Two types of dose-rate dependence of radiation-induced mutation rates in spermatogonia and oogonia of the silkworm.

ACCORDING to the classical theory of radiation genetics, it seemed to be well established that induced mutation rates do not depend upon radiation intensity. However, in 1958 RUSSELL, RUSSELL and KELLY published their data on mouse spermatogonia showing that chronic y-irradiation is mutagenically less effective than acute X-ray irradiation. Soon a€ter this a similar type of dose-rate dependence of radiation-induced mutation rate was confirmed also for the oocytes of young adult mice (RUSSELL, RUSSELL and CUPP 1959). Since then it has been of great concern whether the same principle holds true for organisms other than the mouse. Evidence consistent with the results obtained on mice has been found for Drosophila oogonia by OSTER, ZIMMERING and MULLER (1959) , while the reaction of the spermatogonia of the same insect was less clear. We have been carrying out, since 1959, a similar study with the silkworm, whose germ cells develop approximately synchronously. The use of this material made it possible to test definite stages of germ cell development for dose-rate dependence of mutation rate. Unexpectedly to us, the results of the first experiment did not agree with the above findings of RUSSELL et al.; i.e., mutagenic effectiveness was lower at acute irradiation than at chronic irradiation not only in spermatogonia but also in oogonia. After additional experiments, we found that the mutagenic effectiveness of acute irradiation varies with the stage of the irradiated germ cells, and that in the silkworm a phenomenon in agreement with RUSSELL’S finding on the lower effectiveness of chronic irradiation relative to acute did occur at the stage of primordial germ cells. Furthermore, the findings of our first experiment turned out not to be contradictory to those of RUSSELL, for, although we did not know this at the time, RUSSELL, RUSSELL and KELLY (1959) had also reported a second, inverse, type of dose-rate effect in experiments in which a single high acute dose of radiation gave a lower mutation rate than when the same dose was fractionated.