380 CHAPTER 15A profoundly important and most surprising discovery was that the Hox genes
described in Drosophila are also present in mammals, and some of them occur in
all animal phyla except sponges. This family of genes evolved in the ancestor of
almost all animals, more (probably much more) than 540 million years ago. In
some groups, the entire array of Hox genes has been duplicated: mammals and
other tetrapods, for example, have four sets, although each set lacks one or more of
the genes (FIGURE 15.11). To a large degree, these genes play similar developmen-
tal roles in the various phyla. For example, they specify features along the anterior-
posterior axis of a mammal embryo just as they do in insects. This functional role
of the Hox genes has been phylogenetically conserved.
However, it was soon discovered that in addition to their highly conserved func-
tion, Hox genes and other genes that encode transcription factors can play many
other roles. For example, Ubx is expressed in the developing hind leg of some spe-
cies of Drosophila but not others, and is associated with the presence or absence of
unicellular hairs on part of the leg. High expression suppresses the development
of hairs—a radically different effect of a gene that affects the form of entire body
segments! In water striders, insects that skate on the surface of water, Ubx controls
the length of the middle legs, which are used as propelling oars [58]. The Ubx tran-
scription factor can play diverse roles because it can bind the enhancers of diverse
genes. For this reason, Ubx and some other genes that encode transcription factors
are like a hammer or wrench that can be used for a wide range of different tasks.
Sean Carroll referred to such genes as a genetic toolkit that is shared widely among
animals, and can contribute to evolutionary changes in the regulation of diverse
genes with diverse developmental roles [9]. The effect of Ubx on water strider legs
and Drosophila hairs represents the evolution of a novel use of a preexisting gene
for a new function: it is an example of exaptation (see Chapter 3), or co-option [78].
In some instances, genetic pathways, not just single genes, have been co-opted
in evolution. For example, a subset of the Hox genes that determine the anterior-
posterior differentiation of the vertebrate body (see Figure 15.11) is expressed in the
limbs (FIGURE 15.12A). These genes are expressed from the base to the tip of the
developing limb in the same sequence as their anterior-posterior expression along
the body axis (FIGURE 15.12B), and they determine the proximal-distal differen-
tiation of the limb (e.g., humerus to radius to digits). The same principles apply to
plants. Most species in the potato family (Solanaceae) express the M ADS16 gene,
which encodes a transcription factor only in vegetative tissues, where it affects
cell shape and division rate. In the genus Physalis, known as ground cherries or
Chinese lantern plants, the gene is heterotopically expressed in the sepals after
pollination, and causes these flower parts to grow into a “balloon” that envelops
the fruit (FIGURE 15.13Futuyma Kirkpatrick ) [27]. Evolution, 4e
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Troutt Visual Services
Evolution4e_15.11.ai Date 02-08-17lab pb Dfd Scr Antp Ubx abdA AbdBMouse^1
Hoxb genes2 3 4 5Mouse
embryo6 7 8 9–13MidbrainForebrainHindbrain Spinal cordCervicalLumbarThoracicDrosophila
Hox genes
FIGURE 15.11 Segment-specific patterning functions of Hox
genes in the vertebrate hindbrain and spinal cord. In this sche-
matic diagram of a mouse embryo, the black horizontal bars
indicate segmental patterns of Hoxb gene expression, with
darker color corresponding to areas of relatively high gene
expression. The double-headed arrows connect the genes in
the Hoxb cluster to the homologous Hox genes in Drosophila.
(After [44].)15_EVOL4E_CH15.indd 380 3/22/17 1:30 PM