Tunicate-specific protein Epi-1 is essential for conferring hydrophilicity to the larval tunic in the ascidian Ciona

Animals must avoid adhesion to objects in the environment to maintain their mobility and independence. The marine invertebrate chordate ascidians are characterized by an acellular matrix tunic enveloping their entire body for protection and swimming. The tunic of ascidian larvae consists of a surface cuticle layer and inner matrix layer. Hydrophilic substances coat the cuticle; this modification is thought to be for preventing adhesion. However, the molecule responsible for regulating this modification has not been clarified. We here found that the tunicate-specific protein Epi-1 is responsible for preventing adhesiveness of the tunic in the ascidian Ciona intestinalis Type A. Ciona mutants with homozygous knockouts of Epi-1 exhibited adhesion to plastic plates and to other individuals. The cuticle of the Epi-1 mutants was fragile, and it lost the glycosaminoglycans supplied by test cells, the accessory cells that normally attach to the tunic surface. Although it has an apparent signal peptide for membrane trafficking, we showed that the Epi-1 protein is localized to the cytosol of the epidermal cells. Our study demonstrated that the emergence of the tunicate-specific protein Epi-1 accelerated the immediate ancestor of tunicates to evolve a tunic by providing a way to avoid the adhesiveness of this structure.