A maternally localised Wnt ligand required for axial patterning in the cnidarian Clytia hemisphaerica

Abstract

Regionalised activation of canonical Wnt signalling via β-catenin stabilisation is a key early step in embryonic patterning in many metazoans,including the basally diverging cnidarians, but the upstream maternal cues appear surprisingly variable. In Clytia, regionalised β-catenin stabilisation defining a presumptive `oral' territory is determined by two maternally coded Frizzled family Wnt receptors of opposite localisation and function. We have identified a maternally coded ligand, CheWnt3, the RNA of which is localised to the animal cortex (future oral side) of the egg. Antisense morpholino oligonucleotide experiments showed that CheWnt3 is required maternally for regionalised oral β-catenin stabilisation in the early embryo, being only the second clear example of a maternally required Wnt ligand after Xenopus Xwnt11. In line with the determinant role of the maternally localised Frizzleds, CheWnt3 overexpression by RNA injection initially had little effect on establishing the oral domain. Subsequently,however, overexpression had dramatic consequences for axis development,causing progressive expansion of β-catenin stabilisation to yield spherical `oralised' larvae. Upregulation of both CheFz1 and CheFz3 RNAs in CheWnt3 morpholino embryos indicated that CheWnt3 participates in an active axial patterning system involving reciprocal downregulation of the receptors to maintain oral and aboral territories. Localised introduction of CheWnt3 RNA induced ectopic oral poles in CheWnt3 morpholino embryos, demonstrating its importance in directing oral fate. These findings suggest that the complete ligand-dependent Wnt signalling cascade is involved in axial patterning in ancestral eumetazoans. In Clytia, two variant Frizzled receptors and one Wnt ligand produced from localised RNAs cooperate to initiate regionalised Wnt pathway activation.