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
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Developmental Biology Tutorial |
The Origins of Polarity and Embryo Patterning in Drosophila
Which came first, the fly or the egg?
Extensive genetic analysis in Drosophila has identified maternal
effect genes, which are transcribed during oogenesis and exert their phenotypic
effects during development. They are typically involved in organization
of the body plan of the embryo. How can expression of genes during oogenesis
leave a molecular imprint on the egg? We shall briefly examine the establishment
of the body axes in Drosophila and will return to the important topic
of body plan organization later.
Anterior-posterior asymmetry is evident before dorsal-ventral asymmetry
(Gavis, 1995). Polarization is evident by the localization of the bicoid
and oskar transcripts to opposite ends of the oocyte. (For localization
of oskar transcripts, see Fig. 1, below.) The localization of these
transcripts is dependent upon the oocyte cytoskeleton (Micklem, 1995). Each
egg chamber contains a single interconnecting microtubule complex. During
early oogenesis, the microtubule organizing center (minus end) is located
at the oocyte's posterior end with its plus end extending into the nurse
cells. That complex is then dismantled, and a new one is assembled, with
the microtubule-nucleating activity at the anterior end and the plus ends
of the microtubules extending toward the posterior pole. Both the transport
and anchoring of bicoid and oskar transcripts are MT-dependent.
Figure 1. In situ hybridization
to whole mount Drosophila egg chamber. Oskar mRNA is synthesized
in the polyploid nurse cells (left), transported into the oocyte (right)
and localized to the posterior pole of the oocyte. (Reproduced with permission
of Dr. Ruth Lehmann. Visit Dr.
Lehmann's Web page.)
Organization of the microtubule network depends upon localization of
the oocyte to the posterior end of the egg chamber early in oogenesis. A
signal produced by the oocyte reaches only the follicle cells at that end.
A subset of them (the polar cells) respond to the signal and differentiate
with posterior fates. The posterior polar cells, in turn, signal back to
the oocyte to set up the anterior-posterior axis of the oocyte by reorganizing
the microtubule cytoskeleton (González-Reyes and St Johnston, 1994).
The signal from the oocyte to the follicle cells depends upon gurken.
gurken mRNA localizes to the posterior end of the oocyte during early
oogenesis. gurken encodes a TGF alpha-like signaling molecule that
has an epidermal growth factor (EGF) repeat element. The receptor in the
follicle cells is an EGF receptor-like molecule called top/DER. Any disruption
of this signaling due to mutation prevents determination of the posterior
follicle cells. The oocyte microtubule cytoskeleton fails to become polarized,
bcd mRNA becomes distributed to both the anterior and posterior poles,
and osk mRNA is not localized to the posterior pole (Gonzalés-Reyes
and St. Johnston, 1994).
Remember that oocyte trancripts originate in the nurse
cells. They are transported into the oocyte, apparently directed by
a microtubule minus-end-directed motor. The reorganization of the microtubule
network redirects the minus ends to the anterior end of the oocyte and moves
the transcripts to the anterior end of the oocyte. Most subsequently become
uniformly distributed, with the exception of those that are to become localized
(i.e., bicoid, oskar, gurken and cyclin-B).
gurken signaling occurs early while the microtubule network has its
minus ends in the oocyte. That signaling subsequently reverses the polarity
of the network.
Dorsal-ventral patterning has been shown to be dependent upon a similar
communication mechanism, and the key signaling molecules involved in that
process have been identified. Dorsalization is first detectable in the oocyte
when the nucleus moves to the anterior/dorsal corner of the oocyte as a
consequence of microtubular function. The nucleus is accompanied by gurken
transcripts. As a consequence, Gurken protein is synthesized in this localized
region of the oocyte. The localization of Gurken protein is shown below.
Confocal fluorescence micrograph showing the distribution
of Gurken protein (green; yellow where it co-localizes with cortical actin)
on the presumptive anterior/dorsal side of the developing Drosophila
oocyte.
(Photo courtesy of Dr.
Trudi Schupbach. Image copyright © 1997, Trudi Schupbach.)
Dorsalization occurs in response to a signal produced by gurken (Micklem,
1995). The grk signal, which is produced by the oocyte, is received
by follicle cells via Top/DER. It has been proposed that the binding of
Gurken protein to Top/DER protein in the adjacent follicle cells activates
a receptor tyrosine kinase signal transduction cascade. Follicle cells that
receive the grk signal acquire dorsal fates, whereas the remainder
become ventrally-inclined. Dorsal-ventral differentiation of the follicle
cells causes ventral activation of a second signaling pathway, by which
a signal is transmitted to the fertilized embryo to produce the gradient
of Dorsal protein in embryonic nuclei.
(See Browder et al., Fig. 6.13) Gilbert, 1997, Fig. 14.37;
Kalthoff, 1996, Fig. 21.34; Wolpert et al., 1998, Fig. 5.12, 5.14)
Gonzalés-Reyes et al. (1995) demonstrated that grk mRNA
is not localized when the GV fails to migrate (in top /DER mutant
egg chambers). The oocyte microtubule cytoskeleton is not properly polarized
in egg chambers of these mutants, which lack both A-P and D-V patterning.
These observations indicate that the formation of the D-V axis depends upon
the prior polarization of the A-P axis, which polarizes the microtubule
cytoskeleton that translocates the oocyte nucleus and the grk mRNA.
The displacement of mRNA molecules by the cytoskeleton is an interesting
phenomenon. Once they have been translocated, they must remain in place
in order to produce localized protein product. Localized RNAs in Drosophila
appear to be associated with the oocyte cortex.
The requirement of microtubules for RNA transport is shown by using the
microtubule inhibitor colchicine. As shown by Pokrywka and Stephenson (1995),
oskar mRNA localization is abolished by treatment with colchicine.
