Onset of zygotic transcription and maternal transcript legacy in the rabbit embryo.

Onset of zygotic transcription is progressive from the one-cell stage onward in the rabbit embryo. Maternal transcripts remain fairly stable until the 8-16 cell stage when major transcriptional activation of the zygotic genome takes place. To understand the mechanisms of the maternal-to-zygotic transition in the genetic information governing development, we asked whether a progressive synthesis of zygotic transcripts takes over the maternal molecules, or whether the synthesis of zygotic transcripts is very abrupt and independent of the persistence of the maternal counterparts. To answer this question, we set up mRNA differential display experiments comparing the mRNA content of rabbit embryos at different stages during the preimplantation period. We isolated eight zygotic transcripts whose synthesis is abruptly turned on at the 8-16 cell stage. These transcripts are involved in general cellular metabolism and their maternal counterparts are still present up to the four-cell and even the 8-16 cell stage. This identification of early zygotic transcripts suggests that global long range modifications of chromatin structure result in a rapid increase in transcription rates during the major transcriptional activation of the zygotic genome.