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Zygotic Control of Early Development
by Dr. Derrick Rancourt
Department of Medical Biochemistry
University of Calgary
As we have previously discussed,
although the oocyte translational machinery is complete, in general, translation
does not occur until fertilization (or oocyte maturation), because of numerous
regulatory events. We touched upon some examples of specific translational
regulation. We shall now continue with other examples of specific regulation.
However, the bulk of this topic will be devoted to the zygotic control of
- As development continues beyond fertilization, there is a switch
from control by the maternal genome (maternal mRNA) to the zygotic genome.
The key features of this topic will be that:
- In many organisms transcription of the zygotic genome occurs during
the mid blastula transition. This transition is hallmarked by increases
in cycle times and nuclear-cytoplasmic ratio which appear to dilute negative
transcriptional regulators. Zygotic translation is largely coupled to transcription
and is accompanied by the destruction of maternal mRNAs.
Specific Translational Regulation in Early Development
As you will recall, the translation of maternal mRNAs is controlled generally
by intracellular pH and calcium. Key molecules within the translational
machinery (i.e. initiation factors) could be modified to dampen translation.
As well, the structure of maternal mRNAs can be altered (i.e. polyA tail,
cap methylation, secondary structure) or mRNAs can be sequestered within
the cell or be masked by specific RNP proteins. While alterations in mRNA
represent general mechanisms of translational regulation, they can also
be specific. As we discussed last time, for instance the cytoplasmic polyadenylation
element (CPE) allows certain mRNAs to be preferentially polyadenylated after
fertilization. A number of examples of specific translational regulation
are shown in your textbook.
An excellent example is that of cyclin A in sea urchin. This protein, as
you will recall, is a component of M-phase promoting factor (MPF) and is
important in both mitosis and meiosis. In sea urchin embryos, this protein
is occurs in waves during cleavage such that its concentration is high during
interphase. Since the maternal mRNA is stable, this result suggests that
not only is translation coordinated with the cell cycle but so is its degradation.
(See Browder et al., 1991, Fig. 13.18; Kalthoff, 1996, Figs.
17.11 and 17.12)
In addition to proteins being degraded, maternal mRNAs may also be degraded
according to a specific program. Examples of this are the ribosomal proteins
of Xenopus, the majority of which are degraded at oocyte maturation.
Interestingly, three of these proteins however, continue to be translated
through cleavage and escape degradation.
So in the early developmental program a number of different things can be
happening. Maternal mRNAs can be translated during oocyte maturation, after
which there is sufficient protein to carry the embryo through early development.
Thus this mRNA can be degraded. In other cases, mRNA may need to be stable
and translated a number of times throughout cleavage. Finally some maternal
mRNAs are not needed until the blastula stage or even in some cases as late
The most interesting aspect of this is that all of this regulation appears
to be able to occur with not input from the nucleus (the command centre).
We saw this earlier when we were discussing physical and chemical ennucleation.
Another example is seen in Il;yanassa. You will recall that
during cleavage in Ilyanassa an anucleate polar lobe is formed. The
polar lobe can be removed from the cleaving cells and yet appropriate proteins
will be translated in their appropriate order.
(See Browder et al., 1991, Fig. 5.30, Fig. 13.21; Kalthoff,
1996, Fig. 8.3; Wolpert et al., 1998, Fig. 6.13)
Zygotic Control of Development
If you recall the experiments on interspecies hybridization, you'll remember
that although these experiments demonstrated that early embryonic development
mimicked that of the maternal species, while later after the blastula stage,
signs of paternal contribution became evident. As well, when isozymes are
analysed, the expression of the paternal enzyme occurred around the blastula-gastrula
Hybrids made between the sea urchins Strongylocentrotus and Sphaerechinus
illustrate the onset of zygotic control. In this experiment, paternal contribution
is manifested by the appearance of spicules that do not appear in the maternal
Strongylocentrotus embryo. Here we see two examples of hybrids, both
of which display spicules suggesting paternal contribution, later in development.
The interesting aspect to me however, is that contribution can vary from
embryo to embryo, suggesting that there is a lot of plasticity in development
and that it is not completely hard-wired. It may be hard wired early on
when the maternal program directs development, but later on, it occurs as
a consequence of compromise between different genetic elements. Each time
the compromise is different.
