Anterior-posterior Wnt signaling network conservation between indirect developing sea urchin and hemichordate embryos.

How animal body plans evolved and diversified is a major question in evolutionary developmental biology. To address this question, it is important to characterize the exact molecular mechanisms that establish the major embryonic axes which give rise to the adult animal body plan. The anterior-posterior (AP) axis is the first axis to be established in most animal embryos, and in echinoderm sea urchin embryos its formation is governed by an integrated network of three different Wnt signaling pathways: Wnt/ß-catenin, Wnt/JNK, and Wnt/PKC pathway. The extent to which this embryonic patterning mechanism is conserved among deuterostomes, or more broadly in metazoans, is an important open question whose answers could lead to a deeper appreciation of the evolution of the AP axis. Because Ambulacrarians (echinoderms and hemichordates) reside in a key phylogenetic position as the sister group to chordates, studies in these animals can help inform on how chordate body plans may have evolved. Here, we assayed the spatiotemporal gene expression of a subset of sea urchin AP Wnt patterning gene orthologs in the hemichordate, Schizocardium californicum. Our results show that positioning of the anterior neuroectoderm (ANE) to a territory around the anterior pole during early AP formation is spatially and temporally similar between indirect developing hemichordates and sea urchins. Furthermore, we show that the expression of wnt8 and frizzled5/8, two known drivers of ANE patterning in sea urchins, is similar in hemichordate embryos. Lastly, our results highlight divergence in embryonic expression of several early expressed Wnt genes (wnt1, wnt2 and wnt4). These results suggest that expression of the sea urchin AP Wnt signaling network is largely conserved in indirect developing hemichordates setting the foundation for future functional studies in S. californicum.

Evolutionarily conserved Wnt/Sp5 signaling is critical for anterior-posterior axis patterning in sea urchin embryos.

Studies across a diverse group of metazoan embryos indicate that Wnt signaling often activates the transcription factor Sp5, forming a signaling 'cassette' that plays critical roles in many developmental processes. This study explores the role of Wnt/Sp5 signaling during the specification and patterning of the primary germ layers during early anterior-posterior axis formation in the deuterostome sea urchin embryo. Our functional analyses show that Sp5 is critical for endomesoderm specification downstream of Wnt/ß-catenin in posterior cells as well as anterior neuroectoderm patterning downstream of non-canonical Wnt/JNK signaling in anterior cells. Interestingly, expression and functional data comparisons show that Wnt/Sp5 signaling often plays similar roles in posterior endomesoderm as well as neuroectoderm patterning along the AP axis of several deuterostome embryos, including vertebrates. Thus, our findings provide strong support for the idea that Wnt-Sp5 signaling cassettes were critical for the establishment of early germ layers in the common deuterostome ancestor.