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

Abstract

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 in Ciona intestinalis. During gastrula stages, the anterior neural border marked by Foxc is 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 papilla vs inter-papilla fate decision within the palps forming region. Finally, we showed that modulating BMP signaling led to similar palps phenotypes in another ascidian species Phallusia 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.

Molecular control of cellulosic fin morphogenesis in ascidians

“…To improve tunic and fin blades formaBon amer dechorionaBon, embryos were cultured at 22°C (P. mammillata) or 19°C (C. intes=nalis) in the presence of 0.1% BSA on 1% agarose-coated dishes. Dechorionated embryos were treated with 2.5 µM of the BMP receptor inhibitor DMH1 and with 150 ng/ml of recombinant BMP2 protein from early gastrula stages (St.10) as previously described 22,28 . In vivo transcripBonal assays were performed as described 23,25 .…”

“…Chromogenic in situ hybridizaBon in the different ascidian species were performed using the protocol described earlier 23,28 with dig-labeled RNA probes described in Table S1.…”

“…Median fin blades elongate above a specific epidermal cell populaBon, the tail neurogenic midlines (Fig 2A -C). We first inhibited the BMP signaling pathway that is necessary for ventral tail midline fate acquisiBon 20, 23,28 . Strikingly, treated larvae had no ventral fin (DMH1: 71% of larvae with median fin had only a dorsal fin, n=188; DMSO/BSA-control: 100% of larvae with median fin had both dorsal and ventral fins, n=181; results from four independent experiments; Fig 2D,E).…”