(See Browder et al., 1991, Fig 13.22)
Treatment with actinomycin D demonstrates that the formation of these structures
is dependent upon zygotic transcription.
Activation of Zygotic Transcription
After fertilization, organisms appear to differ in their regulation of
zygotic transcription. Xenopus and Drosophila have a period
of transcriptional quiescence that occurs during cleavage. At this time,
cells are undergoing rapid cellular divisions. In Xenopus, transcription
occurs at the midblastula transition, at which time cleavage has slowed
(See Browder et al., 1991, Fig. 13.23; Shostak, 1991, Fig.
15.14, Wolpert et al., 1998, Figs. 3.33 and 3.34)
Similarly in Drosophila, transcription is observed by the cellular
blastoderm stage which is at cleavage cycle 10. In Drosophila and
Xenopus, it takes roughly 10 and 20 minutes respectively for early
cleavage cells to divide. That's incredibly fast when you bear in mind that
a bacterium takes roughly 20 minutes to do the same thing.
How does Drosophila, which has 100 times more DNA than E. coli,
or Xenopus, which has 1000 times more DNA than E. coli,
manage to replicate its DNA in equal or less time during cleavage?
Because Drosophila and Xenopus are so busy replicating their
genomes, it has been suggested that that the general down regulation of
transcription is related to relatively short interphase periods. It is not
until the cell cycle slows down that transcription is even possible.
In sea urchin, there is no quiescent period. mRNA transcription increases
gradually over the time of cleavage. For instance northern blotting demonstrates
two zygotic forms of histone H2B. The early form which is both maternal
and zygotic declines in its levels immediately following fertilization and
then accumulates during cleavage. Interestingly, sea urchins synthesize
an early set of histones that are found predominantly during cleavage. The
expression of the later set of histones appears at the blastula stage and
by the gastrula stage, the histones that are synthesized are specifically
of the late form.
(See Browder et al., 1991, Fig. 13.26)
- Why synthesize a cleavage-specific set of histones?
Whereas mRNA transcription increases through cleavage, tRNA and rRNA
transcription do not occur until the blastula.
Mouse is similar to sea urchin in that mRNA transcription is detected as
early as the 2 cell stage, even in the late zygote. New rRNA is observed,
as well,at the 2 cell stage while tRNA appears at the 4 cell stage. Treatment
with transcription inhibitors affect cleavage at the two cell stage.
Is There a Unifying Mechanism Regulating Zygotic Transcription?
One thing that bothers developmental biologists is that there isn't a
unifying principle regarding the regulation of zygotic transcription since
the sea urchin and mouse do not display a type of mid-blastular transition.
It has been noted that the cell cycle may be one regulatory point. In mouse
the cell cycle is very long and it can be argued that there is ample time
for transcription to occur. Recall, as well, that that sea urchin has a
number of blastomeres of varying size, which undergo asynchronous cleavage.
- Can asynchronous cleavage be used to explain why the sea urchin
does not appear to have a midblastula transition? How would you test this?
One observation that refutes the notion that zygotic transcription is
linked to the cell cycle is the observation that in mouse transcription
can occur even if the cell cycle is arrested in S phase. Interestingly,
zygotic transcription occurs regardless of replicative state or what cell
cycle stage the embryo is in. As a result, in mouse zygotic transcription
is thought to be regulated by a time dependent mechanism....the so-called
Experiments in Xenopus have refuted that idea that there is a zygotic
clock. When embryos are ligated such that two separate cleavage embryos
result where cleavage is delayed on one side, transcription is observed
only on the side where the blastula resides and not the side of the immature
blastula. Since transcription occurs only on the side of the later embryo,
this suggests that a clock is not working.
(See Browder et al., 1991, Fig. 13.29)
Maternal Cytoplasmic Factors May Regulate Zygotic Transcription
Developmental biologists believe that negative cytoplasmic factors may
in fact regulate zygotic transcription and that these are mediated by nuclear/cytoplasmic
ratios. This idea first emerged from work on Xenopus where it was
observed that transcription from actively transcribing nucleii was repressed
when the nucleus was transferred to a quiescent cytoplasm. This was confirmed
by the experiment just discussed, because only in the embryo with the right
number of cells (therefore correct nuclear-cytoplasmic ratio), does transcription
Other observations that support this theory in Xenopus include:
- polyspermy or removal of cytoplasm advances the MBT;
- haploid embryos have a retarded MBT;
- suppressed gene transcription can be alleviated by injection of extraneous
(See Browder et al., 1991, Fig. 13.28)
(Interestingly, when male and female pronucleii of newly-fertilized mouse
oocytes are microinjected with a DNA construct, transcription is observed
only from male pronuclear injections, suggesting that these negative elements
have not yet begun to occupy the male pronucleus.)
