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
1159
Figure 21–18 The origins of the Drosophila body segments. (A) At
3 hours, the embryo (shown in side view) is at the blastoderm stage and
no segmentation is visible, although a fate map can be drawn showing the
future segmented regions (color). (B) At 10 hours, all the segments are
clearly defined (T1: first thoracic segment; A1: first abdominal segment).
See Movie 21.3. (C) The segments of the Drosophila larva and their
correspondence with regions in the embryo. (D) The segments of the
Drosophila adult and their correspondence with regions in the embryo.
head
parts thorax abdomen
Three Groups of Genes Control Drosophila Segmentation Along
the A-P Axis
The body of an insect is divided along its A-P axis into a series of segments. The
segments are repetitions of a theme with variations: each segment forms highly
specialized structures, but all built according to a similar fundamental plan (Figure
21–18). The gradients of transcription regulators set up along the A-P axis in
the early embryo by the egg-polarity genes are the prelude to creation of the segments.
These regulators initiate the orderly transcription of segmentation genes,
which refine the pattern of gene expression to define the boundaries and ground
plan of the individual segments. Segmentation genes are expressed by subsets of
cells in the embryo, and their products are the first components that the embryo’s
own genome contributes to embryonic development; they are therefore called
zygotic-effect genes, to distinguish them from the earlier-acting maternal-effect
genes. Mutations in segmentation genes can alter either the number of segments
or their basic internal organization.
The segmentation genes fall into three groups according to their mutant phenotypes
(Figure 21–19). It is convenient to think of the three groups as acting in
sequence, although in reality their functions overlap in time. First to be expressed
is a set of at least six gap genes, whose products mark out coarse A-P subdivisions
of the embryo. Mutations in a gap gene eliminate one or more groups of adjacent
segments: in the mutant Krüppel, for example, the larva lacks eight segments. Next
comes a set of eight pair-rule genes. Mutations in these genes cause a series of
deletions affecting alternate segments, leaving the embryo with only half as many
segments as usual; although all the mutants display this two-segment periodicity,
they differ in the precise pattern. Finally, there are at least 10 segment-polarity
genes, in which mutations produce a normal number of segments but with a part
of each segment deleted and replaced by a mirror-image duplicate of all or part of
the rest of the segment.
In parallel with the segmentation process, a further set of genes—the homeotic
selector, or Hox, genes—serves to define and preserve the differences between one
segment and the next, as we describe shortly.
The phenotypes of the various segmentation mutants suggest that the segmentation
genes form a coordinated system that subdivides the embryo progressively
into smaller and smaller domains along the A-P axis, each distinguished
by a different pattern of gene expression. Molecular genetics has helped to reveal
how this system works.
(A) early embryo
T1
T2
(B) late embryo
(C) larva
ANTERIOR
(D) adult
1 mm
MBoC6 m22.26/22.19
0.2 mm
A9
T3 A1 A2 A3 A4 A5 A6 A7A8
POSTERIOR
A Hierarchy of Gene Regulatory Interactions Subdivides the
Drosophila Embryo
Like Bicoid, most of the segmentation genes encode transcription regulator proteins.
Their control by the egg-polarity genes and their actions on one another and
on still other genes can be deciphered by comparing gene expression in normal
and mutant embryos. By using appropriate probes to detect RNA transcripts or
their protein products, one can observe genes switch on and off in changing patterns.
By comparing these patterns in different mutants, one can begin to discern
the logic of the entire gene control system.
The products of the egg-polarity genes provide the global positional signals in
the early embryo (see Figure 21–17). The Bicoid protein, as we have seen, acts as