443The terms MBT and MZT (maternal to zygotic transition) have been used vari-
ously. We follow the clear definitions in Tadros and Lipshitz ( 2009 ) wherein MZT
refers to the gradual transition from maternal to zygotic control and can span a
broad developmental window in the pregastrula embryo. The MBT, in contrast,
refers to a discrete milestone occurring in many species after a species-specific
number of cell divisions and is marked by the coordinated acquisition of cell cycle
checkpoints, cell cycle asynchrony, and the onset of large-scale zygotic transcrip-
tion (Tadros and Lipshitz 2009 ; Gerhart 1980 ; Signoret and Lefresne 1971 ; Newport
and Kirschner 1982a, b; Audic et al. 1997 ; Langley et al. 2014 ). We use the qualified
term “large-scale zygotic transcription” because a subset of zygotic genes is tran-
scribed before the MBT in Xenopus, zebrafish, and Drosophila (Tadros and Lipshitz
2009 ; Baroux et al. 2008 ; Nakakura et al. 1987 ; Yang et al. 2002 ; Skirkanich et al.
2011 ; Lindeman et al. 2011 ; Liang et al. 2008 ; Harrison et al. 2010 ; Edgar and
Schubiger 1986 ; Yasuda et al. 1991 ; Heyn et al. 2014 ; Collart et al. 2014 ; Tan et al.
2013 ; Blythe et al. 2010 ; Lee et al. 2014 ), and this early wave of transcription is
essential for development at the late blastula stage in Xenopus and Drosophila
(Skirkanich et al. 2011 ; Liang et al. 2008 ; Harrison et al. 2010 ). In this chapter, we
focus on the regulation of the cell cycle and zygotic transcription in vertebrate
embryos and briefly touch on information from invertebrate species where it directly
informs our understanding of vertebrate mechanisms. The MBT in nonvertebrate
species has been discussed in more detail in several informative reviews (Yasuda
and Schubiger 1992 ; Tadros and Lipshitz 2009 ; Baroux et al. 2008 ; Blythe and
Wieschaus 2015a; Lee et al. 2014 ; Farrell and O'Farrell 2014 ). Degradation of
maternal mRNAs is addressed in detail elsewhere in this volume (see Chapter 10 ).
9.2 Embryonic Cleavage Divisions
9.2.1 Unique Features of Embryonic Cleavage Cycles
Most actively proliferating cells progress through four distinct phases: the first gap
phase (G1), DNA replication (S-phase), a second gap phase (G2), and mitosis (M).
Newly fertilized embryos from many vertebrate and invertebrate species, including
nonmammalian vertebrates such as fish and frogs, have highly specialized cell
cycles that differ significantly from cells of later development and adult organisms
(Oppenheimer 1936 ; Graham and Morgan 1966 ). These embryonic cell cycles,
known as cleavage divisions, are extremely short and lack the gap phases typical of
growing cells. Without cell growth, these cleavage cycles are reductive divisions
that progressively subdivide a constant volume of cytoplasm from a single large cell
into many smaller cells, increasing the nuclear to cytoplasmic (N:C) ratio with each
round of DNA replication. Rapid cleavage cycles, which establish a large cell popu-
lation necessary for gastrulation, continue until the embryo undergoes the mid-
blastula transition (MBT).
9 Cell Cycle Remodeling and Zygotic Gene Activation at the Midblastula Transition