Genetic injury in hybrid male mice exposed to low doses of 60Co gamma-rays or fission neutrons. II. Dominant lethal mutation response to long-term weekly exposures.

Male B6CF1 mice were exposed to once-weekly doses of either fission neutrons or 60Co gamma-rays for periods up to one year and mated periodically to screen for the induction of dominant lethal mutations. Two independent experiments were performed, each involving a control and three dose levels of both neutrons and gamma-rays. Neutron doses were between 0.125 and 2.67 rad/week and gamma-ray levels were between 5 and 32 rad/week. Data on both pre- and post-implantation fetal deaths were obtained. Analyses that were intended to identify the potential contribution from age- or time-dependent factors, which could include changes in radiosensitivity and in spontaneous rates plus any cumulative damage to the stem cell population did not reveal a consistent significant contribution to the mutation rate/rad/week. Direct comparisons of these data with data from males exposed to single doses confirm that weekly neutron irradiation is significantly more effective than single doses for the induction of postimplant fetal losses, whereas single doses of gamma-rays are more effective than the same dose divided into weekly fractions. Neutron-induced augmentation appears limited in these data to lethal mutations induced in meiotic and postmeiotic cell stages. The relative biological effectiveness (RBE) of neutrons rises from 5 +/- 1 to 12 +/- 1 for single vs. weekly doses. Rates of preimplant loss, although significant, are not a sensitive measure of genetic injury at the low doses used here. They are extremely sensitive to litter size and best estimated in litters of seven or more implants along with appropriate statistical control of concurrent variation in the number of corpora lutea.