Chemosensing versus mechanosensing in nodal and Kupffer's vesicle cilia and in other left–right organizer organs

How is sensing carried out by cilia in the mouse node, zebrafish Kupffer's vesicle and similar left–right (LR) organizer organs in other species? Two possibilities have been put forward. In the former, cilia would detect some chemical species in the fluid; in the latter, they would detect fluid flow. In either case, the hypothesis is that an imbalance would be detected between this signalling coming from cilia on the left and right sides of the organizer, which would initiate a cascade of signals leading ultimately to the breaking of LR symmetry in the developing body plan of the organism. We review the evidence for both hypotheses. This article is part of the Theo Murphy meeting issue ‘Unity and diversity of cilia in locomotion and transport’.