alk8 is implicated in ventral specification. All embryos, unless stated otherwise, are genetically wild type. Embryos in A-D are at 36 hours post fertilization, lateral view, head to the left. (A) Uninjected control. (B) Strongly ventralized embryo (V3) after injection of alk8CA mRNA. Arrowhead indicates absent head region, arrow indicates enlarged blood islands. (C,D) Strongly dorsalized embryo (C4) after injection of alk8(K232R) mRNA (C) or alk8 antisense morpholino oligonucleotide alk8morph2 (D). Arrows indicate wound-up trunk. Note that the phenotype is much stronger than that of the laf mutant (see Fig. 3B). (E)Wild-type looking embryo after injection of the alk8 four-mismatch control antisense morpholino oligonucleotide 4mm-alk8morph2. (F-H) In situ hybridization for krox20 mRNA (Oxtoby and Jowett, 1993), staining rhombomeres 3 and 5; five-somite stage, lateral view, anterior towards the left, dorsal upwards. (E) Uninjected control; (F) injected with alk8morph2; (G) swirl/bmp2b mutant swrta72. alk8morph2-injected and bmp2b mutant embryo display a ventral fusion of both krox20 stripes (arrows), indicative of C5 dorsalization (compare with Dick et al., 2000).