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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(A) marine stickleback
pelvic spine
(B)
Pitx1 expression
Figure 21–37 Morphological diversity
in stickleback fish is caused by
changes in regulatory elements.
(A–D) Pelvic spines are present in marine
(A) but not in freshwater (C) populations.
Correspondingly, Pitx1 is expressed in
the pelvic area in marine (B) but not in
freshwater (D) fish. The lack of expression
in the pelvic area of freshwater populations
is caused by mutations in an enhancer
element. Other enhancers and sites of
expression for Pitx1 are the same in marine
and freshwater sticklebacks. (Courtesy of
Michael D. Shapiro.)
(C) freshwater stickleback
(D)
freshwater sticklebacks have lost this expression as a result of a change at the Pitx1
locus. These changes do not lie in the coding sequence. Instead, each is a small
deletion of a block of adjacent regulatory DNA that controls Pitx1 expression specifically
in the pelvic cells (Figure 21–37).
The Pitx1 protein has important
MBoC6
functions
n2.218/22.38
elsewhere in the body, so that the
DNA sequences that encode this protein must be retained. The regulatory DNA
responsible for Pitx1 expression at these other sites is also unchanged in the two
populations of sticklebacks. The evolution of pelvis development in sticklebacks
shows how the modular nature of regulatory DNA elements that we encountered
in Chapter 7 (see Figure 7–29) allows independent modification of the different
parts of the body, even when formation of those body parts depends on the same
proteins.
In the recent evolution of plants, changes of body structure can be traced in
a similar way to changes in regulatory DNA. For example, these account for a
large part of the dramatic difference between the wild teosinte plant and its modern
descendant, maize, through some 10,000 years of mutation and selection by
Native Americans.
Summary
Drosophila has been the foremost model organism for the study of the genetics of
animal development. Its embryonic pattern is initiated by the products of maternal-effect
genes called egg-polarity genes, which operate by setting up graded distributions
of transcription regulators in the egg and early embryo. The gradient of
Bicoid protein along the A-P axis, for example, helps initiate the orderly expression
of gap genes, pair-rule genes, and segment-polarity genes. These three classes of segmentation
genes, through a hierarchy of interactions, become expressed in some
regions of the embryo and not others, progressively subdividing the embryo along
the A-P axis into a regular series of repeating modular units called segments.
Superimposed on the pattern of gene expression that repeats itself in every segment,
there is a serial pattern of expression of Hox genes that confer on each segment
a different identity. These genes are grouped in complexes and are arranged in
a sequence that matches their sequence of expression along the A-P axis of the body.
Although Hox gene expression is initiated in the embryo, it is subsequently maintained
by the action of chromatin-binding proteins of the Polycomb and Trithorax
group, which stamp the chromatin of the Hox complex with a heritable record of
its embryonic state of repression or activation, respectively. Hox complexes homologous
to that of Drosophila are found in virtually every type of animal, where they
help pattern the A-P axis of the body.
Signaling gradients are also set up along the dorsoventral (D-V) axis. Initially,
Toll signaling generates a nuclear gradient of Dorsal protein, which induces an
extracellular signaling gradient of the TGFβ-family protein Dpp and its antagonist,