Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 134

genes. Individual development increasingly depends on the zygotic gene expres-
sion, although most of the maternal factors persist for varying periods of time in the
embryo until the phylotypic stage. So, for example, after gastrulation, formation of
the complete germ band (the phylotypic stage of insects) in the leafhopper Euscelis
plebejusis is under the control of three maternal cytoplasmic factors ( Vogel, 1983 ).
This stage in the activation of the embryonic genome is known as mid-blastula
transition (MBT). The term was introduced by Kane and Kimmel (1993) to describe
development's passage from maternal to embryonic control. From here, embryonic
cells divide asynchronously and show a level of motility that is necessary for gastru-
lation and cell division. The asynchronic division of cells results from variations in
their nucleocytoplasmic ratios ( Kane and Kimmel, 1993 ).
In Xenopus , the MBT begins by the 12th cell division, when the transcription of
the zygotic genome is detected for the first time ( Lund et al., 2009 ); and in zebrafish,
the MBT ends after the 13th division cycle ( Kane and Kimmel, 1993 ). The MBT
coincides with the silencing and degradation of maternal mRNAs ( Thatcher et al.,
2007; Giraldez, 2010 ).
The transition is associated with maternal and zygotic enzymatic degradation
of maternal mRNAs. Among maternal factors engaged in the clearing of maternal
mRNAs are a few maternal microRNAs (miRNAs). Initially, destruction of maternal
mRNAs is a suicidal phenomenon (maternal proteins eliminate maternal mRNAs),
and later zygotic transcription leads to the production of miRNAs and proteins that
enhance the efficiency of maternal mRNA degradation ( Tadros and Lipshitz, 2009 ).
Among them is the maternal miR-430, which is activated at the MBT and induces
degradation of maternal mRNAs by facilitating their deadenylation ( Giraldez, 2010;
Giraldez et al., 2006 ). In mammals as well, the mouse egg and zygote contain mater-
nal miRNAs, of which the most abundant are miRNAs of the let-7 family. Compared
to the zygote, the two-cell stage embryo has lost 60% of maternal miRNAs, but in
the four- and eight-cell stages, the amount of miRNAs increases more than 10-fold
as a result of the activation of zygotic genes for miRNAs.
That maternal miRNAs are essential for the early development in mammals is
demonstrated by the fact that mouse eggs fail to undergo the first cleavage division
when depleted of all maternal miRNAs ( Tang et al., 2007 ). Recently, a number of
RNA-binding proteins are identified that seem to determine the direction of mater-
nal degradation machinery to mRNAs targeted for degradation ( Walser and Lipshitz,
2011 ).
After the MBTs, in Xenopus , formation of a preorganizer is induced by the mater-
nal β-catenin in the β-catenin-rich dorsal side of the mid-blastula embryo. Soon,
on the vegetal dorsal side of the embryo, appears the Nieuwkoop center, which is
induced by maternal signals of β-catenin, VegT, and Vg1 in the presence of Nodal-
related (Xnr) proteins, as well as a maternal Wnt component ( Vonica and Gumbiner,
2007 ) ( Figure 3.13 ). On the dorsal animal-marginal region of the amphibian embryo
forms the blastula chordin and noggin-expressing (BCNE) center ( Kuroda et al.,
2004 ), from which develops Spemann's organizer and much of the anterior cen-
tral nervous system (CNS) during gastrulation. Establishment and activation of the
BCNE center requires xNorrin, another maternal cytoplasmic factor that is essential