parabolic relationship between age and resistance to this exposure regimen, whereas preirradiated (500 R) mice demonstrate resistance levels which are inversely related to age. The state of the bone marrow, in terms of endogenous colony-forming unit patterns, in previously unirradiated aging mice correlates with their resistance to fractionated exposures, but it does not predict the inverse age-resistance relationship in aging mice given 500 R 30 days earlier. Bone marrow transplantation immediately after a lethal dose of x-rays reduces the residual injury detectable in young adult mice which survive for 30 days but does not remove the age-dependence for residual injury induction. Shielding of the trunk during exposure allows the residual injury measured 30...

The mortality and body weight response of male Wistar rats in the lethal x-ray dose range.

Experiments were conducted to determine the LD50/30 of Wistar strain male rats, aged 72 to 112 days, bred in National Institute of Radiological Sciences. The animals were irradiated at eight exposure levels between 600 and 90O R of X-rays at rate of 23.5 R/min. Probit analysis of mortality data yielded an LD50/30 of 761.6±31.8 R. The body weight of the animals was decreased gradually after irradiation, reaching a minimum value at 4th day after the exposure. The body weight of 30-day survivors shows the second minimum value at 18-21 st day after irradiation.

Enhancement of the Viability of Irradiated Bacteria by Chromosome Transfer

The effect of x-irradiating recipient cells of Escherichia coli K-12 before mating on the formation of recombinants and on the distribution of parental genetic material among recombinants was investigated in both the wild-type (Rec+) and a recombination-deficient (Rec-) mutant. In crosses involving Rec- recipients, recombinants selected for a late donor marker were formed in almost normal numbers. Rec- cells exposed to otherwise lethal doses of x-rays were still able to form viable recombinants for a distal male marker. These recombinants had inherited almost all the transferred donor chromosome, as evidenced by the preponderance of male markers in the recombinants. In contrast, the recombinant-forming ability was about as x-ray-sensitive as the colony-forming ability in Rec+ cells. No preference for donor chromosomal material was observed in recombinants from crosses involving x-irradiated Rec+ cells.