Post-implantation development and cytogenetic analysis of diandric heterozygous diploid mouse embryos

Diandric heterozygous diploid mouse embryos were produced by standard micromanipulatory techniques using eggs from female mice with a normal chromosome constitution and fertilised by homozygous Rb(1.3)1Bnr males containing a pair of large metacentric marker chromosomes in their karyotype. The constructed diandric eggs were transferred to the oviducts of pseudopregnant recipients and subsequently autopsied midday on the eighth day of gestation. From a total of 85 eggs transferred to females that subsequently became pregnant, 30 implanted. Eighteen implantation sites were found to contain resorptions, and 12 egg cylinder stage embryos were recovered. These were cytogenetically examined. In two cases, no mitoses were observed, and in a third embryo of normal size, only a single paternally-derived marker chromosome was present in its mitoses, indicating that this embryo had a normal chromosome constitution. This presumably resulted from a technical error during the micromanipulatory procedure. The remaining nine morphologically small but normal embryos were diploid, and each had two paternally-derived marker chromosomes, thus establishing their ploidy and confirming their diandric origin. G-banding analysis revealed that all of these embryos had an XY sex chromosome constitution. Since the expected XX:XY:YY ratio of 1:2:1 was not observed, it is clear that the XX class embryos were lost at some stage during the pre- or early post-implantation period, though whether they are represented by the resorption sites is not yet established. The YY class would not be expected to be recovered in any case, as these embryos are believed to be lost during early cleavage. The cytogenetic findings reported here are therefore similar to the results of the chromosomal analyses of the human complete hydatidiform moles of dispermic origin, all of which apparently have an XY karyotype. It is unclear why, both in the human and in the mouse, the XX diandric heterozygous diploid group should develop poorly compared to similar embryos with an XY karyotype.