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Developmental Biology Tutorial
Maternal Control of Early Development
Dr. Derrick Rancourt
Department of Medical Biochemistry
University of Calgary
Fertilization acts as a trigger to initiate a program of events starting
with cleavage, and continuing with gastrulation and neurulation, etc. Although
fertilization results in union of maternal and paternal genomes, zygotic
gene activity is not required until the blastula stage. In fact, after fertilization
and through cleavage, the maternal, paternal and zygotic genomes are completely
dispensible. This is because in the egg, there exists a stockpile of maternally
derived mRNAs which govern embryogenesis through cleavage to the blastula
stage. Following the formation of the blastula, zygotic gene transcription
is activated, which carries the embryo through the rest of embryogenesis.
Take-Home Message: Early embryonic events are orchestrated through
the postranscriptional control of maternal mRNA.
Evidence for maternal mRNA control of early development
- maternal trait dominance in interspecific hybrids
- cleavage in enucleate embryos
- transcriptional inhibition does not prevent cleavage
- translational inhibition prevents cleavage
1. Interspecies Hybridization (after Tennant, 1914)
Interspecies hybrids occur when sperm from a closely related species
are used to fertilize an egg. Using this approach, Tennant and others examined
the early developing embryo for dominance of maternal or paternal traits.
In Tennant's experiments using the sea urchins Lytechinus and Eucidaris,
two markers that were used were the rate of cleavage, the time and place
of mesenchymal cells within the archenteron.
(See Browder et al., 1991, Table 13-1; Shostak, 1991, Table
15.1; a similar experiment is discussed on page 407 of Kalthoff, 1996.)
In this experiment, it was found that in the interspecific cros Lytechinus
by Eucidaris, the hybrid developed early features resembling the
female (Eucidaris) partner. In this way it was concluded that maternal
contribution featured more prominently in early embryogenesis.
- What obvious experiment should have been done?
In fact, because the genomes of interspecific hybrids are frequently
unstable, it is unclear whether the paternal (Lytechinus) genome
was simply eliminated from the embryo. While maternal dominance was observed
in a number of different interspecies hybrids (including amphibians and
teleosts), molecular markers were necessary to confirm that the paternal
genome was intact within hybrids. Thus in the mid 1970s these experiments
were revisited using maternal and paternal isozyme markers. Here maternal
specific isozymes were found to be expressed exclusively in the early embryo
until mid blastula after which paternal contribution was observed.
2. Egg Enucleation (after Harvey, 1936)
Here it was demonstrated that sea urchin egg fragments or merogones devoid
of a nucleus, could undergo cleavage and arrest at the blastula stage when
artificially stimulated with sea water.
(See Browder et al., 1991, Figure 13.1; Kalthoff, 1996, Figure
17.9; Shostak, 1991, Figure 9.19)
- What other experiment would you have done here?
Such enucleation experiments have been performed in other models such
as frog. In this case, the male pronucleus is destroyed by irradiating sperm
and removing the female pronucleus physically following fertilization. When
such experiments are performed, the resulting embryos have striking cellular
morphology, except that they are devoid of a nucleus.
(See Browder et al., 1991, Figure 13.2)
3. Transcriptional Inhibition
Due to the technical sophistication of enucleation experiments, in the
late 60s and early 70s, a number of experiments examined the effects of
transcriptional inhibitors (actinomycin D, alpha-amantin) on a number of
experimental embryos. It was observed that following fertilization, treatment
resulted in developmental arrest after cleavage at the time corresponding
to when zygotic gene transcription is first detected.
(See Browder et al., 1991, Table 13-2)
Since cleavage could occur without transcription or a nucleus these observations
suggested that some material other than DNA is directing early development.
As you know from what we've learned so far, some of this material is mRNA.
Could it be protein?
- What's stopping an early embryo from having a maternal program that
is completely derived from protein?
- Can you think of any advantages or disadvantages?
- Other than through translational inhibition, how could you demonstrate
that RNA plays an important part in the program?
Indeed proteins are an integral part of the developing egg, but translational
inhibition experiments demonstrated that new protein had to be synthesized
from maternal mRNA.
4. Translational Inhibition (after Hultin, 1961, 1952)
In the early 60's treatment of sea urchin embryos with puromycin demonstrated
that, after fertilization, protein synthesis was required for early embryogenesis
to proceed. In the absence of translation, embryos died before cleavage.
This observation correlated well with the surge in translation which was
observed immediately after fertilization and suggested that stockpiled maternal
mRNA was the key material required for the early embryonic program. This
program in turn was executed by a translational activation. In
sea urchins this activation is fertilization, which results in 10- to 40-fold
stimulation of translation.
(See Browder et al., 1991, Fig. 13.3).
Recall however, that sea urchins are unusual, in that their oocytes are
arrested as ootids that have completed meiosis. In most other species, oogenesis
is arrested at meiosis II and the major translational activation of maternal
mRNAs occurs at oocyte maturation with only a minor stimulation at fertilization.
As we will discuss in the next lecture, translational activation, is a key
regulatory step, which allows everything to be in a period of stasis one
moment and in a period of abrupt change in another.
- If maturation is the key translational step, why doesn't the oocyte
progress into cleavage?
Send in the Proteins
Since cleavage occurs rapidly, the early embryo requires many structural
proteins, to undergo mitosis and build cells. Thus one can envision a strict
requirement for molecules such as actin and tubulin, histones and ribosomes
(therefore a stockpile of rRNA and tRNA is also required). In addition to
structural proteins, smaller amounts of regulatory proteins (transcription
factors, cell cycle proteins, cytokines/receptors) are also required.
- Are all of these proteins synthesized at the same time or do you
think that a program of translational activation might exist?
Case of the Maternal Effect Genes
Maternal effect genes encode gene products (RNA or protein) that are
required in early development prior to zygotic transcription. As a result
they are distinguished from the zygotic class of genes which can also perturb
early development (post-blastula). Using the rudimentary gene of
Drosophila as an example, by definition, whenever, mothers are homozygous
mutant, their progeny are embryonic lethal despite being genotypically viable.
(See Browder et al., 1991, Table 13-3)
Maternal effect genes have been well-studied in Drosophila. Over
fifty maternal effect genes have been described which affect early embryogenesis.
Many of these have been found to affect dorsal-ventral and anterior-posterior
polarity in the developing embryo. A hallmark example is bicoid.
You will recall that bicoid is a transcription factor that plays
a key role in governing development of the anterior end of the embryo. In
oocytes an anterior to posterior mRNA gradient of bicoid is formed. This
gradient is translated into protein at egg deposition (fertilization).
(See Browder et al. 1991, Fig. 11.37; Gilbert, 1997, Fig. 14.11;
Kalthoff, 1996, Fig. 15.9); Wolpert et al., 1998, Fig. 5.5)
Recall as well, other maternal effect mutations that also affect anterior
development (exuperantia, swallow and staufen). These
gene products affect anterior development indirectly by governing the positioning
of bicoid mRNA in the developing oocyte. exuperantia is required
for the early stages of bicoid localization, swallow for the intermediate
stages while staufen protein acts at the terminal stages just prior to fertilization.
(See Browder et al., 1991, Fig. 11.39; Gilbert, 1997, Table
14.1; Kalthoff, 1996, Table 21.1).
Clearly from this example, key regulatory proteins in oocyte development
have to be present at different times. Therefore while the bulk of translation
may occur at or maturation, a program of maternal mRNA translation must
occur in advance of "the big one". The translational
control of embryogenesis is a very complex topic in developmental biology,
and we shall discuss it next.
Links to Related Material
Early Research Documenting
Stored Maternal mRNAs
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.