The similarities in anteroposterior and dorsoventral patterning genes as well as their conserved relative topological expression patterns and functional roles implicate a common genetic program underlying insect and mammalian nervous system development (Hirth and Reichert, 1999; Arendt and NUbler-Jung, 1999; Reichert and Simeone, 2001). This suggests that orthologous genes were already involved in neural specification in the insect and vertebrate stem species, if not already in a common bilaterian ancestor. Does this mean that the insect and chor-date CNS are homologous structures and therefore of monophyletic origin? Two alternative hypotheses, which are not mutually exclusive, can be envisaged. The first of these postulates that the ancestral bilaterian nervous system was already centralized and had its development governed by conserved genetic mechanisms that are still apparent in extant insects and mammals (monophyletic origin of the brain). The second hypothesis is that the ancestral bilaterian nervous system was controlled by conserved genetic mechanisms that still operate in arthropods and vertebrates, but that centralization of the nervous system occurred independently in protostome and deuterostome lineages (polyphy-letic origin of the brain).
Based on classical phylogeny, which places acoelomates, such as platyhelminthes, and pseudo-coelomates, such as nematodes, nearer to the base of the Bilateria than the coelomate protostomes and deuterostomes, the first hypothesis seems more likely (Figure 8a). Since flatworms and nematodes have a CNS with a brain and a ventral nerve cord, a comparable centralized nervous system would be likely to reflect the ancestral state for both Protostomia and Deuterostomia, and indeed for all Bilateria.
In this view, the evolutionary advance of centralizing the nervous system occurred only once. In contrast, molecular phylogenetic analyses no longer provide evidence that preferentially supports one of the two hypotheses. According to studies based on 18S rRNA sequence comparisons, there are no longer any living bilaterians that can be considered to be evolutionary intermediates between the radially (or biradially) symmetric animals and the bilaterally symmetric protostomes and deuterostomes (Figure 8b). Invertebrate lineages such as platyhel-minthes and nematodes, which were considered to be near the base of the bilaterian tree in classical phylogeny, are now placed next to protostome groups with highly complex body and brain morphology such as mollusks and arthropods in the two new protostome subgroupings, the lophotrochozo-ans and ecdysozoans (Adoutte et al., 2000). Thus, although neurons and nervous systems, which are present in radiate cnidarians and ctenophores, apparently existed before the origin of bilaterian animals,
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