The canonical Wnt pathway in embryonic axis polarity

The canonical Wnt pathway plays crucial roles in multiple developmental processes, including in axis specification. Throughout the animal kingdom, this pathway has been reported to drive patterning of axes as different as the animal–vegetal axis in echinoderms to the dorsal–ventral axis in vertebrates. Intriguingly enough, this pathway appears structurally and functionally well conserved during evolution. However, differences between these phyla are observed that explain how a same pathway can mediate establishment of two such apparently distinct axes. This review compares the axis specification processes used in two evolutionarily distant embryos, the sea urchin and Xenopus.