The expression and distribution of Wnt and Wnt receptor mRNAs during early sea urchin development

The protein β-catenin plays a critically important role in establishing axial polarity during early animal development. In many organisms, β-catenin is degraded preferentially on one side of the cleavage stage embryo. On the opposite side of the embryo, β-catenin is stabilized and accumulates in the nucleus, where it functions in concert with members of the LEF/TCF family to activate the transcription of diverse target genes. Genes that are activated by β-catenin play an essential role in the specification of endomesoderm and in the establishment of key signaling centers in the early embryo. In several organisms, the asymmetric distribution of maternal components of the canonical Wnt pathway has been shown to be responsible for the polarized stabilization of β-catenin. In this study, we identified all Wnt and Wnt receptor mRNAs that are present in unfertilized sea urchin eggs and early embryos and analyzed their distributions along the primary (AV) axis. Our findings indicate that the asymmetric distribution of a maternal Wnt or Wnt receptor mRNA is unlikely to be a primary determinant of the polarized stabilization of β-catenin along the AV axis. This contrasts sharply with findings in other organisms and points to remarkable evolutionary flexibility in the molecular mechanisms that underlie this otherwise very highly conserved patterning process.