Dpp/BMP2-4 Mediates Signaling from the D-Quadrant Organizer in a Spiralian Embryo

In some animal groups, the secondary embryonic axis is patterned by a small group of cells, often called an organizer, that signals to other cells to establish the correct pattern of cell fates. The Dpp/BMP2-4 pathway plays a central role in secondary axis patterning in many animals [1–11], but it has not been examined during early axial patterning in spiralian embryogenesis. This is a deeply conserved mode of development found in mollusks, annelids, nemerteans, entoprocts, and some marine platyhelminth groups (reviewed in [12, 13]). In the spiralian embryo of the mollusk Ilyanassa, we find that the Dpp ortholog (IoDpp) is expressed most strongly on the dorsal side, in cells of the embryonic organizer and its neighbors. Phospho-smad staining indicates that the pathway is active in all lineages during organizer signaling, but activation is strongest on the dorsal side. Knockdown of IoDpp by morpholino oligos prevents the development of all structures that require organizer signaling and ventralizes the embryo. Ectopic activation of the pathway can induce eyes and external shell, which require organizer signaling. These results indicate that Dpp/BMP2-4 signaling is a key part of the spiralian organizer and suggest similarity with other metazoan organizers. However, the fact that IoDpp/BMP2-4 is inducing, rather than repressing, the neuroectoderm is a surprising difference that may be conserved among spiralians. These results connect the spiralian organizer to this general aspect of secondary axis patterning but highlight the significant variation across animals in effects of the pathway on particular cell types and tissues. •In a snail embryo, Dpp/BMP4 is expressed in dorsal cells of the embryonic organizer•Knockdown embryos lack organizer-dependent structures like eyes, shell, and foot•Incubation with ectopic BMP4 induces extra eyes and external shell fragments•Dpp/BMP4 promotes neurogenic ectoderm, rather than repressing it, in this system Lambert et al. show that in a mollusk embryo, the Dpp/BMP2-4 pathway mediates the activity of the D-quadrant organizer, which is found in a number of groups with spiralian development. Unlike some other systems, here the pathway activates, rather than inhibits, neuroectoderm specification.