Recovery of proliferative capability in gamma irradiated Colpoda cucullus (ciliated protist) resting cysts and its radiation hormesis

The resting cyst is one of the quietest life stages (i.e. cryptobiosis) with hardly measurable or even ceased metabolic activity. However, are the resting cysts just resting? Here, to our knowledge, we report for the first time radiation hormesis in resting cysts. In the present study, we focused on the proliferative capability of excysted cells from 500–4000 Gy-gamma-irradiated Colpoda cucullus resting cysts by a bioassay method. Generally, excysted cells from irradiated cysts lose their proliferative capability. However, we observed an increased proliferative capability promoted by low doses of irradiation. Cells, which had excysted from 500-Gy-irradiated cysts, restored their proliferation capability with a recovery period of 12 h and 24 h before excysting. Additionally, cells, which had excysted from 500-4000-Gy irradiated cysts, recovered proliferative capability by re-forming resting cysts. Here, the proliferative capability of excysted cells from 500-Gy-irradiated cysts was even higher than that of non-irradiated.