Nuclear transplantation in the mouse: heritable differences between parental genomes after activation of the embryonic genome.

Paternal and maternal genomes apparently have complementary roles during embryogenesis in the mouse, and both are essential for development to term. However, there is no direct evidence to show that functional differences between parental genomes remain intact after activation of the embryonic genome at the 2-cell stage. In this study we demonstrate that transfer of paternal or maternal nuclei from early haploid preimplantation embryos back to fertilized eggs from which one pronucleus was removed resulted in development to term, but only if the remaining pronucleus was of the parental type opposite to the donor nucleus. Hence, functional differences between parental chromosomes are heritable and they survive activation of the embryonic genome and probable reprogramming of donor embryonic nuclei by epigenetic factors in the egg cytoplasm.