Meiotic maternal chromosomes introduced to the late mouse zygote are recruited to later embryonic divisions

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The aim of this study was to investigate the fate of an additional female genome introduced to a dividing zygote. Maternal chromatin in the form of karyoplasts containing a metaphase II spindle were fused to zygotes blocked in anaphase or telophase of the first cleavage. Permanent preparations made 20-40 min after fusion at anaphase revealed that the donor maternal chromosomes had entered anaphase or telophase in 16 out of 18 cases. A further two groups of embryos that were fused at either anaphase or anaphase/telophase were cultured to the first division. Division occurred 50 min after fusion in both groups of embryos (86 and 85.1%, respectively), of which most divided to two cells (80 and 71.6% of total) and the remainder divided to three cells. About two thirds of two-cell embryos contained an extra nucleus in one blastomere. Nuclei containing donor maternal chromosomes reached a similar size to recipient nuclei in 68% of embryos derived from anaphase-blocked zygotes, in contrast to 31.1% of embryos derived from anaphase/telophase-blocked embryos. Replication of DNA in donor nuclei closely followed the timing and intensity of that in control embryos. When fixed 24 hr after fusion, one third of embryos were still at the two-cell stage, with one or both blastomeres showing a single metaphase plate of the second cleavage. In the remaining embryos, three or four cells were present, some containing two nuclei. Blastocysts developed in 50% of fused embryos and three young were born after transfer of cleaving hybrid embryos to recipients. Chromosome preparations from bone marrow of the young contained 3-4 tetraploid metaphase plates per several hundred plates counted compared with none in control embryos. In conclusion, additional maternal chromosomes can be introduced at the late-dividing zygote and join the embryonic cell cycles during subsequent divisions. This method may provide a useful approach for studying changes specific to the maternal genome during early cell cycles of the mammalian embryo.