Molluskan Dorsal–Ventral Patterning Relying on BMP2/4 and Chordin Provides Insights into Spiralian Development and Evolution

Although a conserved mechanism relying on BMP2/4 and Chordin is suggested for animal dorsal–ventral (DV) patterning, this mechanism has not been reported in spiralians, one of the three major clades of bilaterians. Studies on limited spiralian representatives have suggested markedly diverse DV patterning mechanisms, a considerable number of which no longer deploy BMP signaling. Here, we showed that BMP2/4 and Chordin regulate DV patterning in the mollusk Lottia goshimai, which was predicted in spiralians but not previously reported. In the context of the diverse reports in spiralians, it conversely represents a relatively unusual case. We showed that BMP2/4 and Chordin coordinate to mediate signaling from the D-quadrant organizer to induce the DV axis, and Chordin relays the symmetry-breaking information from the organizer. Further investigations on L. goshimai embryos with impaired DV patterning suggested roles of BMP signaling in regulating the behavior of the blastopore and the organization of the nervous system. These findings provide insights into the evolution of animal DV patterning and the unique development mode of spiralians driven by the D-quadrant organizer.