Fig. 7. A current model for axis determination in Clytia. (A) In early stages (from egg to early blastula), the ligand CheWnt3 and its receptor CheFz1, produced from animally concentrated RNAs, are required for canonical Wnt signalling in the the animal hemisphere. CheFz1 RNA is distributed as an animal-vegetal gradient in the cytoplasm, whereas CheWnt3 RNA is tightly localised to the animal cortex. CheWnt3 protein probably adopts a wider distribution upon translation (see text). The negatively acting receptor CheFz3, produced from a vegetal cortical RNA, is likely to be the major factor restricting canonical Wnt signalling activation to the future oral end. (B) In later stages, tightly localised zygotic CheWnt3 expression at the oral pole becomes a major factor restricting canonical Wnt pathway activation. Wnt3 is also involved in the reciprocal negative regulation between the receptors CheFz1 and CheFz3 by unknown mechansims. Arrows and dotted lines represent signalling pathways and transcriptional regulation pathways, respectively. The coloured boxes represent RNA distribution, the area with a red dashed outline the predicted Wnt3 protein distribution, and green circles β-catenin stabilisation and nuclear localisation.