STEM Today

M ARCH 2015, N O 3

STEM Today

B IOGRAPHY Rachel Carson was a world-renowned marine biologist, author and environmentalist who served as an aquatic biologist and editor-in-chief for the U.S. Fish and Wildlife Service. She has been credited with launching the contemporary environmental movement and awakening the concern of Americans for the environment.

Rachel Carson was born in a small rural Pennsylvania community near the Allegheny River, where she spent a great deal of time exploring the forests and streams around her 65-acre farm. As a young child, Carson’s consuming passions were the nature surrounding her hillside home and her writing. She was first "published" at the age of 10 in a children’s magazine dedicated to the work of young writers.

Other youngsters who first saw their words in print in St. Nicholas included William Faulkner and F. Scott Fitzgerald.

In 1925 Carson entered Pennsylvania College for Women as an English major determined to become a writer. Midway into her studies, however, she switched to biology. Her first experience with the ocean came during a summer fellowship at the U.S. Marine Laboratory in Woods Hole, Massachusetts. Upon graduation from Pennsylvania College, Carson was awarded a scholarship to complete her graduate work in biology at Johns Hopkins University in Baltimore, an enormous accomplishment for a woman in 1929.

Carson’s distinction in both writing and biology won her a part-time position with the U.S. Bureau of Fisheries in 1935 where she was asked to create a series of seven-minute radio programs on marine life called "Romance Under the Waters."

Meantime, she continued to submit writings on conservation and nature to newspapers and magazines, urging from the very beginning the need to regulate the "forces of destruction" and consider always the welfare of the "fish as well as that of the fisherman." Her articles were published regularly by the Baltimore Sun and other of its syndicated papers.

In 1936, Carson was appointed a junior aquatic biologist with the Bureau of Fisheries and became one of only two women then employed with the Bureau at a professional level.Carson’s first book, Under the Sea-Wind, published in 1941, highlighted her unique ability to present deeply intricate scientific material in clear poetic language that could captivate her readers and pique their interest in the natural world. In 1943, Carson was promoted to the position of aquatic biologist in the newly created U.S. Fish and Wildlife Service, where she authored many bulletins directed at the American public. One series, known as "Conservation in Action," was devoted to exploring wildlife and ecology on national wildlife refuges in laymen’s terms.During World War II, Carson participated in a program to investigate undersea sounds, life and terrain designed to assist the Navy in developing techniques and equipment for submarine detection.

Read More Here — Image Credit: National Digital Library of the United States Fish and Wildlife Service.

I N THIS EDITION

Acute - 4: What are the probabilities of hereditary, fertility, and sterility effects from space radiation?

Risks of hereditary, fertility, and sterility effects from space radiation exposure are unlikely at the radiation doses and dose-rates expected in the space environment.

Read More:NASA 1.Hereditary effects of radiation

2.High LET 56 Fe Ion Irradiation Induces Tissue-Specific Changes in DNA Methylation in the Mouse

3.Germline drift in chimeric male mice possessing an F2 component with a paternal F0 radiation history

4.Do paternal exposures to low dose ionizing radiation promote transgenerational epigenetic changes?

5.Genetic effects of HZE and cosmic radiation

Gap’s in NASA Human Reserach Roadmap

Hereditary effects of radiation Hereditary effects of radiation are those effects observed in offspring born after one or both parents have been exposed to radiation before the child was conceived. Hereditary effects are the result of a mutation and chromosome structural changes produced in the reproductive cells of an exposed individual.Radiation exposure of the parents can lead to lethality, malformations, and genetic diseases in future generations. Hereditary effects may appear in the exposed person’s direct offspring, or several generations later, depending on whether the altered genes are dominant or recessive. Hereditary effects must be compatible with germ cell survival and division for transmission to the next generation, unless they are induced in non-dividing forms such as spermatozoa. As these effects can be caused only when ionising radiation reaches the germ cells; penetrating X- or γ -rays can induce germline mutations from outside the body, but α or β particles of short track-length must be present in the gonads and very close to the nuclei of germ cells in order to have a hereditary effect.

The cells of special concern in heredity risk assessment for radiation are those most at risk of accumulating genetic damage, namely spermatogonial stem cells and resting oocytes[1].

The risk of hereditary effects caused by radiation is smaller (1.3%/Sv) than the risk of a lethal, radiation-induced cancer (10%/Sv in high, acute dose or 5%/Sv in low, chronic dose) (ICRP 1990).

