Cytological evaluation of dose-rate effects of radiation on mutation frequency of silkworm gonia: II. Evidence of G 2 accumulation of primary spermatogonia after acute irradiation

Abstract

The effects of acute irradiation on the kinetic indices of primary spermatogonia of the silkworm have been studied in an attempt to gain cytological insight into the mechanisms of dose-fractionation effects on the induction of mutation. 1. ( 1) Kinetic studies, with the use of colchicine treatment and the [ 3H]thymidine-labeling technique, on primary spermatogonial populations in unirradiated larvae showed that primary spermatogonia of this insect had a mean generation time of 34 h, of which the relative time occupied by the various phases of cell cycle was roughly as follows: G 1, ∽ 40%; S, ∽ 40%; G 2, ∽ 18%; and M, ∽ 2%. 2. ( 2) Cytokinetic studies on the effect of acute irradiation (1000 R) on the population of primary spermatogonia showed that during the first 24 h after irradiation the relative frequency of cells in different phases of the cell cycle changed noticeably. Thus, at 24 h as many as 58% of the total population was shown to be in the G 2 population in contrast to 18% of the unirradiated primary spermatogonial population. Furthermore, only 3%, instead of 40%, in the control, was in the S phase at this time. 3. ( 3) It is tentatively concluded that enhanced mutation frequency in dose-fractionation experiments was primarily due to an increase in the relative frequency of cells in the G 2 population. However, the possibility remains that accumulation of cells in late G 1 might also be responsible for the enhancement of mutation frequency by dose fractionations.