Chromosome damage and non-disjunction measured at the first cleavage division in normal and chromosomally mutant female mice irradiated at the diakinesis stage of female meiosis

We have irradiated primary murine oocytes at the diakinesis stage of the first meiotic division with 0.6 Gy X-rays. Fertilized oocytes were cultured overnight to arrest the first cleavage division and display pronuclear chromosomes. All preparations were preferentially stained for centric constitutive heterochromatin and analyzed for structural and numerical radiation effects. Females of 3 different karyotypes were irradiated (all on a Swiss random-bred genetic background): +/+ (221 female pronuclei analyzed), Rb(11.13)4Bnr T(1;13)70H/Rb(1.13)4Bnr T(1;13)70H with 11.13(1) and 1(13) large and small marker bivalents (RbT/RbT, 242 zygotes analyzed) and the same karyotype but with a 1(13)H;1(13) Wa heteromorphic bivalent (RbT/RbT*, 126 zygotes analyzed). Hyperploid chromosome counts were encountered with frequencies of 11.8% (+/+), 11.9% (RbT/RbT) and 16.6% (RbT/RbT*). In this order of karyotypes, the frequencies of dicentrics per zygote were 0.07, 0.16 and 0.11 and the frequencies of fragments 0.13, 0.18 and 0.31. In about half of the supernumerary chromosome spreads, a dicentric chromosome was included. The long marker bivalent 11.13(1) had a non-disjunction frequency of 2.5 times its control value, partially because it was involved in dicentric formation as well. For the RbT/RbT karyotype, the spontaneous maternal non-disjunction level was 5.4%. For the RbT/RbT* karyotype, it can be assumed to be the same or slightly higher because of the 1(13)H;1(13) Wa heteromorphic bivalent. This increased intrinsic sensitivity for non-disjunction was not expressed as an increased sensitivity for aneuploidy after irradiation. This fact and the numerical association between hyperploidy and dicentric formation, both for normal bivalents and for the 11.13(1) marker bivalent, lead us to suppose that in the female mouse, irradiation-caused aneuploidy is effectuated via chromatid exchange. The data presented do not rule out the existence of another mechanism.