Fig. 8

Reciprocal expression and transcriptional repression between Sox3, β-catenin 2, Boz and Sqt. (A) Lateral and animal views, dorsal right, showing endogenous expression of boz (a), sqt (b), sox3 (c), sox19a (d) and sox19b (e). (B) Double in situ hybridisation for boz/sox3 (a) or sqt/sox3 (b). sox3 is detected with BCIP (pale blue), boz/sqt with BM purple. (C) Effect of S37A, Boz and Sqt on sox3 expression. Animal view of sox3 expression at 4.5 hpf, dorsoventral orientation cannot be certain. Endogenous sox3 expression (a) was repressed by injection of boz (b), sqt (c) or S37A β-catenin 2 (d) RNA. MOs directed against boz and/or sqt did not affect the ability of S37A β-catenin 2 RNA to inhibit the expression of sox3 (e-g). (D) Model for the role of Sox3 in restricting organizer formation. T-bars indicate mutual repression between Sox3 and β-catenin 2, boz and sqt. (E) Taking our experimental data together with the endogenous domains of expression illustrated above, this leads us to a model in which at 4-4.5 hpf, high levels of Wnt signalling on the dorsal side (yellow) together with vegetal Nodal signalling inhibit sox3 expression and induce expression of boz, which can also repress the expression of sox3 in the dorsal margin. By 5.5 hpf, sox3 is further depleted throughout the whole margin by Nodal signalling, allowing mesendoderm to form (green). Away from the margin, in the prospective ectoderm, Nodal signalling is too weak to repress sox3 expression and so Sox3 remains present (blue), inhibiting the expansion of mesoderm and organizer into this region (red bars). Scale bars: ∼200 μm.