Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

MECHANISMS OF patterN FORMatION
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it toward the future dorsal side (see Figure 21–14). This mRNA is soon translated
and the Wnt11 protein secreted from cells that form in that region of the embryo
activates the Wnt signaling pathway (see Figure 15–60). This activation is crucial
for triggering the cascade of subsequent events that will organize the dorsoventral
axis of the body. (The A-P axis of the embryo will only become clear later, in the
process of gastrulation.)
Although different animal species use a variety of different mechanisms to
specify their axes, the outcome has been relatively well conserved in evolution:
head is distinguished from tail, back from belly, and gut from skin. It seems that it
does not much matter what tricks the embryo uses to break the initial symmetry
and set up this basic body plan.
Studies in Drosophila Have Revealed the Genetic Control
Mechanisms Underlying Development
It is the fly Drosophila, more than any other organism, that has provided the key to
our present understanding of how genes govern development. Decades of genetic
study culminated in a large-scale genetic screen, focusing especially on the early
embryo and searching for mutations that disrupt its pattern. This revealed that
the key developmental genes fall into a relatively small set of functional classes
defined by their mutant phenotypes. The discovery of these genes and the subsequent
analysis of their functions was a famous tour de force and had a revolutionary
impact on all of developmental biology, earning its discoverers a Nobel Prize.
Some parts of the developmental machinery revealed in this way are conserved
between flies and vertebrates, some parts not. But the logic of the experimental
approach and the general strategies of genetic control that it revealed have transformed
our understanding of multicellular development in general.
To understand how the early developmental machinery operates in Drosophila,
it is important to note a peculiarity of fly development. Like the eggs of other
insects, but unlike most vertebrates, the Drosophila egg—shaped like a cucumber—begins
its development with an extraordinarily rapid series of nuclear divisions
without cell division, producing multiple nuclei in a common cytoplasm—a
syncytium. The nuclei then migrate to the cell cortex, forming a structure called
the syncytial blastoderm. After about 6000 nuclei have been produced, the plasma
membrane folds inward between them and partitions them into separate cells,
converting the syncytial blastoderm into the cellular blastoderm (Figure 21–15).
We shall see that the initial patterning of the Drosophila embryo depends on
signals that diffuse through the cytoplasm at the syncytial stage and exert their
actions on genes in the rapidly dividing nuclei, before the partitioning of the egg
into separate cells. Here, there is no need for the usual forms of cell–cell signaling;
neighboring regions of the syncytial blastoderm can communicate by means of
transcription regulatory proteins that move through the cytoplasm of the giant
multinuclear cell.
Egg-Polarity Genes Encode Macromolecules Deposited in the Egg
to Organize the Axes of the Early Drosophila Embryo
As in most insects, the main axes of the future body of Drosophila are defined
before fertilization by a complex exchange of signals between the developing egg,
Figure 21–15 Development of the
Drosophila egg from fertilization to the
cellular blastoderm stage.
somatic cells
pole cells
(primordial
germ cells)
fertilized egg
many nuclei divide
rapidly in a syncytium
nuclei migrate to
periphery, where cell
boundaries will
eventually form