Abstract
The Wnt/β-catenin (cWnt) signaling pathway initiates pattern formation along the anterior-posterior axis in bilaterians and the oral-aboral axis in cnidarians, the closest outgroup to bilaterians. Specification of both axes is strongly influenced by the animal-vegetal axis of the ovum. Intriguingly, cWnt signaling is activated in vegetal blastomeres of many bilaterians, and in animal blastomeres of cnidarians. To understand how the redeployment of cWnt signaling along the AV axis may have mediated AP axis evolution, I dedicated my dissertation to explore how cWnt signaling is regulated during axis specification in sea urchins (bilaterian) and in Nematostella vectensis (non-bilaterian). Since a major node of cWnt regulation is mediated by the β-catenin destruction complex (βcDC), I focused on analysing its regulation and function in the sea urchin and Nematostella. The βcDC is a multiprotein assembly (primarily composed of Axin, APC, GSK-3β, and CK1α), which inhibits cWnt signaling by regulating the phosphorylation of β-catenin, but its functional conservation is unknown. Moreover, the identity of upstream factors that regulate βcDC at opposite poles in bilaterians and non-bilaterians are also largely unknown. To explore these questions, I annotated the major components of the βcDC in representative metazoans for phylogenetic analyses (Chapter 2). Bilaterian APC has a complex domain structure, but Nematostella APC only has an Axin binding domain and one potential β-catenin binding domain. Surprisingly, Axins in non-bilaterians lack the conserved β-catenin binding domain found in bilaterian Axin. To test Axin function in the βcDC in a non-bilaterian I carried out a comparative analysis using early sea urchin and Nematostella embryos (Chapter 3 & 4). Finally, in Chapter 5 I attempted to identify factors that regulate the localized activation of Dvl in sea urchin embryos. In summary, my studies have generated a new model for the evolution of the βcDC in metazoans and have provided novel insights into the evolutionary transition from non-bilaterian to bilaterian body plans.