Zebrafish trilobite identifies new roles for Strabismus in gastrulation and neuronal movements

Embryonic morphogenesis is driven by a suite of cell behaviours, including coordinated shape changes, cellular rearrangements and individual cell migrations, whose molecular determinants are largely unknown. In the zebrafish, Dani rerio, trilobite mutant embryos have defects in gastrulation movements^1-4 and posterior migration of hindbrain neurons⁵. Here, we have used positional cloning to demonstrate that trilobite mutations disrupt the transmembrane protein Strabismus (Stbm)/Van Gogh (Vang), previously associated with planar cell polarity (PCP) in Drosophila melanogaster^{6, 7}, and PCP and canonical Wnt/β-catenin signalling in vertebrates^{8, 9}. Our genetic and molecular analyses argue that during gastrulation, trilobite interacts with the PCP pathway without affecting canonical Wnt signalling. Furthermore, trilobite may regulate neuronal migration independently of PCP molecules. We show that trilobite mediates polarization of distinct movement behaviours. During gastrulation convergence and extension movements, trilobite regulates mediolateral cell polarity underlying effective intercalation and directed dorsal migration at increasing velocities. In the hindbrain, trilobite controls effective migration of branchiomotor neurons towards posterior rhombomeres. Mosaic analyses show trilobite functions cell-autonomously and non-autonomously in gastrulae and the hindbrain. We propose Trilobite/Stbm mediates cellular interactions that confer directionality on distinct movements during vertebrate embryogenesis.