Effects of Ionizing Radiation on a Slime Mould with Synchronous Mitosis

Abstract
In the slime mould Physarum polycephalum the nuclei of a single plasmodium exhibit synchrony of division. The synthesis of DNA occurs very shortly after nuclear division, resulting in an extremely short G1- and a relatively long G2 -period of interphase. When a single plasmodium was exposed to 9000 r of x-radiation during G2, a delay of division was independent of the time within the G2-period at which the irradiation occurred. From the effects observed with asynchronous shake-cultures of the mould it appears that irradiation during G2 causes a cessation of the onset of DNA-synthesis and a subsequent resumption of the synthesis, which seems related in time to the delay of mitosis. Irradiation during prophase caused a greatly increased delay in nuclear division, and irradiation during the reconstruction period after division delayed the subsequent motisis much more than did exposure during G2. Exposure of the mould to 23 000 rep of 60co gamma-radiation during mitosis and the subsequent period of DNA-synthesis prevented further divisions of the nuclei. In this latter case the replication of DNA was apparently not hindered by irradiation, suggesting that the process of DNA-synthesis per se is not sensitive to radiation and that mitosis and DNA-synthesis can be dissociated by irradiation.