The sex-chromosome constitution and early postimplantation development of diandric triploid mouse embryos.

Diandric triploid mouse embryos were produced by standard micromanipulatory techniques, using eggs isolated from female mice with a normal chromosome constitution that had been mated to homozygous Rb(1.3)1Bnr males (which carry a large metacentric "marker" chromosome, viz., a Robertsonian translocation involving chromosomes 1 and 3). The tripronucleate embryos were transferred to the oviducts of pseudopregnant mice, which were subsequently autopsied at about midday on the 10th day of gestation. Although a relatively small number of the isolated conceptuses consisted of morphologically abnormal egg-cylinder-like structures or empty gestational sacs, most were at clearly distinguishable embryonic stages, from the primitive streak stage to embryos with about 20 pairs of somites present. These embryos all appeared to be morphologically normal but were substantially smaller than normal (diploid) fertilized embryos analyzed at similar stages of development. A total of 63 diandric triploid conceptuses were recovered and analyzed cytogenetically. They were G-banded to determine their sex-chromosome constitution and confirm their diandric triploid status. No obvious difference was observed in the developmental potential of the 58,XXX class of diandric triploids, compared to that of the 58,XXY class. The ratio of 58,XXX to 58,XXY embryos was close to the expected ratio of 1:2, assuming that unfertilized eggs have an equal chance of becoming fertilized by an X- or a Y-bearing spermatozoon and that the additional (i.e., "donor") male pronucleus also has an equal chance of having either an X or a Y sex chromosome present. However, the development of the 58,XYY class appeared to be restricted, even at the stage of gestation analyzed, in that no embryos with this genetic constitution were observed that had progressed beyond the early somite stage. The present findings are discussed in relation to the cytogenetic findings in human triploid conceptuses, the majority of which are spontaneously aborted during the first half of pregnancy. In man, the 69,XYY class (equivalent to the 58,XYY class in our study) is only rarely encountered, and it has been assumed that these triploid embryos are probably lost at a very early stage of gestation.