On the role of the phases of the mitotic cycle for the manifestation of the combined effect from the action of gamma-rays and ethylmethansulphonate in Chlorella vulgaris

Synchronous cultures of Chlorella vulgaris were irradiated with gamma rays at dose of 6 and 15 kR (LD50, respectively LD90) and immediately after that were treated with ethylmethanesulphonate (EMS). The experiments were performed at different phases of the cell cycle. The mutagenic effect was assessed on the basis of 5 basic types of mutations on the 11th day size, morphological, pigmental, mosaic and sectorial types). The combined treatment of the two mutagenic factors show - as compared to the thoretically expected - a different effect in dependence with the cell cycle phase. This effect is lower at the 0 moment of the development of the cell culture, as well as at the 16th and the 18th hours after treatment with 15 kR + EMS and 6 kR + EMS respectively. This phenomenon is probably connected with the accumulation of small physiological injuries which intensify the lethal effect. The treatment of cells in the S-phase leads to an effect somewhat higher than the summary one, while a super-additive effect was established upon treating cells with 15 kR + EMS at the 4th hour. Consequently, for the manifestation of the effect of combined treatment of importance is not only the phase of the cell cycle, but the radiation dose as well. The combined treatment at higher radiation dose tends to narrow in all cases the mutation spectrum, as compared with the combination of 6 kR + EMS. The phase of the cell culture is of importance for the broading of the induced spectrum of mutations only at the combination with the lower radiation doses. The broadest spectrum of mutations is obtained at the treatment in G1. Determined differences have been established also in the relation between the basis types of mutations in dependence of the cell cycle, as well as the appearance of new types of mutations, which are not to be encountered upon independent treatment of the mutagenic factors. The results obtained show the importance of the metabolic activity during the treatment of mutagenic factors for their manifestation. (A.B.)