![]() ![]() One important consequence is that traits homologous in arthropods and vertebrates must have been present in the common ‘urbilaterian’ ancestor from which protostomes and deuterostomes diverged. Evidence from the new molecular-based phylogeny implicates the absence of intermediate taxa at the basis of protostome–deuterostome lineage separation ( Figure 1) ( Adoutte et al, 2000). The question of whether the last common ancestor of bilaterians had an anatomically complex central nervous system (CNS) is controversial. Taken together, these findings demonstrate conserved genetic patterning mechanisms in insect and vertebrate brain development and suggest a monophyletic origin of the brain in protostome and deuterostome bilaterians. This supports the notion that a dorsoventral body axis inversion occurred after the separation of protostome and deuterostome lineages in evolution. Homologous genes involved in dorsoventral regionalization of the CNS in vertebrates and insects show remarkably similar patterning and orientation with respect to the neurogenic region (ventral in insects and dorsal in vertebrates). Furthermore, cross-phylum rescue experiments demonstrate that insect and mammalian members of the otd/Otx and ems/Emx gene families can functionally replace each other in embryonic brain patterning. A third intermediate domain on the anteroposterior axis of the vertebrate and insect brain is characterized by the expression of the Pax2/5/8 orthologues, suggesting that the tripartite ground plans of the protostome and deuterostome brains share a common evolutionary origin. Thus, in both insects and vertebrates, the correct regionalization and neuronal identity of the anterior brain anlage is controlled by the cephalic gap genes otd/Otx and ems/Emx, whereas members of the Hox genes are involved in patterning of the posterior brain. Recent molecular genetic analyses of Drosophila melanogaster and mouse central nervous system (CNS) development revealed strikingly similar genetic patterning mechanisms in the formation of the insect and vertebrate brain. ![]()
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