Insect radiosensitivity: dose curves and dose-fractionation studies of dominant lethal mutations in the mature sperm of 4 insect species.

Males of 4 species of insects: Musca domestica L. (housefly) (Diptera), Oncopeltus fasciatus (Dallas) ( milkweed bug) (Hemiptera), Anagasta kuhniella ( Zeller ) ( mealmouth ) (Lepidoptera) and Heliothis virescens (Fab.) (tobacco budworm ) (Lepidoptera) were irradiated as adults. Dose--response curves for the induction of dominant lethal mutations in the mature sperm were constructed. The curves were analyzed mathematically and compared with theoretical computer simulated curves requiring 1, 2, 4, 8 and 16 'hits' for the induction of a dominant lethal mutation. The 4 species belonging to 3 different orders of insects showed a wide range in radiation sensitivity and vastly different dose--response curves. The house fly was the most sensitive and displayed a '1-hit' curve. The milkweed bug was intermediate in sensitivity and the curve exhibited '4-hit' kinetics. The mealmoth and tobacco budworm were most radioresistant and the response curves resembled 8-16--hit simulated curves. When the data were analyzed by several mathematical models we found that a logistic response curve gave reasonably good fit with vastly different parameters for the 4 species. Dose-fractionation experiments showed no reduction in the frequency of lethal mutations induced in any species when an acute dose was fractionated into 2 equal exposures separated by an 8-h period. Various reasons for large differences in radiosensitivity are discussed, e.g., (1) differences in repair mechanisms, (2) variation in oxygen concentration, (3) holokinetic versus monokinetic chromosomes and (4) intrinsic differences in mitotic cell cycles which affect the behavior of radiation-induced chromosome breaks.