Biology Reference
In-Depth Information
4.14.2.2 Pair-Rule Genes
The
pair-rule genes
were so named because mutant flies have a repetitive aber-
ration throughout the germ band, with the removal of integral, alternate seg-
ment-width areas (
Figure 4.7
). The pair-rule genes are transiently expressed in
seven or eight stripes during cellularization of the blastoderm. However, each is
otherwise unique in its expression. Pair-rule genes are essential, directly or indi-
rectly, for the initial establishment of segmentation.
4.14.2.3 Segment-Polarity Genes
The
segment-polarity genes
seem to determine anterior-posterior domains
within each segment (
Figure 4.7
). Segment-polarity genes are required either
continuously or over extensive periods to maintain the segmentation pattern.
Most or all are required to maintain patterns in the imaginal tissues.
4.14.2.4 Homeotic Genes
In 1894, William Bateson coined the word
homeosis
to describe the situation in
which “something has been changed into the likeness of something else” (
Lewis
1994
). Bateson was attempting to provide evidence in support of Darwin's the-
ory of evolution and homeotic variations seemed to Bateson to be the kind of
dramatic changes that could explain how evolution occurred.
E. B. Lewis (1994)
concluded that homeosis provided a rich legacy: “Besides giving us the homeo-
box, it has opened up a completely new approach to the study of development.
And over the past 15 years, it as led to the realization that the body plan of
most animals, and presumably of plants as well, is controlled by a set of master
regulatory genes, first identified by their homeotic mutations.”
The periodic pattern of body segments generated by segmentation genes
(gap genes, pair-rule genes, and segment-polarity genes) has to be converted
into segments with wings, legs, and antennae (
Figure 4.7
). Thus, in insects, tho-
racic segment 2 is different from thoracic segment 3 and abdominal segment 2
will be different from the terminal abdominal segments, which typically have
genital structures. This fine-tuning is determined by
homeotic
or
Hox genes
.
Hox genes contain a special sequence called the
homeobox
. The homeobox con-
sists of
≈
180bp that is translated into a 60-amino-acid domain. The sequences
of the different homeoboxes are nearly identical and they mediate the binding
of homeotic proteins to specific DNA sequences and thus regulate the expres-
sion of many downstream genes. It has been proposed that just two homeotic
genes,
even-skipped
+
and
Fushi tarazu
+
, directly control the expression of the
majority of genes in the
Drosophila
genome (
Mannervik 1999
). Homeodomain
proteins occur in all eukaryotes, where they perform important functions during
development.
