Fig. 6.

Symmetry break and asymmetric expression of BMP-like genes and BMP antagonists in Nematostella embryos. (A,B) Early gastrula stage (oral view) showing radial expression of a BMP antagonist, the Nematostella homolog chordin (chd, A), and of the Nematostella BMP2 homolog dpp (B). (C) Double in situ hybridization of chordin and dpp showing that during the mid-gastrula stage, a symmetry break occurs and both genes become expressed on the same side of the blastopore. (D,E) During the planula stage, expression of chordin remains lateral to the blastopore (D), whereas dpp is largely expressed in an endodermal stripe and in a spot at the border of the blastopore (E), on the side of chordin expression. (F) Double in situ hybridization of chordin and dpp in a planula larva showing that both genes remain expressed asymmetrically, on the same side, but segregate with respect to ectoderm and endoderm. Asterisks mark the blastopore. Scale bar: 100 μm. (G) Schematic of the planula stage illustrating the asymmetric expression of chordin and dpp on one side, and of gdf5-like, a member of the BMP family, and of gremlin, a BMP antagonist, on the opposite side. Note that a number of other genes (not shown for clarity) are also expressed asymmetrically, indicative of a directive axis. (H) Double negative-feedback loop between Dpp and Chordin as suggested by morpholino-mediated gene knockdown experiments (Saina et al., 2009). Images in A-G are reproduced with permission (Rentzsch et al., 2006).