Increased Frequency of Copy Number Variations Revealed by Array Comparative Genomic Hybridization in the Offspring of Male Mice Exposed to Low Dose-Rate Ionizing Radiation.

There is very little information on the transgenerational or genetic effects of low dose-rate ionizing radiation. We report the detection of the transgenerational effects of chronic low dose-rate irradiation in mice, at the molecular level in the whole genome, using array comparative genomic hybridization technology. We observed that the number of the mice with de novo copy number variations (specifically, deletions) was significantly increased in the offspring of C57BL/6J male mice exposed to 20 mGy/day gamma-rays for 400 days (total dose: 8000 mGy), as compared to non-irradiated controls. We did not detect any difference in the size of the de novo deletions between the irradiated and the non-irradiated groups. An analysis of the life span of the offspring suggested a possibility that de novo copy-number variations may be associated with shorter life spans.

DISTRIBUTION OF RADIOCESIUM IN BLACK PINE TREE FORESTS IN ROKKASHO, AOMORI, JAPAN.

Since the Fukushima Dai-ichi Nuclear Power Plant (FDNNP) accident in 2011, 134Cs had been detected in atmospheric deposition samples collected in Rokkasho and Aomori in Japan, located ~400-km north of the FDNNP, during March 2011-May 2015. To study the distribution of radiocesium (134, 137Cs) in a Japanese black pine tree forest, we measured the activity concentration of 134, 137Cs in whole black pine trees, including roots collected during 2017-19 along with soil samples in 2015 and 2017-19. The results show that most of the deposited 134Cs was retained in the forest floor in 2015, with depth distribution different from that of the weapon testing fallout. Proportions of the weapon fallout 137Cs in trunks, branches, needles and roots were 44, 17, 13 and 26%, respectively, indicating that >50% of 137Cs in above-ground part existed in the trunks. The total inventories of 134Cs and 137Cs in the forest were estimated to be 8.9 and 2.5·103 Bq m-2, respectively, and ~4.8% of 137Cs inventory was originated from the accident. Inventory of 134Cs in the forest-including the black pine trees-was 1.6 times larger than cumulative deposition outside of the forest due to the canopy effect.

Frequencies of Chromosome Aberrations are Lower in Splenic Lymphocytes from Mice Continuously Exposed to Very Low-Dose-Rate Gamma Rays Compared with Non-Irradiated Control Mice.

Chromosome aberrations have been one of the most sensitive and reliable biomarkers of exposure to ionizing radiation. Using the multiplex fluorescence in situ hybridization (M-FISH) technique, we compared the changes, over time, in the frequencies of translocations and of dicentric chromosomes in the splenic lymphocytes from specific pathogen-free (SPF) C3H/HeN female mice continuously exposed to 0.05 mGy/day (18.25 mGy/year) gamma rays for 125 to 700 days (total accumulated doses: 6.25-35 mGy) with age-matched non-irradiated controls. Results show that the frequencies of translocations and of dicentric chromosomes increased significantly over time in both irradiated and non-irradiated control mice, and that the frequencies were significantly lower, not higher, in the irradiated mice, which differs from our previous reports of increased chromosome aberration frequencies at higher radiation dose rates of 1 mGy/day and 20 mGy/day. These results will be useful when considering the radiation risk at very low-dose rates comparable to regulatory dose limits.

Frequencies of Chromosome Aberrations are Lower in Splenic Lymphocytes from Mice Continuously Exposed to Very Low-Dose-Rate Gamma Rays Compared with Non-Irradiated Control Mice.

Chromosome aberrations have been one of the most sensitive and reliable biomarkers of exposure to ionizing radiation. Using the multiplex fluorescence in situ hybridization (M-FISH) technique, we compared the changes, over time, in the frequencies of translocations and of dicentric chromosomes in the splenic lymphocytes from specific pathogen-free (SPF) C3H/HeN female mice continuously exposed to 0.05 mGy/day (18.25 mGy/year) gamma rays for 125 to 700 days (total accumulated doses: 6.25-35 mGy) compare with age-matched non-irradiated controls. Results show that the frequencies of translocations and of dicentric chromosomes increased significantly over time in both irradiated and non-irradiated control mice, and that the frequencies were significantly lower, not higher, in the irradiated mice, which differs from our previous reports of increased chromosome aberration frequencies at higher radiation dose rates of 1 mGy/day and 20 mGy/day. These results will be useful when considering the radiation risk at very low-dose rates comparable to regulatory dose limits.

Effects of Continuous In Utero Low- and Medium-Dose-Rate Gamma-Ray Exposure on Fetal Germ Cells.

