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Fig. 9. Hypothetical scenario for the evolution of the vertebrate hindbrain. In the
hypothetical common ancestor of the chordates, the neural tube is assumed to
be regionalized along the anteroposterior axis by the nested expression
patterns of the Hox genes. In the common ancestor of vertebrates, segmental
patterns of reticular neuron development are established in the hindbrain
region. The appearance of hindbrain segmentation results in repeated sets of
serially homologous reticular neurons. In support of this hypothesis,
reticular-like neurons are already present in amphioxus
(Fritzsch, 1996), although
this organism does not possess hindbrain segmentation. The anteroposterior
specification of branchiomotor neurons is already under the control of a Hox
code in the common ancestor. In vertebrates, the registering of hindbrain
segmentation and Hox code regulation appears in the gnathostome lineage, as
suggested by the analysis of the lamprey branchiomotor neuron spatial pattern
and Hox regulation. In amniotes, large interneurons have been lost, together
with the overt serial homology of these neurons. This scenario of vertebrate
hindbrain evolution postulates independent mechanisms for neuronal patterning,
established as evolutionary events distinct from hindbrain segmentation
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