476
9.3.7 Summary
Vertebrate embryos delay the first major wave of zygotic transcription from hours to
days after fertilization, although the number of cell divisions until zygotic gene
activation varies. Large-scale changes in chromatin architecture appear to anticipate
and/or accompany these changes, although the timing and strategy for regulation of
gene expression may vary between organisms. The capacity for transcription exists
already in the egg, indicating that repressive mechanisms are programmed to delay
zygotic transcription. In oviparous species a repressive activity linked to chromatin
assembly is titrated out as the DNA content and/or nuclear volume increases relative
to the cytoplasm, but there is also evidence for N:C independent timing mechanisms
that may regulate zygotic gene activation, maternal RNA clearance, and cell cycle
remodeling. Recent work has identified specific mRNAs and microRNAs robustly
expressed before the MBT that are essential for critical developmental processes
that define the maternal to zygotic transition, including clearance of maternal RNAs
and activation of the earliest steps in germ layer formation. Whether expression of
these early RNAs depends on the N:C ratio or instead is regulated by an N:C inde-
pendent timer remains to be investigated.
Acknowledgements We thank Shelby Blythe, Jing Yang, Daniel Kessler, Mary Mullins, Karla
Neugebauer, Patricia Heyn, Steven Harvey, and Derek Stemple for helpful discussions. We thank
Jing Yang for sharing unpublished data and Karla Neugebauer and Patricia Heyn for permission to
reprint their published data. We are indebted to authoritative reviews from many authors, as cited
above, and thank Shelby Blythe and Eric Wieschaus for sharing their review prior to publication.
M.Z. was supported in part by the Developmental Biology Training Grant at Penn (T32HD007516).
J.S. was supported in part by the Cell and Molecular Biology Training Grant at Penn
(T32-GM07229). M.A.L. was supported by the NIH (R01GM083988) and a Searle Scholar award.
P.S.K. was supported by the NIH (R01HL110806 and R01MH100923) and the Institute for
Regenerative Medicine at the University of Pennsylvania.
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