On the cellular and developmental lethality of a Xenopus nucleocytoplasmic hybrid.

Nucleocytoplasmic hybrid (cybrid) embryos result from the combination of the nucleus of one species, and the egg cytoplasm of another species. Cybrid embryos can be obtained either in the haploid state by the cross-fertilization or intra-cytoplasmic injection of an enucleated egg with sperm from another species, or in the diploid state by the technique of interspecies somatic cell nuclear transfer (iSCNT). Cybrids that originate from the combination of the nucleus and the cytoplasm of distantly related species commonly expire during early embryonic development, and the cause of this arrest is currently under investigation. Here we show that cells isolated from a Xenopus cybrid (Xenopus (Silurana) tropicalis haploid nucleus combined with Xenopus laevis egg cytoplasm) embryo are unable to proliferate and expand normally in vitro. We also provide evidence that the lack of nuclear donor species maternal poly(A)(+) RNA-dependent factors in the recipient species egg may contribute to the developmental dead-end of distantly-related cybrid embryos. Overall, the data are consistent with the view that the development promoted by one species' nucleus is dependent on the presence of maternally-derived, mRNA encoded, species-specific factors. These results also show that cybrid development can be improved without nuclear species mitochondria supplementation or replacement.