Dynamic Development

CONTENTS

Main Page Dynamic Development

The Foundations of Developmental Biology

Gametogenesis

From Sperm and Egg to Embryo

Genetic Regulation of Development

Organizing the Multicellular Embryo

Generating Cell Diversity


Dynamic Development at a Glance


Learning Resources

Research Resources

The Developmental Biology Journal Club

Developmental Biology Tutorial

Maternal Control of Early Development

by
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

  1. maternal trait dominance in interspecific hybrids
  2. cleavage in enucleate embryos
  3. transcriptional inhibition does not prevent cleavage
  4. 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.

Question:

  • 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)

Question
:

  • 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?

Questions:

  • 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.

Question:

  • 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.

Question:

  • 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.


Digging Deeper

Links to Related Material

Early Research Documenting Stored Maternal mRNAs


References

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. Sunderland, MA.

Kalthoff, K. 1996. Analysis of Biological Development. McGraw-Hill. New York.

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. London.


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Leon Browder & Laurie Iten (Ed.) Dynamic Development
Last revised Wednesday, July 15, 1998