Cross-phylum regulatory potential of the ascidian Otx gene in brain development in Drosophila melanogaster

The origin of molecular mechanisms of cephalic development is an intriguing question in evolutionary and developmental biology. Ascidians, positioned near the origin of the phylum Chordata, share a conserved set of anteropost erior patterning genes with vertebrates. Here we report the cross-phylum re gulatory potential of the ascidian Otx gene in the development of the Droso phila brain and the head vertex structures. The ascidian Otx gene rescued t he embryonic brain defect caused by a null mutation of the Drosophila ortho denticle (otd) gene and enhanced rostral brain development while it suppres sed trunk nerve cord formation. Furthermore, the ascidian Otx gene restored the head vertex defects caused by a viable otd mutation, ocelliless, via s pecific activation and repression of downstream regulatory genes, These cro ss-phylum regulatory potentials of the ascidian Otx gene are equivalent to the activities of the Drosophila and human otd/Otx genes in these developme ntal processes. These results support the notion that basal chordates such as ascidians have the same molecular patterning mechanism for the anterior structures found in higher chordates, and suggest a common genetic program of cephalic development in invertebrate, protochordate and vertebrate.