Recovery capacity of highly regenerative organisms exposed to gamma radiation: comparison between the duckweed Lemna minor and planarian Schmidtea mediterranea

Abstract

Gamma radiation can induce DNA damage in all organisms. However, highly regenerative organisms such as the duckweed Lemna minor and the planarian Schmidtea mediterranea, have high developmental plasticity because of their meristem and stem cells respectively, enabling recovery. To determine the cellular mechanisms underlying recovery in these organisms exposed to genotoxic gamma radiation, DNA repair activity, and levels of mitosis and apoptosis were studied. Both organisms were exposed to gamma radiation for one week, followed by a seven-day recovery with sampling at day 0, 3 and 7. The applied dose rates were 68, 116 and 153 mGy/h for duckweed and 18, 29 and 83 mGy/h for planarians. First, the recovery capacity was determined by evaluating growth characteristics of duckweed and amputated planarians. Subsequently, DNA repair activity and apoptosis were analyzed at the transcriptional level via qPCR. The mitotic activity of the planarian stem cells was measured via anti-phospho-histone H3 immunostaining. In both organisms, recovery initiated earlier for lower exposure levels. DNA repair and apoptosis levels both increased before the recovery initiated in L. minor, while these analyses did not demonstrate a dose-dependent relation with the growth in S. mediterranea. However, before regeneration of the planarians was initiated, their mitotic activity increased. These results suggest that active DNA repair and apoptosis in L. minor and increased mitosis in S. mediterranea precede the onset of recovery.