Six3/6 acts downstream of canonical Wnt signaling to regulate anterior neural border-derived sensory organ formation in ascidian embryos

Three protruding papillae at the anteriormost region of the ascidian Ciona intestinalis larva constitute an adhesive, mechano- and chemo-sensory organ, the palps, essential for settlement and metamorphosis. The palps derive from the anterior neural plate border whose formation 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 abolished palp formation. Reciprocally, inhibiting Wnt led 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 the other deuterostomes, Six3/6 is expressed in an equivalent anterior domain away from posteriorizing Wnt activity. Consequently, 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.