An autoradiographic study of unscheduled DNA synthesis in the germ cells of male mice treated with X-rays and methyl methanesulfonate

Unscheduled DNA synthesis (UDS) in the germ cells of male mice after in vivo treatment with X-rays or methyl methanesulfonate (MMS) was assayed by use of a quantitative autoradiographic procedure. MMS induced UDS in meiotic through type III elongating spermatid stages, whereas X-rays induced UDS in meiotic through round spermatid stages. No UDS was detected in the most mature spermatid stages present in the testis with either MMS or X-rays. Taking into account differences in DNA content of the various germ-cell stages studied, we concluded that X-rays induced a maximum UDS response in spermatocytes at diakinesis--metaphase I. The level of UDS induced by MMS was about the same in all the stages capable of repair. Chromosome damage and UDS were measured simultaneously in the same spermatocytes at diakinesis 90 min after X-irradiation or MMS treatment. The level of UDS in most of the X-irradiated cells paralleled the extent of chromosome damage induced. A statistical analysis of these results revealed a positive correlation. As expected, MMS induced no chromosome aberrations above control levels. Therefore no correlation was determined between UDS and chromosome damage in this case. The distribution of UDS over the chromosomes treated at diakinesis with MMS or X-rays was studied. It was found that UDS occurred in clusters in the irradiated cells, whereas it was uniformly distributed in the MMS-treated cells.