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

1170 Chapter 21: Development of Multicellular Organisms
Drosophila
Hox complex
ancestral
Hox complex
mammalian
Hox complex
HoxA
HoxB
HoxC
HoxD
anterior
Lab
Bcd,
Pb Zen Dfd Scr (Ftz) Antp Ubx AbdA AbdB
Hox1 Hox2 Hox3 Hox4 Hox5 Hox6 (central) Hox7 (posterior)
A1 A2 A3 A4 A5 A6 A7 A9 A10 A11 A13
B1 B2 B3 B4 B5 B6 B7 B8 B9
B13
C4 C5 C6
C8 C9 C10 C11 C12 C13
D1 D3 D4 D8 D9 D10 D11 D12 D13
hindbrain
spinal cord
posterior
Figure 21–32 The Hox complexes of an
insect and a mammal, compared and
related to body regions. The genes of the
Antennapedia and Bithorax complexes of
Drosophila are shown in their chromosomal
order in the top line. The corresponding
genes of the four mammalian Hox
complexes are shown below, also in
chromosomal order. The gene expression
domains in fly and mammal are indicated
in a simplified form by color in the cartoons
of animals above and below. There is a
remarkable parallelism. However, the details
of the patterns depend on developmental
stage and vary somewhat from one
mammalian Hox complex to another. Also,
in many cases, genes shown here as
expressed in an anterior domain are also
expressed more posteriorly, overlapping the
domains of more posterior Hox genes.
The complexes are thought to have
evolved as follows: first, in some common
ancestor of worms, flies, and vertebrates,
a single primordial homeotic selector gene
underwent repeated duplication to form
a series of such genes in tandem—the
ancestral Hox complex. In the Drosophila
sublineage, this single complex became
split into separate Antennapedia and
Bithorax complexes. Meanwhile, in the
lineage leading to the mammals, the whole
complex was repeatedly duplicated to give
four Hox complexes. The parallelism is not
perfect because apparently some individual
genes have been duplicated and others
lost. Still others have been co-opted for
different purposes (genes in parentheses in
the top line) over the time that has elapsed
since the complexes diverged. (Based on a
diagram courtesy of William McGinnis.)
anterior
posterior
mesoderm
a posterior character. Conversely, loss of posterior Hox genes allows the posterior
tissue where they are normally expressed to adopt an anterior character (Figure
21–33). Because of a redundancy between genes in the four Hox gene clusters,
the transformations observed in mouse Hox mutants are not always so straightforward
as those in the fly, and they are often incomplete. Nonetheless, it seems
clear that the fly and the mouse use essentially the same molecular machinery to
impart individual characteristics to successive regions along at least a part of the
A-P axis.
MBoC6 m22.46/22.32
Some Transcription Regulators Can Activate a Program That
Defines a Cell Type or Creates an Entire Organ
Just as there are genes that regulate pattern formation and segmental identity,
there are genes whose products act as triggers for the development of a specific
cell type or even a specific organ, initiating and coordinating the whole complex
program of gene expression that is required. An example is the MyoD/myogenin