Mechanical strain, novel genes and evolutionary insights: news from the frog left-right organizer

Animal asymmetries are widespread, from lobster claws to human handedness. Controlled by the left-sided Nodal signaling cascade, asymmetric morphogenesis and placement of vertebrate organs (heart, gut, etc.) are executed during embryogenesis. Fish, amphibians and mammals use a ciliated epithelium to break bilateral symmetry and induce the Nodal cascade. Cilia tilt and polarize to the posterior cell pole, such that clockwise rotation causes a leftward flow at the cell surface. Recent progress in Xenopus showed that mechanical strain drives cilia lengthening and polarization. Studying mutant alleles causing human organ situs defects and following novel EvoDevo approaches, new genes were discovered and functionally characterized in the frog, facilitated by a unique set of experimental tools.