Dorsal axis duplication as a functional readout for Wnt activity.

The easy accessibility, distinctive features of early cleavage stage embryos and simple manipulation methods make Xenopus embryos an ideal model organism to study gene function and deciphering signaling pathways. For many years, investigators have analyzed putative dorsalizing factors by their ability to induce secondary dorsal structures when misexpressed in early Xenopus embryos. This assay, among others, has contributed substantially to our knowledge about Wnt signaling pathways and is still the assay of choice to quickly determine whether a factor acts positively or negatively in the Wnt signaling pathway. This chapter describes two experimental approaches to determine canonical Wnt signaling: induction of a secondary axis and analyses of target gene expression.