The smad5 mutation somitabun blocks Bmp2b signaling during early dorsoventral patterning of the zebrafish embryo

Signaling by members of the TGF beta superfamily is thought to be transduce d by Smad proteins. Here, we describe a zebrafish mutant in smad5, designat ed somitabun (sbn). The dominant maternal and zygotic effect of the sbn(tc2 4) mutation is caused by a change in a single amino acid in the L3 loop of Smad5 protein which transforms Smad5 into an antimorphic version, inhibitin g wild-type Smad5 and related Smad proteins. sba mutant embryos are strongl y dorsalized, similarly to mutants in Bmp2b, its putative upstream signal. Double mutant analyses and RNA injection experiments show that sbn and bmp2 b interact and that sbn acts downstream of Bmp2b signaling to mediate Bmp2b autoregulation during early dorsoventral (D-V) pattern formation. Comparis on of early marker gene expression patterns, chimera analyses and rescue ex periments involving temporally controlled misexpression of bmp or smad in m utant embryos reveal three phases of D-V patterning: an early sbn- and bmp2 b-independent phase when a coarse initial D-V pattern is set up, an interme diate sbn- and bmp2b-dependent phase during which the putative morphogeneti c Bmp2/4 gradient is established, and a later sbn-independent phase during gastrulation when the Bmp2/4 gradient is interpreted and cell fates are spe cified.