Studies on Radioresistance II

The chromosome condition and radiosensitivity was investigated in a subline of the Yoshida sarcoma induced by pre-irradiation. The subline, characterized by increased radiosensitivity, was found to be of mixed diploidhypotetraploid population: it has a modal chromosome number of 37 in earlier generations and 38 in later generations in the diploid region, and of 70 to 73 in the tetraploid region. It was found that the polyploid cells showed a remarkable decrease in frequency during the serial transfers for 4 months without irradiation. There is a possibility that the rise of the subline was induced by frequently repeated heavy X-irradiations with an integral dose of 87, 480r during 94 generations.Relationship between the chromosome number and radiosensitivity of the tumor is discussed.Addendum: After this paper was completed and ready to send to press, Dr. T. H. Yosida reported in the monthly meeting of the Genetic Society at Sapporo the rise of a tetraploid subline of the Yoshida sarcoma and the gradual decrease of tetraploid cells during serial transfers.