Variations in radiosensitivity during development of the frog Limnodynastes tasmaniensis

Twenty‐two stages of the life cycle of the laboratory‐reared frog Limnodynastes tasmaniensis , from the fertilised egg to the adult frog, have been subjected to acute irradiation from cobalt‐60 ( 60 Co) gamma rays to determine the lethal dose (LD 50 ) values. The period of observation was 40–160 days, depending on the stage irradiated. The most sensitive stage was the fertilised egg, with an LD 50/40 of 0.6 Gy. Thereafter, radioresistance increased in an uneven pattern, with periods of relatively rapid increase alternating with periods of little change. The most resistant stages of the life cycle were medium to large tadpoles at limb bud and toe development stages (LD 50/60 ∼ 25 Gy). At metamorphosis, resistance decreased significantly to an LD 50/160 of 18.3 Gy, with a very similar result, 18.7 Gy, for young adult frogs. The latter figure, when compared with LD 50 values for other frogs, is found to be in good agreement with other results obtained using gamma rays. Preliminary experiment on chronic caesium‐137 ( 137 Cs) gamma irradiation are described.