The transient but pronounced ventral tail bending is found in many chordate embryos and constitutes an interesting model of how tissue interactions control embryo shape (Lu et al., 2020). Here, we identify one key upstream regulator of ventral tail bending in the ascidian Ciona embryo. We show that during early tailbud stage, ventral epidermal cells exhibit a boat-shaped morphology (boat cell) with a narrow apical surface where phosphorylated myosin (pMLC) accumulated. We further show that interfering with the function of the BMP ligand Admp leads to pMLC localizing to the basal instead of the apical side of ventral epidermal cells and a reduced number of boat cells. Finally, we show that cutting ventral epidermal midline cells at their apex using a ultraviolet laser relaxes ventral tail bending. Based on these results, we propose a novel function for Admp in localizing pMLC to the apical side of ventral epidermal cells, which causes the tail to bend ventrally by resisting antero-posterior notochord extension at the ventral side of the tail.

Chordate tailbud embryos have similar morphological features, including a bending tail. A recent study revealed that the actomyosin of the notochord changes the contractility and drive tail bending of the early Ciona tailbud embryo. Yet, the upstream regulator of tail bending remains unknown. In this study, we find that Admp regulates tail bending of Ciona mid-tailbud embryos. Anti-pSmad antibody signal was detected at the ventral midline tail epidermis. Admp knock-down embryo completely inhibited the ventral tail bending and reduced the number of the triangular-shaped cells, which has the apical accumulation of the myosin phosphorylation and inhibited specifically the cell-cell intercalation of the ventral epidermis. The degree of myosin phosphorylation of the ventral cells and tail bending were correlated. Finally, the laser cutter experiments demonstrated the myosin-phosphorylation-dependent tension of the ventral midline epidermis during tail bending. We conclude that Admp is an upstream regulator of the tail bending by controlling myosin phosphorylation and its localization of ventral epidermal cells. These data reveal a new aspect of the function of the Admp that might be evolutionarily conserved in bilaterian animals.