<i>Six3/6</i> acts downstream of canonical Wnt signaling to regulate the formation of anterior neural border-derived structures in ascidian embryos

Three protruding papillae at the anterior-most region of larva of the ascidian Ciona intestinalis constitute an adhesive, mechanosensory and chemosensory organ, known as the palps, essential for settlement and metamorphosis. The palps derive from the anterior neural plate border, formation of which is regulated through the combined actions of FGF, BMP and Wnt signaling pathways before neurulation. Here, we show that, following the formation of this precursor territory, canonical Wnt signaling acts again to regulate palp formation. Activating Wnt during neurulation abolishes palp formation. Reciprocally, inhibiting Wnt leads to the formation of an ectopic fourth palp-like structure posteriorly. Moreover, we show that Six3/6, expressed posteriorly to the palps in the anterior brain and oral siphon, is activated by Wnt and likely mediates its action. In other deuterostomes, Six3/6 is expressed in an equivalent anterior domain away from posteriorizing Wnt activity. We conclude that Wnt regulates the size of the anterior neurectoderm in Ciona as in other deuterostomes, but in a different manner. Deciphering changes in Wnt-mediated transcriptional regulation at the invertebrate/vertebrate transition is crucial for understanding the evolution of anterior nervous system patterning and the emergence of vertebrate unique characters.