Radiation-induced inhibition of eclosion in the tobacco hornworm, Manduca sexta

Control of eclosion in Manduca sexta (laboratory reared at 23-25 degrees C on 18L:6D) was investigated by utilizing the ED50 (X-ray exposure required to prevent eclosion in 50% of the irradiated animals) throughout pupal-adult development as a measure of radiation sensitivity. An initial period (day 0-6) of nearly constant radiosensitivity (ED50 range: 13.0-14.4 kr) was followed by a brief period of increased radiosensitivity between day 6.5-7.75 (ED50:8.23 kr). Thereafter, a pronounced decrease in radiosensitivity was noted through the day of eclosion (day 8: ED50 = 19 kr; day 22: ED50 = 75 kr). The association between hemolymph ecdysone levels and maximum radiosensitivity observed on day 7 was studied. Animals administered beta-ecdysone on days 1, 4, and 12, and irradiated at various times post-injection, exhibited significant increases in radiosensitivity exhibited by Manduca sexta on days 0-7 is in part dependent upon the titer of ecdysone in hemolymph. The role of the brain as a radiosensitive region was investigated in day 10 animals by selectively transplanting ED100-irradiated and nonirradiated brains into ED100-irradiated and nonirradiated animals. The presence of a radiosensitive component in addition to the brain is proposed since the radiation-induced inhibition of eclosion could not be completely explained in terms of brain damage alone. Selective shielding of day 10 animals X-irradiated at an ED100 level demonstrated the absence of radiosensitive regions in the abdomen and their presence in both the head and thorax.