alk8 acts downstream of bmp2b and bmp7 and upstream of smad5. All embryos are shown at 36 hours post-fertilization, lateral view, head towards the left. The laf, swr and snh embryos shown in panels C-F,K,L were genotyped after photography. (A) Wild-type sibling. (B) laftm110 mutant. Arrows indicate absent ventral tail fin and enlarged pericardial cavity. (C) Rescued laftm110 mutant, injected with alk8 mRNA. (D) Rescued laftm110 mutant, injected with smad5 mRNA; arrow indicates starting swelling of the precardial cavity. (E) laftm110 mutant, injected with bmp2b mRNA. (F) laftm110 mutant, injected with bmp7 mRNA. Arrowheads indicate small head and enlarged blood islands, both signs for ventralization; arrows indicate absent ventral tail fin and enlarged heart cavity, both laf-characteristic features (compare with B). (G-J) Genetically wild-type embryos after injection with bmp2b mRNA (G), co-injection of bmp2b mRNA and the alk8 antisense morpholino oligonucleotide alk8morph2 (H), injection of smad5 mRNA (I), or co-injection of smad5 mRNA and alk8morph2 (J). The embryos in G,I,J display strong ventralization (V3; arrowheads indicate absent head and enlarged blood island); the embryo in H shows strong dorsalization (C4; arrow indicates wound-up trunk), similar to the alk8morph2-injected embryo shown in Fig. 2D. (K) Rescued bmp2b mutant swrta72, injected with alk8CA mRNA. (L) Rescued bmp7 mutant snhty68, injected with alk8CA mRNA. Arrows in K,L indicate reduced or absent ventral tail fin, indicating mild dorsalization (C1); arrowheads indicate smaller head, indicating weak ventralization. (M,N) Sibling embryos from sbndtc24 heterozygous mother; displaying C4 dorsalization, characterized by wound-up tail and trunk (arrows); (M) uninjected embryo; (N) alk8CA mRNA-injected embryo.