Evolution__3rd_Edition

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Changes in arthropod limbs are
associated with ...
. . . changes in Hox gene
interactions
CHAPTER 20 / Evolutionary Developmental Biology 585
Hox genes are expressed in two phases during the development of fish fins. These
phases might, for instance, help to cause an outward growth of bones to form the fin. In
tetrapods, the Hox genes are also expressed in a third, later phase during limb development.
The third phase is associated with the further growth outwards of the limb bones,
to form the limb and hands. Thus, part of the mechanism by which fins may have
evolved into limbs may have been for certain Hox genes to be switched on for a third
time in the developing limb. Earlier in the chapter we met the concept of heterochrony
(Section 20.2), which was based on classic morphological research. Here we can see a
genetic example, in which a change in the timing of a developmental genetic process
leads to evolutionary change in morphology.
Morphological evolution may be caused by a change in which genes a Hox gene
interacts with. For example, insects differ from some other arthropods in lacking legs on
their abdomens. An insect has legs on its thorax and not its abdomen, but myriapods
and many crustaceans have abdominal legs. During evolution, leg development came
to be switched off in the embryonic insect abdomen. The genetic mechanism, simplified,
is that the Hox genes ultrabithorax (Ubx) and Abd-A are expressed down the abdomen
of insects, crustaceans, and myriapods. They are regional controllers of development.
In insects, Ubx and Abd-A repress the gene distal-less (Dll); Dll is the gene that directs
leg development. In myriapods and crustaceans, Ubx and Abd-A do not repress Dll.
Two hypotheses can explain events such as the loss of limbs from the insect
abdomen. One is a change in a transcription factor such as Ubx. In the evolution of
insects, Ubx may have changed such that it became able to repress the genes, such as Dll,
controlling limb development. The other hypothesis is that the enhancer of Dll may
have changed during insect evolution. The enhancer may have ceased to bind Ubx.
Alternatively, the enhancer may have continued to bind Ubx, but has changed its
interaction with it such that Ubx now switches off limb development in the abdomen
rather than switching it on. Some evidence supports the first hypothesis (Levine 2002).
Crustacean Ubx is unable to repress Dll in fruitflies. That result suggests that Ubx itself
has changed between crustaceans and insects. If Ubx were unchanged, crustacean Ubx
should have the same effect in fruitflies as normal fruitfly Ubx.
In summary, we have seen three developmental mechanisms that are thought to
have contributed to evolutionary changes in morphology. One is the change in the
spatial expression of genes. A second is the change in which genes are switched on or off
by transcription factors that have not themselves changed; this is achieved by changes
in enhancers. A third is the change in transcription factors, such that they change their
interactions with enhancers.
20.8 Evolution of genetic switches enables evolutionary
innovation, making the system more “evolvable”
The examples in the previous section illustrate how evolutionary changes in gene
regulatory networks can underlie morphological evolution. In the hoxc6 example, in
which the number of cervical vertebrae changed between mice and geese, the change
concerned the regulatory relations between the hoxc6 gene and some higher control