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A
Hox expression
Motor neurons
3 day chick embryo
Midbrain
r1
Paralogs
r1
r2
Forebrain
r2
r3
r4
r5
r6
r7
r3
Trigeminal Nerve
1
Branchial arches
r4
r8
2
Facial Nerve
Abducens
nerve
r5
Limb bud
r6
3
Glossopharyngeal
nerve
r7
S20
4
S10
Somites
r8
Vagus Nerve
B
r1
r1
r2
r2
Trigeminal
r3
Trigeminal
r3
Facial
r4
rx
Facial
r5
Abducens
r6
r6
r7
r7
Glossopharyngeal
Glossopharyngeal
wild type
hoxa1 -/-
FIGURE 2.6 Rhombomeres are repeated morphological subdivisions of the hindbrain. A. The rhom-
bomeres are numbered from the anterior-most unit, r1 , just posterior to the midbrain (mesencephalon), to the
posterior most unit, r7 , at the junction of the hindbrain with the spinal cord. The members of the Hox gene
cluster are expressed in a 3¢ to 5¢ order in the rhombomeres. The segmentation in this region of the embryo is
also observed in the cranial nerves, and the motoneurons send their axons through defined points at alternat-
ing rhombomeres. B. Rhombomere identity is determined by the Hox code. Hox gene knockouts in mice affect
the development of specific rhombomeres. Wild-type animals have a stereotypic pattern of motoneurons in
the hindbrain. The trigeminal (V) cranial nerve motoneurons are generated from r2 and r3 , while the facial
nerve motoneurons are produced in r4 and the abducens motorneurons are produced by r5 . Deletion of the
Hoxa1 gene in mice causes the complete loss of rhombomere 5 and a reduction of rhombomere 4 (rx). The
abducens motoneurons are lost in the knockout animals, and the number of the facial motoneurons is reduced.
development of rhombomeres and the neurons they
produce (Lufkin et al., 1993; Gavalas et al., 2003).
Specifically, the rhombomere 4 domain is dramatically
reduced and does not form a clear boundary with
rhombomere 3. Rhombomere 5 is completely lost, or
fused with rhombomere 4, into a new region called
“rx.” The abducens motoneurons fail to develop in
these animals, and the facial motor neurons are also
defective. However, some of the neurons derived from
this region of the hindbrain now begin to resemble the
trigeminal motor neurons (Figure 2.6). Thus, when
Hoxa1 is lost from the hindbrain, rhombomere 4 and 5
are partly transformed to a rhombomere 2/3 identity.
Thus, at least at some level, the Hox genes of mice
appear to confer regional anterior-posterior identity on
a region of the nervous system in a manner similar to
the homeotic genes of Drosophila .
Earlier in this section, we showed a picture of an
arthropod that had no Hox genes; all segments were
essentially identical. Is this true of vertebrates? What
would the hindbrain look like without Hox genes?
Studies in both Drosophila and vertebrates have found
that the specificity of the Hox genes for promoters on
their downstream targets is significantly enhanced
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