High LET 56 Fe Ion Irradiation Induces Tissue-Specific Changes in DNA Methylation in the Mouse Epigenetic mechanisms heritably affect gene expression patterns over many cell divisions or even transgenerationally without changes to the DNA sequence. These epigenetic mechanisms include DNA methylation and histone modifications. DNA methylation can be altered by environmental exposures including ionizing radiation in a manner that can drive changes in cellular phenotype and have also been shown to play key roles in the initiation and progression of carcinogenesis.

The majority of human radiation exposures occur in a much lower dose range than the daily 2 Gy dose fraction typically used in the radiation oncology clinic . These more common, lower dose, exposures occur in the course of diagnostic medical procedures such as X-rays or computed tomography scans where a typical dose for a procedure might be in the mGy range, although for some diagnostic procedures partial body doses may be higher. Exposures can also occur in the context of nuclear power production, accidents, or in other occupational settings including astronauts who might receive a dose of 0.4-0.8 mGy/d on an average day.

To date, in vitro studies characterizing the effect of radiation exposure on DNA methylation have primarily evaluated the effect of low LET irradiations and few of them have used moderate or low dose exposures. Of those lower dose studies, global hypomethylation and hypermethylation have both been observed with either dose dependence or independence . Similarly, effects of in vivo low dose irradiation using mouse models have shown tissue, sex, or locus-specific hypomethylation or hypermethylation using doses of less than 100 cGy. Although these studies of the low dose effects of irradiation on DNA methylation use varied methodology to evaluate various different loci, they suggest that radiation exposure, even at very low doses can alter DNA methylation and that these epigenetic changes can alter phenotype.

Authors have demonstrated in their previous in vitro studies radiation quality-dependent changes in DNA methylation [2]. In this study, repeat element and global DNA methylation changes were observed in cultured normal human fibroblast and colon carcinoma cells at a delayed time following exposure to 10 or 100 cGy of low LET X-rays or protons, or high LET iron (56 Fe) ions.

56

Fe ions are one of the most prevalent and biologically effective ion species in space radiation to which astronauts are exposed during missions [3,4,5].

Using quantitative pyrosequencing assays, we have evaluated the methylation status of the DAPK1, EVL, 14.3.3, p16, MGMT, and IGFBP3 genes as well as the gene expression level for each locus. These genes were selected for their roles in such radiation responses or carcinogenic processes as apoptosis, metastasis, cell cycle regulation, and DNA repair (Table I).

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Gap’s in NASA Human Reserach Roadmap

Assay Controls and Tissue-Specific DNA Methylation Profiles Authors evaluated the liver and the lung of all 16 non-irradiated control mice using PCR and pyrosequencing assays.

These measurements demonstrated no consistent or significant differences in DNA methylation levels between the two tissues for any of the loci evaluated (data not shown).

Tissue-Specific Effects of 56 Fe Ion Irradiation on Specific Locus DNA Methylation To define the effect of in vivo radiation exposure on DNA methylation in the mouse, authors measured the levels of methylation for the six genes of interest-DAPK1, EVL, 14.3.3, p16, MGMT, and IGFBP3-in the liver of the irradiated mice as compared to the liver of the nonirradiated concurrent control mice.

No changes in liver DNA methylation were observed for any locus as a direct effect of irradiation at 1 day post irradiation for any dose, nor were any changes observed as a delayed effect at 7, 30, and 120 days post irradiation

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Gap’s in NASA Human Reserach Roadmap (data not shown).

Evaluation of the same genes in the lung of the same mice revealed a strikingly different story (Fig. 1). At 1 day post exposure, significant DNA hypermethylation was observed for all six loci following the 10 cGy radiation exposure (DAPK1, 26%; EVL, 34%; 14.3.3, 24%;p16, 32%; MGMT, 37%; IGFBP3, 40%).

Although hypermethylation was also observed for the moderate 30cGy irradiation none of these increases were statistically significant. Counter to these observations, no significant change in DNA methylation relative to concurrent control was observed for any locus as a result of the highest, 100 cGy, exposure.

At 7 days post irradiation authors also observed changes for the lung, but surprisingly the consistent trend was for DNA hypomethylation relative to control. For DAPK1 and 14.3.3, again the most dramatic response was to 10 cGy (250% and 266%, respectively) and again no effect of the 100 cGy dose was observed; a mirror image of the trend observed at 1 day post irradiation. The other four genes each demonstrated hypomethylation levels that were similar among all doses of radiation. At 30 days post exposure DAPK1, EVL, 14.3.3, p16, and MGMT had returned to a methylation pattern similar to that observed on day 1 with little or no effect of the 100 cGy exposure and hypermethylation relative to control for the lower doses (33-57%).

