Islet is a key determinant of ascidian palp morphogenesis

The anterior-most ectoderm of ascidian larvae contains the adhesive papillae, or palps, which play an important role in triggering the metamorphosis of swimming tadpoles. In Ciona intestinalis, the palps consist of three conical protrusions within a field of thickened epithelium that form late in embryogenesis, as tailbuds mature into larvae. The palp protrusions express the LIM-homeodomain transcription factor Islet. Protrusion occurs through differential cell elongation, probably mediated by Islet, as we find that ectopic expression of Islet is sufficient to promote cell lengthening. FGF signaling is required for both Islete xpression and palp morphogenesis. Importantly, we show that Islet expression can rescue the palpdeficient phenotype that results from inhibition of FGF signaling. We conclude that Isletis a key regulatory factor governing morphogenesis of the palps. It is conceivable that Isletis also essential for the cellular morphogenesis of placode-derived sensory neurons in vertebrates.