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

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... fering 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.