IGFBP3 was not different from control. By 120 days post exposure, the magnitude of change in DNA methylation dropped dramatically. Nonetheless, subtle, consistent hypomethylation ranging between 27% and 222% of control was observed for the 10 cGy and 30 cGy exposures, reinforcing the impression of a cyclic or oscillatory low dose epigenetic response. Again the 100 cGy dose elicited little or no response. IGFBP3 was the exception to these 120 day observations again and not different from control.

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Gap’s in NASA Human Reserach Roadmap Tissue-Specific Effects of 56 Fe Ion Irradiation on Repeat Element DNA Methylation Authors measured the tissue-specific effect of in vivo radiation exposure on TF and A1 monomer methylation.As with the gene promoters, no changes in DNA methylation were observed for the TF or A1 monomer in the liver for any experimental group (data not shown). Similarly, evaluation of the lung demonstrated very low levels of hypomethylation at the earlier time points for both monomers and subtle hypermethylation for only the TF monomer at 30 days post irradiation (Fig. 2). These changes in methylation were significant in only three instances.

Gene Expression in the Lung After 56 Fe Ion Irradiation Authors also analyzed the concomitant RNA expression levels for the same mice to determine whether the observed changes in DNA methylation represented functional changes in expression that alter transcript abundance (Fig. 3).

At 1 day after irradiation DAPK1expression was significantly decreased relative to concurrent control for all three radiation doses (228%, 226%, and 216%; 10, 30, and 100 cGy, respectively) and at 7 days for the 10 and 30 cGy doses, as well (241%, 222%, 10, 30 cGy, respectively).

EVL expression levels were decreased at 1, 7, and 30 days post irradiation for all radiation doses, but those changes were only significant at 30 days post exposure (216%, 220%, 222%; 10, 30, and 100 cGy, respectively). Expression of 14.3.3 and p16 was not significantly affected at any time. For no dose, time point or gene do the changes in expression reach twofold magnitude, however, suggesting that the observed changes in mRNA levels may not be biologically relevant or representative of a radiation response. Decreases in expression of the TF monomer were also observed at 1, 30, and 120 days with no clear dose dependence of the measured changes.

In this study, authors evaluated six genes and two repeat elements whose expression is aberrantly regulated

by DNA methylation in the context of carcinogenesis. The goal was to determine whether low to moderate dose 56 Fe ion radiation exposure would alter DNA methylation and subsequent expression of these genes in a tissue-specific manner. The results of these studies demonstrate that changes in DNA methylation are tissue and locus specific as well as dose and time dependent.

Further, these changes were detected up to 120 days after exposure. However, these changes in DNA methylation did not correlate with significant changes in mRNA level at the same time points, nor were they observed for all loci studied.

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D ID YOU KNOW ?

A RE F LIGHT ATTENDANTS AT H IGHER R ISK FOR M ISCAR RIAGE ?

Investigators analyzed 840 pregnancies among 673 female flight attendants and examined company records of 2 million single flights flown by these women. From these data, researchers estimated a marker of circadian disruption working during normal sleeping hours and exposure to cosmic and solar particle event radiation for each flight, and assessed the physical demands of the job.

Using the airline records, researchers estimated each flight attendant’s chronic sleep disturbance during the first trimester of pregnancy. They added up the time the flight attendants were working in the air when they would normally be asleep at home. Results indicated that flight attendants who flew more than 15 hours during normal sleep hours in the first trimester were at increased risk for miscarriage.

To estimate the flight attendants’ exposure to background cosmic radiation, researchers used the CARI software program from the U.S. Federal Aviation Administration, which is designed to provide radiation dose estimates for past flights. Additionally, NIOSH researchers collaborated with National Aeronautics and Space Administration (NASA) to compare the flight attendants’ flights with NAIRAS data- a model that estimates radiation exposure to commercial airline crew from solar particle events.

Analysis of exposure to background cosmic and solar particle event radiation suggested that exposure to 0.1mGy or more may be associated with increased risk of miscarriage. Solar particle events were infrequent, but during one of the solar particle events studied, radiation dose reached 0.45 mGy on a single flight. These data suggest that if a pregnant flight attendant works on a flight that travels through a solar particle event, she could be exposed to more radiation than is recommended during pregnancy.

