Patterning of an ascidian embryo along the anterior–posterior axis through spatial regulation of competence and induction ability by maternally localized PEM

Abstract

In ascidian embryos, anterior–posterior (A–P) patterning of the vegetal cells is regulated by posteriorizing activities of a localized egg region known as posterior-vegetal cortex/cytoplasm (PVC). PEM is an essential component of the PVC and is involved in the posterior-specific cell cleavage pattern. Here we report a novel function of PEM independently of its function in cleavage regulation; it controls cell fate by excluding competence to respond to the FGF signal for notochord induction from posterior-vegetal cells. PEM was found to regulate the nuclear accumulation of β-catenin, an upstream activator of the competence factor. PEM also influences A–P patterning in the animal hemisphere. It was found to regulate FGF signal expression and restrict the occurrence of brain induction only in the anterior region. Our results suggest a model in which PEM patterns the embryo along the A–P axis through regulation of the spatial distribution of competence and induction ability.