Japanese "nameko" mushrooms (Pholiota microspora) produced via sawdust-based cultivation exhibit severe genetic bottleneck associated with a single founder.

Pholiota microspora ("nameko" in Japanese) is one of the most common edible mushrooms, especially in Japan, where sawdust-based cultivation is the most dominant method accounting for 99% of the production. The current strains for sawdust cultivation in Japan are considered to have been derived from a single wild strain collected from Fukushima, Japan, implying that commercial nameko mushrooms are derived from a severe genetic bottleneck. We tested this single founder hypothesis by developing 14 microsatellite markers for P. microspora to evaluate the genetic diversity of 50 cultivars and 73 wild strains isolated from across Japan. Microsatellite analysis demonstrated that sawdust-cultivated strains from Japan were significantly less genetically diverse than the wild strains, and the former displayed a significant bottleneck signature. Analyzing the genetic relationships among all genotypes also revealed that the sawdust-cultivated samples clustered into one monophyletic subgroup. Moreover, the sawdust-cultivated samples in Japan were more closely related than full-sibs. These results were consistent with the single founder hypothesis that suggests that all commercial nameko mushrooms produced in Japan are descendants of a single ancestor. Therefore, we conclude that cultivated P. microspora originated from a single domestication event that substantially reduced the diversity of commercial nameko mushrooms in Japan.

Changes in the distribution of radiocesium in the wood of Japanese cedar trees from 2011 to 2013.

The changes in the distribution of (137)Cs in the wood of Japanese cedar (Cryptomeria japonica) trunks within three years after the Fukushima Dai-ichi Nuclear Power Plant (FDNP) accident in 2011 were investigated. Thirteen trees were felled to collect samples at 6 forests in 2 regions of the Fukushima prefecture. The radial distribution of (137)Cs in the wood was measured at different heights. Profiles of (137)Cs distribution in the wood changed considerably from 2011 to 2013, and the process of (137)Cs distribution change in the wood was clarified. From 2011 to 2012, the active transportation from sapwood to heartwood and the radial diffusion in heartwood proceeded quickly, and the radial (137)Cs distribution differed according to the vertical positon of trees. From 2012 to 2013, the vertical diffusion of (137)Cs from the treetop to the ground, probably caused by the gradient of (137)Cs concentration in the trunk, was observed. Eventually, the radial (137)Cs distributions were nearly identical at any vertical positions in 2013. Our results suggested that the active transportation from sapwood to heartwood and the vertical and radial diffusion in heartwood proceeded according to the vertical position of the tree and (137)Cs distribution in the wood approached the equilibrium state within three years after the accident.

Atmospheric direct uptake and long-term fate of radiocesium in trees after the Fukushima nuclear accident.

Large areas of forests were radioactively contaminated by the Fukushima nuclear accident of 2011, and forest decontamination is now an important problem in Japan. However, whether trees absorb radioactive fallout from soil via the roots or directly from the atmosphere through the bark and leaves is unclear. We measured the uptake of radiocesium by trees in forests heavily contaminated by the Fukushima nuclear accident. The radiocesium concentrations in sapwood of two tree species, the deciduous broadleaved konara (Quercus serrata) and the evergreen coniferous sugi (Cryptomeria japonica), were higher than that in heartwood. The concentration profiles showed anomalous directionality in konara and non-directionality in sugi, indicating that most radiocesium in the tree rings was directly absorbed from the atmosphere via bark and leaves rather than via roots. Numerical modelling shows that the maximum (137)Cs concentration in the xylem of konara will be achieved 28 years after the accident. Conversely, the values for sugi will monotonously decrease because of the small transfer factor in this species. Overall, xylem (137)Cs concentrations will not be affected by root uptake if active root systems occur 10 cm below the soil.