Practice question

β-catenin nuclear entry is absolutely required for endomesoderm specification in vegetal hemisphere of sea urchin embryo. Vegetal Disheveled normally blocks GSK-3β mediated degradation, allowing TCF-dependent transcription of Pmar1 and downstream endodermal genes such as FoxA, GataE and Brachyury. If nuclear accumulation experimentally prevented by overexpressing GSK-3β, dominant-negative TCF, or sequestering β-catenin in cytoplasm, vegetal gene regulatory network never initiates. Embryos fail to gastrulate, form no gut or mesenchyme, and develop as hollow ciliated balls composed entirely of animal-like ectoderm, called animalized or dauerblastula phenotype. This demonstrates necessity of β-catenin.