Practice question

Vegetal T-box factor VegT induces Nodal-related genes in vegetal endoderm via Smad-activated enhancers, while nuclear beta-catenin on dorsal side binds TCF to activate Siamois. Siamois and VegT cooperatively bind Xnr enhancers, synergistically upregulating Xnr1,2,5,6 on dorsal side highest, ventral side moderate, establishing gradient. BMP and Activin also modulate but transcriptional activation fundamentally requires both beta-catenin dorsal cue and VegT mesendoderm competence factor. Double knockdown eliminates mesoderm entirely. Thus intersection explains dorsal-high Nodal gradient establishing organizer position and patterning mesoderm along dorsoventral axis.