Fig. 4. The timing, extent and localization of expression of Xnrs is regulated by the Wnt pathway and is necessary for inducing dorsal mesoderm in equatorial cells. (A) Real-time RT-PCR to show the relative levels of expression of the nodal genes Xnr1, 2, 3 and 4 in dorsal and ventral halves of wild-type and ß-catenin^– embryos frozen at the late blastula and early gastrula stages (9, 9.5 and 10). The dorso-ventral pattern of expression of Xnr1 seen in wild-type halves, is lost in ß-catenin^– embryos. ODC is used as a loading control (data not shown), and each bar is normalized to the level of ODC. (B) The design of the experiment shown in C. Equators from wild-type and ß-catenin^– mid-blastula stage embryos were co-cultured with with ß-catenin^– vegetal masses from mid-blastulae (ß-cat^–/ß-cat^– early) or early gastrula stage (ß-cat^–/ß-cat^– late). Controls were mid blastula ß-catenin^– equators co-cultured with wild-type mid-blastula vegetal masses (ß-cat^–/WT). The co-cultures were then separated and equators were cultured until siblings reached the late neurula stage and they were then frozen for analysis. (C) Real-time RT-PCR to show the relative levels of expression of MyoD in equators from the experiment described in B. ODC is used as a loading control (data not shown), and each bar is normalized to the level of ODC.