Amphibian interorder nuclear transfer embryos reveal conserved embryonic gene transcription, but deficient DNA replication or chromosome segregation.

Early interspecies nuclear transfer (iNT) experiments suggested that a foreign nucleus may become permanently damaged after a few rounds of cell division in the cytoplasm of another species. That is, in some distant species combinations, nucleocytoplasmic hybrid (cybrid) blastula nuclei can no longer support development, even if they are back-transferred into their own kind of egg cytoplasm. We monitored foreign DNA amplification and RNA production by quantitative PCR (qPCR) and RT-qPCR in interorder amphibian hybrids and cybrids formed by the transfer of newt (Pleurodeles waltl) embryonic nuclei into intact and enucleated frog (Xenopus laevis) eggs. We found a dramatic reduction in the expansion of foreign DNA and cell numbers in developing cybrid embryos that correlated with reduced gene transcription. Interestingly, expansion in cell numbers was rescued by the recipient species (Xenopus) maternal genome in iNT hybrids, but it did not improve P. waltl DNA expansion or gene transcription. Also, foreign gene transcripts, normalized to DNA copy numbers, were mostly normal in both iNT hybrids and cybrids. Thus, incomplete foreign DNA replication and/or chromosome segregation during cell division may be the major form of nuclear damage occurring as a result of nuclear replication in a foreign cytoplasmic environment. It also shows that the mechanisms of embryonic gene transcription are highly conserved across amphibians and may not be a major cause of cybrid lethality.