Reference:Centers for Disease Control and Prevention(CDC)

Gap’s in NASA Human Reserach Roadmap Germline drift in chimeric male mice possessing an F2 component with a paternal F0 radiation history Radiation can induce genomic instability through epigenetic mechanisms that could be passed from one generation to the next. Studies were conducted using a mouse model system where chimeric male mice were exposed to radiation and their radiation history was traced to the F(2) generation. Changes were shown in gene expression and enzyme activity in the many kinases in organs of offspring where the parents had been exposed to radiation. These changes are indicative of the induction of genomic instability in the offspring and suggest the potential for increased cancer risk in the radiated mice.

The materials and methods regarding the mice, irradiations and post-irradiation breedings to produce F1 males and their F2 embryos were as described in Baulch et al. (2001)[7] except that in this study the mated females from both 5 and 6 weeks after paternal F0 irradiation were allowed to deliver their F1 litters. Post-irradiation week 5 was considered the control mating for this experiment, since pre-implantation embryo chimera assays using 1.0 Gy of 137 Cs γ-radiation demonstrated that F1 and F2 paternal F0 post-irradiation week 5 embryos exhibit no competitive cell proliferation disadvantage. Additionally, comparison of two different spermatogenic stages from the same sires reduces extraneous biological variability and emphasizes the difference in effect on heritable competitive cell proliferation disadvantage of the two spermatogenic stages based upon the sensitivity of each spermatogenic stage to acute γ-irradiation.

Out of 30 chimeric offspring, a total of 10 XY ↔ XY chimeric males were obtained (Table I). One male from each experimental group showed no evidence of neo+ Sperm. One male from the group with a paternal F0 post-irradiation week 6 history showed no evidence of neo − sperm from a radiation history as demonstrated by peak elevation of neo + ratios at all four time points measured (0.44.0.64, Table II).

These three animals were excluded from the final statistical analyses. As a result, the data used to evaluate germline chimeric drift were obtained from three males derived from F2 embryos with three different F1 sires having a paternal F0 post-irradiation week 5 history and four males derived from F2 embryos with three different F1 sires having a paternal F0 post-irradiation week 6 history. These seven males that had a paternal F0 postirradiation week 5 or 6 history were mated six times over 13 weeks to obtain a minimum of three litters per chimeric sire (Table II).

Fewer neo − offspring were produced over time by the chimeric males with a germline component descended from offspring conceived at paternal F0 post-irradiation week 6, corresponding to irradiation of the sensitive paternal F0 type B spermatogonia. In contrast, there was little overall change in the number of neo − offspring produced by the males with a germline component descended from paternal F0 post-irradiation week 5, corresponding to irradiation of the relatively radiation-resistant early spermatocytes.

This observation is consistent with results of the pre-implantation embryo chimera assay, which also revealed little or no effect on the cell proliferation of F1 or F2 embryos produced by sperm at paternal F0 post-irradiation week 5. The decreasing relative numbers of neo − offspring that were observed in the litters from the week 6 chimeric males are unlikely to have resulted from prenatal death since there were no significant changes in litter size over the 13 weeks of the experiment (Table II).

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Gap’s in NASA Human Reserach Roadmap

Other studies with the same system were initiated that indicated radiation-induced cellular reprogramming resulting in changes in gene and protein expression. These changes were followed in the offspring through the F(3) generation (Vance et al. 2002[9]). These studies suggested that genomic instability had been induced in the offspring of the irradiated mice. The frequency of the changes in the offspring were high so that they could not be explained based on known genetic transmission and could only be explained by epigenetic mechanisms.

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D ID YOU KNOW ?

E FFECTS OF LONG -TERM EXPOSURE TO µG ON THE RE PRODUCTIVE CAPABILITY OF SCROTAL MAMMALS , INCLUD ING HUMANS

NASA has expressed the need to determine whether the ability of organisms to reproduce is impacted in the µG environment. With regard to the male, the capacity to reproduce is determined by : 1) The ability of the testis to produce, in sufficient numbers, spermatozoa with normal morphology and complement of haploid DNA. 2) Whether these sperm can mature, become motile, and ultimately bind to and fertilize the egg.

In ground-based studies, hindlimb suspension (HLS) has been widely used as a model for mimicking the physiological changes (loss of bone and muscle mass) that occur in µG and employed as a ground control for µG flight experiments.

With regard to an impact of µG on the testis and spermatogenesis, a rapid decline in circulating testosterone has been consistently observed in rats (in biosatellite experiments) and astronauts during Space Shuttle flights.

