Fig. 3. Increased WntD expression blocks ventral invagination by interfering with twist and snail expression. The panels in the left column, except panel I, are wild-type embryos. The panels in the right column are embryos expressing WntD by the nanos-Gal4-UAS system. The in situ hybridization probes used are indicated on the left. Panels C and D are cross-sections with dorsal side up, and all other panels are ventral views with anterior to the left. Arrow indicates ventral furrow; arrowhead indicates cephalic furrow. (A,B) The overall expression level of wntD was higher in nanos-Gal4-UAS-wntD embryos than in wild-type embryos and the expression was ubiquitous. The pictures were underexposed to show the cell morphology. The embryo in panel B had no ventral furrow, whereas the cephalic furrow appeared normal. (C,D) Cross-sections of gastrulating embryos showing that no mesoderm was formed during gastrulation in embryos overexpressing WntD. (E,F) The twist expression pattern was much reduced in embryos overexpressing WntD. In wild-type embryos, twist expression is approximately 22 cells wide along the dorsoventral axis at the onset of gastrulation. The embryo shown in E already had some of the cells invaginated. (G) A wild-type embryo showing the normal snail pattern. (H-J) The panels show the reduced snail pattern with increasing severity in embryos overexpressing WntD. Some embryos showed narrower patterns of expression whereas others showed no expression in the anterior regions. (K-N) WntD overexpression also causes sim and rhomboid to show abnormal expression patterns. In wild-type embryos, the expression of sim and rhomboid in the ventral cells is repressed by Snail. Moreover, the positioning of sim also requires Snail. Thus, the abnormal patterns of sim and rhomboid in panels L and N correlate well with the reduced snail pattern.