The TGF-beta superfamily comprises two distinct branches: the Activin/Nodal and BMP
pathways. During development, signaling by this superfamily regulates a variety of
embryological processes, and it has a conserved role in patterning the dorsal-ventral
body axis. Recent studies show that BMP signaling establishes the dorsal-ventral axis
in some mollusks. However, previous pharmacological inhibition studies in the annelid
Capitella teleta, a sister clade to the mollusks, suggests that the dorsal-ventral
axis is patterned via Activin/Nodal signaling. Here, we determine the role of both
the Activin/Nodal and BMP pathways as they function in Capitella axis patterning.
Antisense morpholino oligonucleotides were targeted to Ct-Smad2/3 and Ct-Smad1/5/8,
transcription factors specific to the Activin/Nodal and BMP pathways, respectively.
Following microinjection of zygotes, resulting morphant larvae were scored for axial
anomalies. We demonstrate that the Activin/Nodal pathway of the TGF-beta superfamily,
but not theBMP pathway, is the primary dorsal-ventral patterning signal in Capitella.
These results demonstrate variation in the molecular control of axis patterning across
spiralians, despite sharing a conserved cleavage program. We suggest that these findings
represent an example of developmental system drift.