The decline in testosterone has been observed, even in the absence of increased cortisol, suggesting that a corticosteroidassociated stress response is unlikely to be responsible for the androgen decline and the associated bone loss in such animals.Some studies found increased cortisol in µG or during HLS, whereas other studies showed no significant change.

In animals exposed to µG for 14 days, circulating testosterone and seminiferous tubule diameter were significantly reduced and the number of germ cells per tubule cross section decreased compared with the HLS ground controls for COSMOS-2044.

After a 22-day exposure to µG on COSMOS-605, rats showed an increase in relative testicular weight, calculated as testicular weight ÷ body weight.

However,if the actual mean testicular weights are recalculated from their data, then a decrease in testicular weights relative to controls was observed in µG. Although the data for the HLS animals (simulated flight controls) for COSMOS-1887 cannot be used (no inguinal ligation was performed), there were significant reductions in circulating testosterone levels, testis weight, and numbers of spermatogonia in the flight animals vs. the free-roaming controls.

Similar results were obtained for male rats flown on the 7-day Space Shuttle Space Lab 3 (STS-51B) mission .

Reference:Long Term (6-Week) Hindlimb Suspension Inhibits Spermatogenesis in Adult Male Rats

Gap’s in NASA Human Reserach Roadmap Do paternal exposures to low dose ionizing radiation promote transgenerational epigenetic changes?

Ionizing radiation induces germ line genomic instability and may have adverse effects on the offspring [11]. Men who received radiation treatment for childhood cancers have an elevated sperm DNA fragmentation index, and are at increased risk of having fertility problems when compared to controls [13].

Transgenerational effects from paternal exposure to radiation through occupation, airport scans, medical treatment and diagnosis, and other man-made sources of radiation presently remain mostly unknown. An unsolved epidemiologic finding relates to the Sellafield case. Public concerns in the 1980s prompted the UK government to investigate the excess of malignant diseases in children living in the vicinity of the Sellafield nuclear plant. A population-based analysis confirmed the high incidence of leukemia and lymphoma in the young residents of Seascale, the village near the Sellafield plant, when compared to those in national registries and surrounding areas [14].

In a cohort study of children attending school at Seascale, an increased rate of leukemia and other cancers was observed among children born in Seascale, but not in children who moved to the village after birth [15]. A casecontrol study indicated that children of fathers working at the nuclear plant at the time of conception had a three times higher risk of developing leukemia or non-Hodgkin’s lymphoma before the age of 25. Interestingly, the same study suggested a preconceptional dose-response relationship [16].

An independent casecontrol analysis confirmed the association between excess risk of childhood leukemia and lymphoma in the area when paternal radiation exposure occurred at the time of conception, but not when radiation exposure occurred three to six months before conception [17].

These studies ultimately led to "Gardner’s Hypothesis", which proposes a causal relationship between paternal exposure to ionizing radiation and cancer risk in the offspring [16, 18]. Gardner’s hypothesis has been widely criticized, and was ultimately rejected [19-21]; in part, because after a comparison with other studies, such as those on the atomic bomb survivors in Japan, no evidence was found for increased cancer incidence in children from exposed fathers [22-24]. Some attributed the increased risk of childhood leukemia near the nuclear plant to population mixing and a yet unidentified infectious agent [17, 25, 26].

Furthermore, additional studies on populations near nuclear plants in other countries did not show significant effects on the young population living in the vicinity of nuclear plants, with some exceptions [27]. Other clusters of childhood cancers were reported near the Krummel nuclear power plant in Germany [28], the Dounreay nuclear reactor in Scotland [25], and the nuclear fuel reprocessing plant near La Hague, France [29].

Although the possibility that paternal exposure to ionizing radiation increases the susceptibility of the offspring to cancer remains controversial, it cannot be excluded that epigenetic effects may play a role in the unexplained excess of cancer incidence observed in children from fathers working in the nuclear industry.

Notably, studies in human and animal populations exposed to radiation from the Chernobyl nuclear power plant accident in 1986 showed DNA damage in sperm and an overall increase in generation of reactive oxygen metabolites [30, 31]. Further analyses in the offspring of fathers exposed to low doses of radiation during cleanup of the nuclear plant showed an elevated frequency of chromosome aberrations, which may lead to increased morbidity over their lifetimes [32]. Animal data provides evidence for transgenerational epigenetic changes from exposure to high and low dose ionizing radiation.

