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Simple Summary: In 2011, the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident resulted in the release of vast quantities of radioactive substances. In this study, we performed an ultrastructural analysis of the testis of the wild large Japanese field mouse (Apodemus speciosus), inhabiting the area near the ex-evacuation zones of the FDNPP and chronically exposed to various levels of low-dose-rate (LDR) radiation, to evaluate the effects of ionizing radiation on the male reproductive system and the possible consequences for future generations. Our results showed a preserved morphology of seminiferous tubules. However, ultrastructural changes, such as wide intercellular spaces, cytoplasmic vacuolization, vacuolated mitochondria, and increased lipid droplet clusters, were found, which could be linked to the mechanisms related to spermatogenesis. Long-term chronic LDR radiation exposure associated with the FDNPP accident had no pronounced adverse effect on spermatogenesis in A. speciosus, even if testis presented some ultrastructural changes. Our findings could be used in future studies to evaluate the effects of radiation on reproductive health in wild animals. Since the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, great attention has been paid to the impact of chronic low-dose-rate (LDR) radiation exposure on biological systems. The reproductive system is sensitive to radiation, with implications connected to infertility. We investigated the testis ultrastructure of the wild large Japanese field mouse (Apodemus speciosus) from three areas contaminated after the FDNPP accident, with different levels of LDR radiation (0.29 µSv/h, 5.11 µSv/h, and 11.80 µSv/h). Results showed good preservation of the seminiferous tubules, comparable to the unexposed animals (controls), except for some ultrastructural modifications. Increases in the numerical density of lipid droplet clusters in spermatogenic cells were found at high levels of LDR radiation, indicating an antioxidant activity rising due to radiation recovery. In all groups, wide intercellular spaces were found between spermatogenic cells, and cytoplasmic vacuolization increased at intermediate and high levels and vacuolated mitochondria at the high-level. However, these findings were also related to the physiological dynamics of spermatogenesis. In conclusion, the testes of A. speciosus exposed to LDR radiation associated with the FDNPP accident showed a normal spermatogenesis, with some ultrastructural changes. These outcomes may add information on the reproductive potential of mammals chronically exposed to LDR radiation. [ABSTRACT FROM AUTHOR]

Simple Summary: Other than DNA damage, it has become clear that ionizing radiation can also induce epigenetic effects. Thus, intergenerational effects from chronic low-dose exposures are possible concerns. It is important to elucidate the effects of radiation on germ cell maturation, fertilization and embryonic development. We analyzed global DNA methylation patterns in the testes and expression of Izumo1 in the testes and Izumo1r (Izumo1 receptor) in the ovaries of mice after neonatal acute high-dose radiation (HDR) and chronic low-dose radiation (LDR). The results of global DNA methylation patterns in the testis showed that the LDR group maintained its DNA methylation level, while the HDR group showed decreased DNA methylation levels with age. The expression level of Izumo1 in the testis recovered after the end of irradiation in the LDR group, while it remained low regardless of age in the HDR group. Although the expression of Izumo1r in the ovary decreased with age in the LDR group, it remained low in the HDR group. Our results indicate that LDR can induce different DNA methylation patterns, and both low- and high-dose radiation before sexual maturity might affect gametogenesis and fertility. The intergenerational effects from chronic low-dose exposure are matters of concern. It is thus important to elucidate the radiation-induced effects of germ cell maturation, fertilization and embryonic development. It is well known that DNA methylation levels in CpG sites in gametes are reprogrammed in stages during their maturity. Furthermore, the binding of Izumo on the surface of sperm and Juno on the surface of oocytes is essential for fertilization. Thus, there is a possibility that these genes are useful indicators to evaluate fertility in mice after irradiation exposure. Therefore, in this study, we analyzed global DNA methylation patterns in the testes and gene expression of Izumo1 and Izumo1r (Juno) in the gonads of mice after neonatal acute high-dose ionizing radiation (HDR) and chronic low-dose ionizing radiation (LDR). One-week-old male and female mice were irradiated with a total dose of 4 Gy, with acute HDR at 7 days at a dose rate of 30 Gy/h and LDR continuously at a dose rate of 6 mGy/h from 7 to 35 days. Their gonads were subsequently analyzed. The results of global DNA methylation patterns in the testes showed that methylation level increased with age in the control group, the LDR group maintained its DNA methylation level, and the HDR group showed decreased DNA methylation levels with age. In the control group, the gene expression level of Izumo1 in the testis did not show age-related changes, although there was high expression at 100 days of age. However, in the LDR group, the expression level recovered after the end of irradiation, while it remained low regardless of age in the HDR group. Conversely, gene expression of Izumo1r (Izumo1 receptor) in the ovary decreased with age in the control group. Although the gene expression of Izumo1r decreased with age in the LDR group, it remained low in the HDR group. Our results indicate that LDR can induce different DNA methylation patterns, and both high- and low-dose radiation before sexual maturity might affect gametogenesis and fertility. [ABSTRACT FROM AUTHOR]