Degradation of maternal cdc25c during the maternal to zygotic transition is dependent upon embryonic transcription*

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Abstract
To gain a better understanding of the molecular differences that may contribute to cleavage arrest and the poorer development associated with laboratory produced embryos, a series of experiments were conducted to quantitate the message levels of the cell cycle controller cdc25c, over the maternal to zygotic transition (MZT) in 4‐cell in vivo‐ and in vitro‐derived porcine embryos. The experiments were designed to measure both maternal and embryonic derived cdc25c transcripts by quantitative reverse transcription‐competitive polymerase chain reaction (RT‐cPCR), determine the point of the transition to zygotic genome activation, and study the interaction between initial embryonic transcription and maternal cdc25c degradation. Analysis of in vivo‐ and in vitro‐derived embryos revealed no difference in cdc25c message level for any of the times P4CC (P > 0.05). Comparison of control embryos from 5‐ to 33‐hr P4CC revealed a reduction in transcript quantities in the 10‐hr P4CC group that was maintained at later time points (P < 0.05). Embryos cultured in the RNA polymerase inhibitor, α‐amanitin, from cleavage to 5‐, 10‐, 18‐, 25‐, or 33‐hr P4CC displayed no difference in cdc25c levels when compared to controls at similar time points (P > 0.05). However, if embryos were first exposed to α‐amanitin after 18‐hr P4CC with this treatment continuing to 33 hr, the levels of cdc25c transcript were reduced (P < 0.04) when compared to those embryos that were first exposed to the inhibitor at either 5‐ or 10‐hr P4CC. This finding and the comparison of these same embryos to the 0–33‐hr α‐amanitin and control groups allowed us to conclude that cdc25c transcription began between 10‐ and 18‐hr P4CC, with the degradation of maternal cdc25c message dependent on transcriptional initiation. Mol. Reprod. Dev. 60: 181–188, 2001.