Gamma ray-induced mutations in Drosophila melanogaster oocytes: the phenomenon of dose rate.

RIOR to 1958, it had been generally accepted that radiation-induced mutation IP frequencies are independent of dose rate. This was based mainly on findings for irradiated spermatozoa in Drosophila (PATTERSON 1931 ; TIMOFEEFF-RESSOVSKY and ZIMMER 1935 j RAY-CHAUDHURI 1939,1944). In 1955, HERSKOWITZ and ABRAHAMSON reported that the frequencies of sex-linked recessive lethals induced in oocytes of Drosophila females might be intensity-dependent, since a higher frequency of lethals was obtained after an acute dose of X rays than after a fractionated treatment. However, subsequent experiments on Drosophila females with somewhat different doses and dose rates failed to show this difference in frequency of sex-linked lethals ( HERSKOWITZ 1960). In 1958, RUSSELL, et al. observed, for irradiated mouse spermatogonial cells, that the frequency, per roentgen, of induced specific-locus mutations was about three times higher after an acute (80 to 9Or per minute) X irradiation than a chronic (80 to 90r per week) cobalt-60 gamma-irradiation (RUSSELL and KELLY 1958; RUSSELL, RUSSELL and KELLY 1958). They suggested that although the mutation process in spermatozoa is apparently independent of dose rate, this might not be the case in metabolically active cells such as spermatogonia. No doserate effect was observed in irradiated mouse spermatozoa, paralleling the results for Drosophila spermatozoa. PHILLIPS (1 961 ) independently confirmed the doserate effect for spermatogonia in male mice. Experiments performed with female mice again demonstrated an effect of radiation intensity, and the effect was greater on oocytes than on spermatogonia (RUSSELL, RUSSELL, GOWER and MADDUX 1958; RUSSELL, RUSSELL and CUPP 1959). Following this, other organisms were examined to see if corroborative evidence for a dose-rate effect might be found. TAZIMA, KONDO and SADO (1961) measured the frequencies of specific-locus mutations induced by gamma rays in spermatogonia and oogonia of the silkworm, Bombyx mori, and found two kinds of doserate effects, depending on the age of the treated larvae. BALDWIN (1962, 1965) measured visible mutation frequencies recovered from treated oogonial cells of the wasp, Dahlbominus, followed by acute X irradiation and chronic gamma irradiation. The mutation frequency obtained from the acute dose was about 1.5 times greater than that from the chronic dose.