After the accident at the Fukushima Daiichi Nuclear Power Plant (F1NPP) in March 2011, much attention has been paid to the biological consequences of the released radionuclides into the surrounding area. We investigated the morphological changes in Japanese fir, a Japanese endemic native conifer, at locations near the F1NPP. Japanese fir populations near the F1NPP showed a significantly increased number of morphological defects, involving deletions of leader shoots of the main axis, compared to a control population far from the F1NPP. The frequency of the defects corresponded to the radioactive contamination levels of the observation sites. A significant increase in deletions of the leader shoots became apparent in those that elongated after the spring of 2012, a year after the accident. These results suggest possibility that the contamination by radionuclides contributed to the morphological defects in Japanese fir trees in the area near the F1NPP.

Following the Fukushima Dai-ichi Nuclear Power Plant accident, radiation effects on nonhuman biota in the contaminated areas have been a great concern. The induction of chromosomal aberrations in splenic lymphocytes of small Japanese field mice (Apodemus argenteus) and house mice (Mus musculus) inhabiting Fukushima Prefecture was investigated. In mice inhabiting the slightly contaminated area, the average frequency of dicentric chromosomes was similar to that seen in mice inhabiting a noncontaminated control area. In contrast, mice inhabiting the moderately and heavily contaminated areas showed a significant increase in the average frequencies of dicentric chromosomes. Total absorbed dose rate was estimated to be approximately 1 mGy d(-1) and 3 mGy d(-1) in the moderately and heavily contaminated areas, respectively. Chromosomal aberrations tended to roughly increase with dose rate. Although theoretically, the frequency of chromosomal aberrations was considered proportional to the absorbed dose, chromosomal aberrations in old mice (estimated median age 300 days) did not increase with radiation dose at the same rate as that observed in young mice (estimated median age 105 days).

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Since the Fukushima Dai-ichi Nuclear Power Plant accident, radiation effects on nonhuman biota in the contaminated areas have been a major concern. Here, we analyzed the frequencies of chromosomal aberrations (translocations and dicentrics) in the splenic lymphocytes of large Japanese field mice (Apodemus speciosus) inhabiting Fukushima Prefecture. A. speciosus chromosomes 1, 2, and 5 were flow-sorted in order to develop A. speciosus chromosome-specific painting probes, and FISH (fluorescence in situ hybridization) was performed using these painting probes to detect the translocations and dicentrics. The average frequency of the translocations and dicentrics per cell in the heavily contaminated area was significantly higher than the frequencies in the case of the noncontaminated control area and the slightly and moderately contaminated areas, and this aberration frequency in individual mice tended to roughly increase with the estimated dose rates and accumulated doses. In all four sampling areas, the proportion of aberrations occurring in chromosome 2 was approximately >3 times higher than that in chromosomes 1 and 5, which suggests that A. speciosus chromosome 2 harbors a fragile site that is highly sensitive to chromosome breaks induced by cellular stress such as DNA replication. The elevated frequency of chromosomal aberrations in A. speciosus potentially resulting from the presence of a fragile site in chromosome 2 might make it challenging to observe the mild effect of chronic low-dose-rate irradiation on the induction of chromosomal aberrations in A. speciosus inhabiting the contaminated areas of Fukushima.

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Since the Fukushima Dai-ichi Nuclear Power Plant (F1-NPP) accident in 2011, radiation effects on wildlife in the contaminated areas have been a major concern. The outskirts of the F1-NPP are mainly rural areas, where many rice fields, streams and reservoirs are located. We searched for wild medaka (small aquarium fish) around the F1-NPP and found two wild medaka habitats (S1 and S2). S1 is a stream located 4 km from the F1-NPP, where the ambient dose equivalent rate was 0.4–0.9 μSv/h (2013–14), and S2 is a reservoir located 7.5 km from the F1-NPP, where the ambient dose equivalent rate was 9.8–22 μSv/h (2013–14 and 2017–18). Dosimeters were placed for one day at the locations where the medaka were captured, and the absorbed dose rates were estimated. Radiation effects on wild medaka were examined using micronucleus assay between 2013 and 2018. No significant difference in frequency of micronucleated gill cells was observed among the wild medaka from S1, S2 and our cultivated medaka that were used as a control.

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After the Fukushima Daiichi Nuclear Power Plant accident, we studied the chromosomal aberrations (dicentrics and translocations) in the splenic lymphocytes of wild mice inhabiting Fukushima prefecture. Here, we report the frequencies of chromosomal aberrations in large Japanese field mice (Apodemus speciosus) captured from 2012 to 2016 in a heavily contaminated area. The chromosomal aberrations were detected using newly developed 4-color FISH (fluorescence in situ hybridization) with A. speciosus chromosome 1-, 3-, 4- and 5-specific painting probes. The frequencies of chromosomal aberrations in mice captured in July 2012 and October 2014 were significantly higher than that in the mice inhabiting the non-contaminated control area; however, the frequency of chromosomal aberrations in mice captured in January 2016 was not. The frequency of chromosomal aberrations in individual mice tended to increase with certain dose rates and accumulated doses. Regression tree analyses suggested increasing chromosomal aberration rate in mice exposed to chronic radiation at dose rates of more than 1.1 mGy day–1 and at accumulated doses of more than 200 mGy. It is concluded that ambient dose rates in the most severely contaminated area of Fukushima prefecture and radiation doses to wild mice inhabiting this area decrease with time; consequently, chromosomal aberrations induced by radiation have not been detected 5 years after the accident.