Other transcripts, however, either are not localized or have different destinations.
The differences between transcripts imply that individual RNA molecules
may have "postal codes" (zip codes for the Americans) that direct
them to particular addresses within the cell (and attendants that keep them
there). The delivery system that reads and utilizes the postal code is not
well understood, but it has the ability to select among transcripts. For
example, exuperantia and swallow are required for localization
of bicoid. However, oskar mRNA is localized normally in exuperantia
and swallow mutant egg chambers. Presumably, proteins that anchor
specific transcripts to the microtubules are involved. Glotzer et al.
(1997) have also demonstrated that colchicine prevents localization of oskar
mRNA if labeled transcripts were injected into oocytes at sites distal to
the posterior pole. However, when transcripts were injected close to the
posterior pole, the injected mRNA localized even in the presence of colchicine.
This result suggests that long-range transport of the messenger is dependent
upon microtubules but that microtubule-independent processes are involved
in short-range transport and anchoring of osk mRNA to the posterior
pole.
What do these proteins respond to on mRNA? What is the postal code comprised
of? In the case of osk and bcd mRNAs, distinct elements in
the 3' UTRs have been identified that direct their transport and localization.
The localization of oskar mRNA has important implications for formation
of the pole plasm. The pole plasm contains elements that are necessary for
translation of nanos mRNA.
This establishes a gradient in Nanos protein that is necessary for patterning
of the posterior of the embryo. The pole plasm also contains determinants
of the germ line. Oskar protein induces pole plasm assembly (Ephrussi and
Lehmann, 1992). If osk mRNA is mislocalized to the anterior pole,
it induces polar plasm there. However, in mutant ovaries in which oskar
mRNA is not localized, ectopic pole plasm does not form (Ephrussi and Lehmann,
1992). These results suggest that unlocalized osk mRNA fails to be
translated and that localization is necessary for its translation.
Why are unlocalized oskar transcripts inactive? The evidence suggests
that their translation is repressed outside the posterior domain. This repression
is dependent upon the binding of a protein called Bruno to elements in the
3' UTR of oskar mRNA. In the absence of a functional bruno
gene, oskar is translated prematurely. The Oskar protein that is
synthesized accumulates throughout the oocyte, causing posteriorization
of the embryo (Kim-Ha et al., 1995).
Learning Objectives
- Summarize the roles of the cytoskeleton in establishment of the asymmetry
of the Drosophila oocyte. Document the experimental evidence to
support these roles.
- What is the role of the posterior polar cells in polarization?
- What is gurken? Describe the odyssey of gurken mRNA during
oogenesis.
- What is the first overt sign of dorsalization of the oocyte?
- What is top/Der?
- What happens to the distribution of gurken mRNA in top/Der
mutants?
- What are the developmental implications of the localization of oskar
mRNA?
- Why are unlocalized oskar transcripts inactive?
Digging Deeper:
Additional Reading
Markussen, F.H., Michon, A.M., Breitwieser, W. and Ephrussi, A. 1995.
Tranlsational control of oskar generates short OSK, the isoform that
induces pole plasm assembly. Development 121: 3723-3732.
Neuman-Silberberg, F.S. and Schupbach, T. 1996. The Drosophila TGF-alpha-like
protein Gurken: expression and cellular localization during Drosophila
oogenesis. Mech. Dev. 59: 105-113.
Newmark, P.A., Mohr, S.E., Gong, L. and Boswell, R.E. 1997. mago nashi mediates
the posterior follicle cell-to-oocyte signal to organize axis formation
in Drosophila. Development 124: 3197-3207.
Ray, R.P. and Schupbach, T. 1996. Intercellular signaling and the polarization
of body axes during Drosophila oogenesis. Genes & Dev. 10: 1711-1723.
Wilson, J.E., Connell, J.E. and Macdonald, P.M. 1996. aubergine enhances
oskar translation in the Drosophila ovary. Development 122:
1631-1639.
References
Browder, L.W., Erickson, C.A. and Jeffery, W.R. 1991. Developmental
Biology. Third edition. Saunders College Pub. Philadelphia.
Ephrussi, A. and Lehmann, R. 1992. Induction of germ cell formation by oskar.
Nature 358: 387-392.
Gavis, E. R. 1995. Gurken meets torpedo for the first time. Current Biology
5: 1252-1254.
Gilbert, S.F. 1997. Developmental Biology. Fifth Edition. Sinauer.
Sunderland, MA.
Glotzer, J.B., Saffrich, R., Glotzer, M. and Ephrussi, A. 1997. Cytoplasmic
flows localize injected oskar RNA in Drosophila oocytes. Curr. Biol. 7:
326-337.
Gonzáles-Reyes and St Johnson, D. 1994. Role of oocyte position
in establishment of anterior- posterior polarity in Drosophila. Science
266: 639-642.
Gonzáles-Reyes, A., Elliott, H. and St Johnson, D. 1995. Polarization
of both major body axes in Drosophila by gurken-torpedo signalling.
Nature 375: 654-658.
Kalthoff, K. 1996. Analysis of Biological Development. McGraw-Hill.
New York.
Kim-Ha, J., Kerr, K. and Macdonald, P.M. 1995. Translational regulation
of oskar mRNA by bruno, an ovarian RNA-binding protein, is essential. Cell
81: 403-412.
Micklem, D.R. 1995. mRNA localisation during development. Dev. Biol. 172:
377-395.
Pokrywka, N. J. and Stephenson, E. C. 1995. Microtubules are a general component
of mRNA localization systems in Drosophila oocytes. Developmental
Biology 167: 363-370.
Wolpert, L., Beddington, R., Brockes, J., Jessell, T., Lawrence, P. and
Meyerowitz, E. 1998. Principles of Development. Current Biology.
London. |
