Regulation of anterior neurectoderm specification and differentiation by BMP signaling in ascidians

Three palps make the most anterior structure of the ascidian larva. These ectodermal derivatives have both a sensory and adhesive functions essential for metamorphosis. They derive from the anterior neural border and their formation is regulated by signaling pathways such as FGF and Wnt. Since they also share gene expression profiles with vertebrate anterior neural tissue and cranial placodes, their study should shed light on the emergence of the unique vertebrate telencephalon. Here, we show that BMP signaling regulates two phases of palps formation inCiona intestinalis. During gastrula stages, the anterior neural border marked byFoxcis specified in a domain of inactive BMP signaling, and activating BMP prevented its formation. Later on, inhibiting BMP led to the formation of a single large palp, most likely of dorsal identity. Our results indicate that BMP signaling regulates papillavsinter-papilla fate decision within the palps forming region. Finally, we showed that modulating BMP signaling led to similar palps phenotypes in another ascidian speciesPhallusia mammillata. This led us to screen transcriptomic data and identify novel palps markers. Collectively, we provide a better molecular description of palps formation in ascidians that will be instrumental for comparative studies within ascidians and between ascidians and other chordates.