![]() ![]() ![]() ![]() The commitment of the first embryonic cells to more restricted developmental fates (e.g., endoderm, neuroectoderm and mesoderm) is a pivotal step in animal embryogenesis that leads to the establishment of body plans and influences subsequent development 1, 2, 3. Together, these findings suggest that conditional specification of an ERK1/2 + embryonic organiser is ancestral in spiral cleavage and was repeatedly lost in annelid lineages with autonomous development. The organising role of ERK1/2 in Owenia is shared with molluscs, but not with autonomous annelids. This cell likely acts as an organiser, inducing mesodermal and posterodorsal fates in neighbouring cells and repressing anteriorising signals. In Owenia, ERK1/2-mediated FGF receptor signalling specifies the endomesodermal progenitor. To identify the mechanisms driving this change, we study Owenia fusiformis, an early-branching, conditional annelid. During spiral cleavage-a stereotypic embryogenesis ancestral to 15 invertebrate groups, including molluscs and annelids-most lineages specify cell fates conditionally, while some define the primary axial fates autonomously. Yet how these two major developmental modes evolved remains unclear. Animal development is classified as conditional or autonomous based on whether cell fates are specified through inductive signals or maternal determinants, respectively. ![]()
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