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Background  Left-right (LR) organ asymmetries are a common feature of metazoan animals. In many cases, laterality is established by a conserved asymmetric Nodal signaling cascade during embryogenesis. In most vertebrates, asymmetric  nodal  induction results from a cilia-driven leftward fluid flow at the left-right organizer (LRO), a ciliated epithelium present during gastrula/neurula stages. Conservation of LRO and flow beyond the vertebrates has not been reported yet.    Results  Here we study sea urchin embryos, which use  nodal  to establish larval LR asymmetry as well. Cilia were found in the archenteron of embryos undergoing gastrulation. Expression of  foxj1  and  dnah9  suggested that archenteron cilia were motile. Cilia were polarized to the posterior pole of cells, a prerequisite of directed flow. High-speed videography revealed rotating cilia in the archenteron slightly before asymmetric  nodal  induction. Removal of cilia through brief high salt treatments resulted in aberrant patterns of  nodal  expression. Our data demonstrate that cilia - like in vertebrates - are required for asymmetric  nodal  induction in sea urchin embryos.    Conclusions  Based on these results we argue that the anterior archenteron represents a  bona fide  LRO and propose that cilia-based symmetry breakage is a synapomorphy of the deuterostomes.    Electronic supplementary material  The online version of this article (doi:10.1186/s12861-016-0128-7) contains supplementary material, which is available to authorized users.

Cilia are required for asymmetric nodal induction in the sea urchin embryo