Monocilia have been proposed to establish the left-right (LR) body axis in vertebrate embryos by creating a directional fluid flow that triggers asymmetric gene expression. In zebrafish, dorsal forerunner cells (DFCs) express a conserved ciliary dynein gene (left-right dynein-related1, lrdr1) and form a ciliated epithelium inside a fluid-filled organ called Kupffer's vesicle (KV). Here, videomicroscopy demonstrates that cilia inside KV are motile and create a directional fluid flow just prior to the onset of asymmetric gene expression in lateral cells. Laser ablation of DFCs and surgical disruption of KV provide direct evidence that ciliated KV cells are required during early somitogenesis for subsequent LR patterning in the brain, heart and gut. Antisense morpholinos against lrdr1 disrupt KV fluid flow and perturb LR development. Furthermore, lrdr1 morpholinos targeted to DFC/KV cells demonstrate that Lrdr1 functions in these ciliated cells to control LR patterning. This provides the first direct evidence, in any vertebrate, that impairing cilia function in derivatives of the dorsal organizer, and not in other cells that express ciliogenic genes, alters LR development. Finally, genetic analysis reveals novel roles for the T-box transcription factor no tail and the Nodal signaling pathway as upstream regulators of lrdr1 expression and KV morphogenesis. We propose that KV is a transient embryonic 'organ of asymmetry' that directs LR development by establishing a directional fluid flow. These results suggest that cilia are an essential component of a conserved mechanism that controls the transition from bilateral symmetry to LR asymmetry in vertebrates.