Animal models: Evidence for transgenerational epigenetic effects from paternal exposures to ionizing radiation High doses of ionizing radiation in mice, administrated before mating, cause an accumulation of DNA double strand breaks in somatic cells of the offspring, which is accompanied by global hypomethylation, changes in the levels of methyltransferases, and altered microRNA expression [10, 33].

An acute gamma-irradiation of male mice destabilizes the sperm genome and F1 brain genome, indicating a transgenerational instability triggered by a certain threshold dose of acute paternal irradiation [34].

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Gap’s in NASA Human Reserach Roadmap Koturbash et al. [33] speculate that sperm cells damaged by radiation may interfere with the epigenetic programming of the fertilized egg, causing genomic instability and potential carcinogenesis in the progeny of the exposed parent.

Additionally, an epigenetic bystander effect occurs in the cells of unirradiated organs of rats after cranial radiation with high doses. These changes included loss of global DNA methylation, altered levels of miRNAs, and downregulation of DNA methyltransferases and methyl CpG binding protein 2 (Mecp2) [35, 36].

Since these epigenetic effects occur in organs that neighbor the exposed tissues, it is possible that germ cells may also be affected through a bystander process, thereby causing heritable defects in the offspring. The same research group investigated the effects of chronic low dose radiation exposure on somatic cells in an in vivo murine model, and found that it induced epigenetic changes, such as genome-wide hypomethylation; while acute low dose administration showed no direct measurable effects [37].

Bernal et al. [38] recently showed, with the use of the agouti viable yellow (Avy ) mouse model, that maternal exposure to doses of X-rays used in diagnostic CT-scans (0.7-7.6 cGy) alters the epigenome in the offspring. The offspring were irradiated at implantation stage, while an effect of paternal irradiation on the epigenome of the offspring has not yet been determined. The results of this study demonstrate that low doses of X-rays induce doseand sex-dependent increases in DNA methylation at the Avy locus, causing a significant shift in the coat color distribution of the offspring from yellow to brown [39].

Dietary antioxidants taken during pregnancy negate the radiation-induced increase in DNA methylation, indicating that low doses of ionizing radiation increase DNA methylation at the Avy locus in part through the generation of ROS. Persistent induction of ROS as a response to radiation exposure has been suggested earlier [39], and free radical injury can profoundly alter DNA methylation levels [11, 40]. Thus, events such as exposure to X-rays during early pregnancy may alter the cellular redox state in pluripotent stem cells, determining the ultimate methylation status at the Avy locus at birth. Once the uteroplacental circulation is established, dietary antioxidants may reduce the abundance of highly reactive ROS, and reduce the epigenetic consequences. Although this is speculative, this intriguing possibility needs to be investigated.

Genetic effects of HZE and cosmic radiation For examining the genetic effects of High Energy Heavy ions (HZE) and cosmic radiation, adult males and larvae of fruit flies (Drosophila melanogaster) were loaded on the Space Shuttle, Endeavor (STS-47), and sex-linked recessive lethal mutation in the male germ cells (spermatozoa and spermatogonia that produce them) and chromosomerecombination-derived mutation on the larva wing basal somatic cells were investigated.

The strains used were a standard wild type strain (Canton-S) and a radiation sensitive strain (mei-41). Two-hundred adult males and about 6000 larvae of each strain were loaded and exposed to the cosmic environment. At the same time, almost the same number of adult flies and larvae were bred in the same environmental condition (temperature and humidity) as ground control groups. The flight period was about eight days. The returned male flies were mated with virgin female flies from a detection strain for investigating sex-linked recessive lethal mutation. X chromosomes with lethal genes were detected on the second generation. In the returned flies, the frequency of chromosomes with lethal genes was two times higher in the wild type strain and three times higher in the radiation sensitive strain than that of the ground control groups. Most of the returned larvae were in the pupal stage on the return day, and began to emerge on the next day[42].

The emerged adult flies were stored in 70 percent alcohol solutions, and their wing samples were prepared for investigating the wing mutation spots derived from chromosome mutation. Mutation frequency in the wild type strain was almost the same between the flight and ground groups. The frequency in the Muller-5 individuals separated from the radiation sensitive strain was one and half times higher in the flight group than in the ground control group.

However, the frequency in the radiation sensitive strain was significantly lower in the flight group than the ground group. The higher recessive lethal mutation frequency in the flight group suggests the synergistic effect of radiation and minute gravity on mutation induction in germ cells. However, this synergy effect was not observed on the chromosome mutation induction in somatic cells.

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Gap’s in NASA Human Reserach Roadmap

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