Nuclear β-catenin plays crucial roles in the establishment of the embryonic axis and formation of mesendoderm tissues in ascidians and other animals. However, the cue responsible for nuclear accumulation of β-catenin in the vegetal hemisphere is still unknown in ascidians. Here, we investigated the roles of Wnt5α and Dsh in the nuclear accumulation of β-catenin and activation of its downstream genes in the ascidian Halocynthia roretzi. Wnt5α knockdown embryos lost nuclear accumulation of β-catenin at the 64-cell stage but not at the 32-cell stage, and expression of Hr-lim, one of the targets of β-catenin, was impaired in the anterior region of the embryo. Zygotic Wnt5α expression in the anterior-vegetal blastomeres was primarily responsible for these defects. Dsh knockdown showed no effect on nuclear localization of β-catenin, but inhibited Hr-lim expression in the posterior region. These results suggest that maintenance of nuclear Hr-β-catenin after the 64-cell stage is regulated by zygotic Hr-Wnt5α, and that expression of its target genes is modulated by both Hr-Wnt5α and Hr-Dsh. Our results also highlight the importance of nuclear accumulation of β-catenin up to the 32-cell stage through a still unclarified mechanism.
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