Chemical induction of presumed dominant-lethal mutations in postcopulation germ cells and zygotes of mice. II. Sensitivity of different postcopulation-precleavage stages to three alkylating chemicals.

The relative sensitivities of various postcopulation-precleabage and pronuclear stages to dominant-lethal effects of isopropyl methanesulfonate (IMS), ethyl methanesulfonate (EMS), and triethylenemelamine (TEM) were investigated. The pattern of sensitivity differed with the chemical. IMS was most effective when pronuclear formation was already completed and the majority of the zygotes were presumably undergoing DNA synthesis. EMS, on the other hand, induced its most pronounced effects when eggs in the course of second meiotic division and zygotes in early pronuclear stages were treated. The greatest effect of TEM was observed when zygotes were treated at the early pronuclear stage. EMS and TEM, in contrast to IMS, are similar to radiations in that zygotes undergoing DNA synthesis are more resistant to them than are the early pronuclear stages. In the case of IMS, effects induced in the most sensitive postcopulation-precleavage stage were 6 to 9 times greater than in the most sensitive precopulatory dictyate oocytes or male germ cells. On the other hand, in the case of EMS and TEM, the most sensitive precopulatory male germ cells, but not the dictyate oocytes, were more sensitive than the most sensitive postcopulation stages.