The regulation of convergent extension during gastrulation in zebrafish

Coordinated cell movements during zebrafish gastrulation shape the embryo, forming three germ layers and establishing the embryonic body plan. Through convergent extension (CE) movements, mesodermal and neurectodermal cells intercalate at the dorsal midline, extending the anteroposterior axis. CE has been shown to be regulated by non-canonical Wnt signalling, homologous to planar cell polarity (PCP) signalling in Drosophila. In this thesis, I undertook several different approaches to understand the molecular mechanisms by which non-canonical Wnt/PCP signalling regulates CE. Firstly, I characterised the novel PCP gene pklb, one of three zebrafish homologues of the PCP gene prickle (pk). pk1a was shown to regulate CE. Like pk1a, pklb is expressed during gastrulation. Abrogation of pklb function enhances CE defects in the non-canonical Wnt/PCP pathway mutants silberblicklwntl 1 (sib) and trilobiteistbm (tri). Secondly, I investigated the hypothesis that non-canonical Wnt signalling interacts with the Drosophila axon-pathfinding ligand Slit and its transmembrane receptor Robo in regulating CE. The zebrafish homologues slitla, slit2, slit3, robol, robo2 and robo3 are expressed during gastrulation, while gain of function of slit2 in zebrafish was shown to severely disturb CE movements. Abrogation of slitla, slit2, robo2 and robo3 function via morpholino knockdown and a dominant-negative approach yielded CE phenotypes in wildtype embryos and enhanced CE defects on non-canonical Wnt/PCP mutant/morphant backgrounds, suggesting that non- canonical Wnt/PCP and Slit/Robo signalling pathways cooperate in the context of CE. Finally, I characterised a mutant identified in a screen for dominant enhancers of sib that we have r provisionally named airbag (abg). Homozygous abg embryos exhibit a slightly shorter body axis and thickened yolk extension. The pk1a, pklb and stbm morphant phenotypes are enhanced on the abg background. I explored the abg phenotype in contexts separate to CE prior to the mapping of this mutation