The effects of radiation exposure on germ cells and the gonads have been well studied at acute high-dose exposures, but the effects of chronic low-dose-rate (LDR) irradiation, particularly relevant for radiation protection, on germ cells and the gonads are largely unknown. Our previous study revealed that chronic exposure of mice to medium-dose-rate (MDR, 200 or 400 mGy/day) gamma-rays in utero for the entire gestation period (18 days) induced only a mild degree of general growth retardation, but with very drastic effects on the gonads and germ cells. In the current study, we further investigated the histomorphological changes in the gonads and the number of germ cells from gestation day (GD) 18 fetuses irradiated with MDR throughout the entire gestation period. The germ cells in the testes and ovaries of the MDR-irradiated fetuses were almost obliterated. Gestation day 18 fetuses exposed to LDR (20 mGy/day) radiation for the entire gestation period showed decreases in the number of the germ cells, which were not statistically significant or only marginally significant at most. Further investigations on the effects of LDR irradiation in utero using more sensitive methods are necessary.

Web-building spider Nephila clavata (Nephilidae: Arachnida) can represent 137Cs contamination of arthropod communities and bioavailable 137Cs in forest soils at Fukushima, Japan.

Large areas of Fukushima's forests were contaminated with radiocesium (137Cs) after the Fukushima Dai-ichi Nuclear Power Plant accident. Most of the contaminated forests have not been decontaminated, and bioavailable 137Cs is likely to circulate within the forest environment's food web. Nephila clavata (Nephilidae: Arachnida) is a top predator in the forest arthropod community, and this web-building spider potentially consumes many arthropod species presented in the grazing and detrital food chains. We tested whether 137Cs in the spider could serve as a proxy for 137Cs contamination of these arthropod communities. We also examined whether N. clavata could serve as a proxy for soil bioavailable 137Cs. Nephila clavata was similarly or more contaminated with 137Cs compared with lower-trophic-level arthropods such as herbivores and other predators at the same trophic level. Thus, the 137Cs activity of N. clavata could represent the extent to which the arthropod community was contaminated with 137Cs. Data from nine 137Cs-contaminated sites in Fukushima showed a significant positive correlation between soil bioavailable 137Cs and N. clavata's 137Cs activity05 but the coefficient of determination was only moderate (R2 = 0.43), suggesting that N. clavata is only a weak proxy of soil bioavailable 137Cs. Our results also showed that the bioavailable fraction of 137Cs in Fukushima was strongly correlated with the total inventory and that the K and Na contents of the soil determined the soil-to-spider transfer factor for 137Cs and the 137Cs activity in N. clavata, respectively. These results improve our understanding of 137Cs transfer from the soil to arthropod species.

Life-Shortening Effect of Chronic Low-Dose-Rate Irradiation in Calorie-Restricted Mice.

Calorie restriction is known to influence several physiological processes and to alleviate the late effects of radiation exposure such as neoplasm induction and life shortening. However, earlier related studies were limited to acute radiation exposure. Therefore, in this study we examined the influence of chronic low-dose-rate irradiation on lifespan. Young male B6C3F1/Jcl mice were divided randomly into two groups, which were fed either a low-calorie (65 kcal/ week) or high-calorie (95 kcal/week) diet. The latter is comparable to ad libitum feeding. The animals in the irradiated group were continuously exposed to gamma rays for 400 days at 20 mGy/day, resulting in a total dose of 8 Gy. Exposure and calorie restriction were initiated at 8 weeks of age and the diets were maintained for life. The life-shortening effects from chronic whole-body irradiation were compared between the groups. Body weights were reduced in calorie-restricted mice irrespective of radiation treatment. Radiation induced a shortened median lifespan in both groups, but to a greater extent in the calorie-restricted mice. These results suggest that calorie restriction may sensitize mice to chronic low-dose-rate radiation exposure to produce a life-shortening effect rather than alleviating the effects of radiation.

Effects of local-scale decontamination in a secondary forest contaminated after the Fukushima nuclear power plant accident.

We investigated whether local-scale decontamination (removal of the litter layer, superficial soil layer, and understory) in a secondary forest contaminated by the Fukushima nuclear power plant accident reduced 137Cs contamination of the soil and litter. We also measured 137Cs concentrations in plants and in the web-building spider Nephila clavata (Nephilidae: Arachnida), as an indicator species, to examine 137Cs contamination in arthropods. One month after decontamination, the total 137Cs contamination (soil + litter) was reduced by 20% (100 kBq·m-2) relative to that in an adjacent untreated (i.e., contaminated) area, which was however not statistically significant. Four months after decontamination, 137Cs in the decontaminated area had increased to a level similar to those in the untreated area, and the air radiation dose in the decontaminated area was about 2.1 µSv·h-1, significantly higher than that in the untreated area (1.9 µSv·h-1). This may have been attributed to a torrential rain event. Although no statistically significant reduction was observed, most spiders had a lower 137Cs contamination than that before the decontamination. This implied that the decontamination may have reduced 137Cs transfer from soil via litter to N. clavata through the detrital food chains, but may not have reduced the amount of 137Cs transfer through grazing food chains because the concentration of 137Cs in living tree leaves was not reduced by the decontamination. In autumn, about 2 kBq·m-2 of 137Cs was supplied from foliage to the ground by litterfall. The results suggested that removal of the litter and superficial soil layers in a contaminated forest may be ineffective. The present study suggests that the local-scale decontamination in a secondary forest had no effect on the reduction of 137Cs contamination in the treated area.

