Dishevelled (Dvl) proteins are key transducers of Wnt signaling encoded by members of a multi-gene family in vertebrates. We report here the divergent, tissue-specific expression patterns for all three Dvl genes in Xenopus embryos, which contrast dramatically with their expression patterns in mice. Moreover, we find that the expression patterns of Dvl genes in the chick diverge significantly from those of Xenopus. In addition, in hemichordates, an outgroup to chordates, we find that the one Dvl gene is dynamically expressed in a tissue-specific manner. Using knockdowns, we find that Dvl1 and Dvl2 are required for early neural crest specification and for somite segmentation in Xenopus. Most strikingly, we report a novel role for Dvl3 in the maintenance of gene expression in muscle and in the development of the Xenopus sclerotome. These data demonstrate that the expression patterns and developmental functions of specific Dvl genes have diverged significantly during chordate evolution.

SUMMARYFGFs act in vertebrate mesoderm induction and also play key roles in early mesoderm formation in ascidians and amphioxus. However, in sea urchins initial characterizations of FGF function do not support a role in early mesoderm induction, making the ancestral roles of FGF signaling and mechanisms of mesoderm specification in deuterostomes unclear. In order to better characterize the evolution of mesoderm formation, we have examined the role of FGF signaling during mesoderm development in Saccoglossus kowalevskii, an experimentally tractable representative of hemichordates. We report the expression of an FGF ligand, fgf8/17/18, in ectoderm overlying sites of mesoderm specification within the archenteron endomesoderm. Embryological experiments demonstrate that mesoderm induction in the archenteron requires contact with ectoderm, and loss-of-function experiments indicate that both FGF ligand and receptor are necessary for mesoderm specification. fgf8/17/18 gain-of-function experiments establish that FGF8/17/18 is sufficient to induce mesoderm in adjacent endomesoderm. These experiments suggest that FGF signaling is necessary from the earliest stages of mesoderm specification and is required for all mesoderm development. Furthermore, they suggest that the archenteron is competent to form mesoderm or endoderm, and that FGF signaling from the ectoderm defines the location and amount of mesoderm. When considered in a comparative context, these data support a phylogenetically broad requirement for FGF8/17/18 signaling in mesoderm specification and suggest that FGF signaling played an ancestral role in deuterostome mesoderm formation. RESEARCH ARTICLEFGFs and hemichordate mesoderm from five pouches that evaginate from the archenteron in a process called enterocoely (Bateson, 1884); this trait is shared with echinoderms and amphioxus (Conklin, 1932) and is suggested to be primitive for deuterostomes (Remane, 1963;Valentine, 2004). These morphogenetic similarities with echinoderms and basal chordates, and close similarities with chordates in early body plan patterning, suggest that analysis of mesoderm specification in enteropneusts could help reconstruct ancestral deuterostome developmental mechanisms for mesoderm induction.To investigate a potentially conserved role of FGF signaling in deuterostome mesoderm induction, we examined the role of FGF signaling during early development of the direct-developing hemichordate Saccoglossus kowalevskii (Bateson, 1884;Bateson, 1886;Colwin and Colwin, 1953;Lowe et al., 2004;Gerhart et al., 2005;Röttinger and Lowe, 2012). We tested the function of the FGF ligand FGF8/17/18 and the FGF receptor FGFR-B (Rebscher et al., 2009) in hemichordate mesoderm formation, and our work demonstrates that FGF8/17/18 signals from ectoderm to the underlying archenteron to induce mesoderm. These findings suggest that an ortholog of the fgf8/17/18 subfamily was essential for mesoderm induction in the deuterostome common ancestor and have important implications for the evolution of mesoderm induct...