Abstract

Animals with bilateral symmetry (i.e., Bilateria) show the highest diversity of body plans. Remarkably, nearly half of these body plans occur in a large clade of invertebrates termed Spiralia, which exhibits a unique mode of development termed spiral cleavage as the ancestral and defining feature. How spiral cleavage is controlled and how it generates such a diversity of adult morphologies despite its seeming conservation is unknown but fundamental to our understanding of animal development and evolution. To investigate these questions, my lab has established the “basal” annelid Owenia fusiformis as a tractable research system. In this species, the FGF receptor triggers the ERK signalling pathway to specify a dorsoposterior fate in the cell that acts as the axial organiser. This event triggers a cascade that involves Notch and BMP signalling and transcriptional regulators involved in trunk mesoderm and dorsoposterior development. In other annelids, however, the dorsoventral specification does not depend on the FGF -ERK-BMP axis but on Activin/Nodal, which appears to control cell differentiation along the animal-vegetal axis in O. fusiformis. Notably, this transition to Activin/Nodal coincides with a shift from conditional to maternally-regulated autonomous development and the accelerated formation of adult characters, such as a segmented trunk. Together, our work reveals widespread developmental system drift and heterochronic shifts in axial patterning and organogenesis in Spiralia and during spiral cleavage, without these implying, paradoxically, major alterations in the early patterns of zygotic divisions.