Relationship between radiocesium contamination and the contents of various elements in the web spider Nephila clavata (Nephilidae: Arachnida).

The accident at the Fukushima Dai-ichi nuclear power plant seriously contaminated a large area in northeast Japan with a large amount of radioactive material. Consequently, various organisms, including arthropods, in the ecosystem have been contaminated with radiocesium ((137)Cs) through the food chain. We previously showed that the web spider Nephila clavata was contaminated with (137)Cs and that the level of contamination, which varied among spider individuals, was independent of the amount of prey consumed. The present study aimed to clarify the mechanisms that could determine the level of (137)Cs contamination in N. clavata. We first demonstrated the patterns of contents of over 30 elements in N. clavata that were collected at two forest sites (PS and ES) in Fukushima and then focused on the relationships between the contents of the alkali metals Li, Na, K, and Rb and the accumulation of (137)Cs in the spiders; Cs is an alkali metal and is expected to act similarly to Li, Na, K, and Rb. We also focused on the content of the non-alkali element, Cu, which is an essential element for oxygen transport in spiders. We found that Na content correlated positively with (137)Cs accumulation at both sites, which suggested that (137)Cs accumulation in N. clavata was related with the dynamics of Na. The K-, Rb-, and Cu-(137)Cs relationships were site specific; the relationships were significant at site PS, but not significant at site ES. Factors causing the site specific relationships and the probable pathway for (137)Cs transfer from soil to plants and then to higher trophic levels are discussed in terms of the transfer processes of the alkali metals.

Radiocesium contamination of the web spider Nephila clavata (Nephilidae: Arachnida) 1.5 years after the Fukushima Dai-ichi Nuclear Power Plant accident.

We measured the concentrations of radiocesium ((134)Cs and (137)Cs) in a large web spider, Nephila clavata L. Koch (Nephilidae: Arachnida), collected at three sites at different distances from the Fukushima Dai-ichi Nuclear Power Plant about 1.5 y after the accident in March 2011. The radiocesium concentrations in spiders were highest in a streamside secondary forest 33 km northwest of the power plant: mean ± a standard deviation of 2.401 ± 1.197 Bq g(-1) dry for (134)Cs and 3.955 ± 1.756 Bq g(-1) dry for (137)Cs. In a hillside secondary forest 37 km northwest of the power plant, the mean concentrations of (134)Cs and (137)Cs were 0.825 ± 0.247 Bq g(-1) dry and 1.470 ± 0.454 Bq g(-1) dry, respectively. In a pine forest 62 km west of the power plant, very low radiocesium concentrations were detected, but in only a few individuals. The concentrations of (134)Cs and (137)Cs in spiders collected at each site tended to be correlated with the air radiation dose rate at each site. Since spiders are key components of food webs in forests, the high concentrations in this species at contaminated sites suggested that the radiocesium from the accident has transferred through food chains and reached to higher trophic level of the food chains.

Complex feeding tracks of the sessile herbivorous insect Ophiomyia maura as a function of the defense against insect parasitoids.

Because insect herbivores generally suffer from high mortality due to their natural enemies, reducing the risk of being located by natural enemies is of critical importance for them, forcing them to develop a variety of defensive measures. Larvae of leaf-mining insects lead a sedentary life inside a leaf and make conspicuous feeding tracks called mines, exposing themselves to the potential risk of parasitism. We investigated the defense strategy of the linear leafminer Ophiomyia maura Meigen (Diptera: Agromyzidae), by focusing on its mining patterns. We examined whether the leafminer could reduce the risk of being parasitized (1) by making cross structures in the inner area of a leaf to deter parasitoids from tracking the mines due to complex pathways, and (2) by mining along the edge of a leaf to hinder visually searching parasitoids from finding mined leaves due to effective background matching of the mined leaves among intact leaves. We quantified fractal dimension as mine complexity and area of mine in the inner area of the leaf as interior mine density for each sample mine, and analyzed whether these mine traits affected the susceptibility of O. maura to parasitism. Our results have shown that an increase in mine complexity with the development of occupying larvae decreases the probability of being parasitized, while interior mine density has no influence on parasitism. These results suggest that the larval development increases the host defense ability through increasing mine complexity. Thus the feeding pattern of these sessile insects has a defensive function by reducing the risk of parasitism.