- Since blastular cells differ in size within the Xenopus embryo,
how can nuclear/cytoplasmic ratios regulate transcription?
Cytoplasmic Inhibitor Theory Confirmed using Drosophila Genetics
Because Drosophila genetics has uncovered a number of zygotic
lethal mutations, a number of markers of early zygotic transcription have
been discovered. These represent the gap and pair rule genes which will
be discussed in greater detail in the next section of lectures. By definition,
zygotic lethal genes represent anything that is lethal when homozygous.
The earliest zygotic lethals such as the gap gene Krupple and the
pair rule gene fushi tarazu (ftz)are expressed as early as
cell cycle 10, when the cellular blastoderm is forming.
Expression is of these genes is detected by in situ hybridization,
by cycle 10 both genes appear to be expressed according to their normal
pattern. Interestingly, expression can bedetected as early as cell cycle
8, however at this time expression is limited to a few scattered nucleii
in the periphery of the embryo. This result suggests that while the bulk
of early transcription occurs at cycle 10, the regulation of zygotic transcription
is leaky, and transcription at cycle 8 is stochastic. This observation is
consistent with the nuclear/cytoplasm theory as negative regulators within
embryo may be locally depleted in a stochastic manner. (Recall that at cycle
8, the Drosophila embryo is still a syncitium.
A second experiment which confirms the first, relates to the use of a mutant
gs(1)N26. In this mutant nucleii, within the syncitium migrate non-uniformly
to the periphery, first in the anterior and later in the posterior. In this
mutant ftz expression is first observed in the anterior of the embryo
and later followed by correct posterior expression. Because the nuclear/cytoplasm
ratio is increased anteriorly first, depletion of cytoplasmic inhibitors
occur there first.
Tramtrack is a Maternal Inhibitor of Zygotic Gene Expression
Tramtrack (ttk) is a maternal effect lethal gene that
encodes a zinc finger protein which in part acts as a repressor of ftz
transcription. Interestingly, when the dosage of ttk loaded into
the oocyte is altered, a significant effect on zygotic ftz expression
is observed. When maternal ttk levels are lowered using heterozygous
mothers, ftz transcription is observed one cell cycle earlier (In
this experiment, homozygous mothers cannot be used since very abnormal embryos
result.). Alternately, if extra ttk protein is loaded into the oocyte
using exogenously added ttk genes, ftz expression is delayed.
The Switch from Oogenic to Zygotic Translation
Compared with maternal mRNA in oocytes, the translation of zygotic mRNA
is largely linked to transcription. The efficiency at which zygotic translation
occurs relates to the degradation of maternal mRNA after fertilization.
In mouse, this degradation is very rapid occurring the two cell stage, after
which translation of these products occur directly from zygotic sources.
This translation correlates with the appearance of new proteins derived
from zygotic mRNA. This example serves to indicate that many proteins have
both a maternal and a zygotic phase.
(See Browder et al., 1991, Fig. 13.25)
- How could you distinguish between maternal and zygotic phases? What
type of proteins do you think would have both maternal and zygotic phases?
Interestingly, maternal effect genes isolated in Drosophila have
indicated that the programmed degradation of maternal mRNAs may also be
related to the nuclear/cytoplasmic ratio. How this occurs is unknown.
Browder, L.W., Erickson, C.A. and Jeffery, W.R. 1991. Developmental
Biology. Third edition. Saunders College Pub. Philadelphia.
Gilbert, S.F. 1997. Developmental Biology. Fifth Edition. Sinauer.
Kalthoff, K. 1996. Analysis of Biological Development. McGraw-Hill.
Shostak, S. 1991. Embryology. HarperCollins. New York.
Wolpert, L., Beddington, R., Brockes, J., Jessell, T., Lawrence, P. and
Meyerowitz, E. 1998. Principles of Development. Current Biology.