Practice question

Vegetal micromeres in sea urchin inherit high levels of Dishevelled and consequently nuclear beta-catenin due to vegetal cortex localization established before fertilization. Beta-catenin activates endomesoderm gene regulatory network including Pmar1, which represses HesC repressor, allowing expression of Wnt8, blimp1b, krox, and Delta ligand. These cells become primary mesenchyme producing larval skeleton and act as organizers inducing adjacent macromeres to become endoderm forming gut and secondary mesenchyme pigment cells. Without beta-catenin accumulation, embryos become animalized ciliated balls lacking mesodermal skeleton and endodermal gut, demonstrating requirement for mesoderm and endoderm induction.