The conserved expression and function of otd/Otx and Hox genes suggest that invertebrate and vertebrate brains are all characterized by a rostral region specified by genes of the otd/Otx family and a caudal region specified by genes of the Hox family. However, in ascidians and vertebrates, a Pax2/5/8 expression domain is located between the anterior Otx and the posterior Hox expression regions of the embryonic brain (Holland and Holland, 1999; Wada and Satoh, 2001). In vertebrate brain development, this Pax2/5/8 expression domain is an early marker for the isthmic organizer positioned at the midbrain-hindbrain boundary (MHB), which controls the development of the midbrain and the anterior hindbrain (Liu and Joyner, 2001; Rhinn and Brand, 2001; Wurst and Bally-Cuif, 2001). The central role of this MHB region in brain development together with the conserved expression patterns of Pax2/5/8 genes in this region have led to the proposal that a fundamental characteristic of the ancestral chordate brain was its tripartite organization characterized by Otx, Pax2/5/8, and Hox gene expressing regions (Wada et al., 1998).
An analysis of brain development in Drosophila has uncovered similarities in the expression and function of the orthologous genes that pattern the vertebrate MHB region (Hirth et al., 2003). Thus, a Pax2/5/8 expressing domain was found to be located between the anterior otd/Otx expressing region and the posterior Hox expressing region in the embryonic brain. In Drosophila, as in vertebrates, this Pax2/5/8 expressing domain is positioned at the interface between the otd/Otx2 expression domain and a posteriorly abutting unplugged/ Gbx2 expression domain. Moreover, inactivation of otd/Otx or of unplugged/Gbx2 results in comparable effects on mispositioning or loss of brain-specific expression domains of orthologous genes in both embryonic brain types. These developmental genetic similarities indicate that the tripartite ground plan, which characterizes the developing vertebrate brain, is also at the basis of the developing insect brain (Figure 6). This, in turn, has led to the suggestion that a corresponding, evolutionarily conserved, tripartite organization also characterized the brain of the last common ancestor of insects and chordates (Hirth et al., 2003).
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