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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Bicoid
ANTERIOR
egg-polarity genes
POSTERIOR
Figure 21–20 An example of the
regulatory hierarchy of egg-polarity,
segmentation, and Hox genes. As
discussed in the text, there are three
groups of segmentation genes. The
photographs show mRNA expression
patterns of representative examples of
genes of each type. (Courtesy of Stephen
Small.)
gap genes
Hunchback
pair-rule genes
Evenskipped
segment-polarity genes
Hox genes
Engrailed
Antennapedia
and transient: as the embryo proceeds through gastrulation and beyond, the
pattern disintegrates. The genes’ actions, however, have passed on an enduring
memory of their patterns of expression by inducing the expression of certain segment
polarity genes along with Hox genes (discussed shortly). After a period of
pattern refinement mediated by cell–cell interactions, the expression patterns of
these new groups of patterning genes is stabilized to provide positional labels that
serve to maintain the segmental organization of the larva and adult fly.
The segment-polarity gene Engrailed provides a good example. Its RNA transcripts
form a series of 14 bands in the cellular blastoderm, each approximately
MBoC6 n22.209/22.20
one-cell wide. These stripes lie immediately anterior to similar stripes of expression
of another segment polarity gene, Wingless. As the cells in the developing
embryo continue to grow, divide, and move, a mutually reinforcing signal
between the Wingless expressing cells and the Engrailed expressing cells maintains
narrow stripes of their expression (see Figure 21–21). After three cell cycles,
newly expressed regulators stabilize an Engrailed expression pattern that will last
A
N
TERI
O
R
Wingless protein
Hedgehog protein
Engrailed protein
P
OSTERIOR
Figure 21–21 Mutual maintenance of
Hedgehog and Wingless expression.
Engrailed is a transcription regulator (blue)
that drives the expression of Hedgehog.
Hedgehog encodes a secreted protein
(red) that activates its signaling pathway
in neighboring cells and thereby drives
them to express the Wingless gene. In
turn, Wingless encodes a secreted protein
(green) that acts back on neighbors of the
Wingless-expressing cell to maintain their
expression of Engrailed and Hedgehog. As
indicated, the same control loop repeats
along the A-P axis of the fly. (Based on
S. Dinardo et al., Curr. Opin. Genet. Dev.
4:529–534, 1994.)