[Cloning method forDrosophila cells in vitro. Sensitivity to X-rays of different cell lines]

1. The sensitivity to X-rays of 5 differentDrosophila cell lines has been studied. Statistical analysis of experimental data for doses between 10,000 and 16,000 roentgen showed that, for each cell line, the probability of survial of each cell is independent. According to the relationship observed between the dose and the survival frequency, the five lines studied can be classified into 3 groups. Moreover, the cell population of each line is heterogeneous with respect to radiosensitivity, some cells being more radioresistant than others. 2. UnirradiatedDrosophila cells seeded at a very low multiplicity are unable to multiply unless they are cultured among heavily irradiated cells. Each cell has an independant probability of initiating a clone. This probability,p, is different for each one of the 3 cell lines examined:p ₁=0.69,p ₃=0.35,p ₆=0.02. These differences are likely to have a genetic origin. 3. The ability of a single cell to initiate a clone, when cultured among cells whose multiplication has been stopped by X ray irradiation, provides a simple and efficient cloning method. The conditions in which a given cell line could be cloned were defined after determination of the dose-survival relationship and estimation ofp. The cloning procedure is described. The homogeneity of the karyotype and cell morphology can be used to monitor the homogeneity of the cell population.