Strabismus-mediated primary archenteron invagination is uncoupled from Wnt/β-catenin-dependent endoderm cell fate specification in Nematostella vectensis (Anthozoa, Cnidaria): Implications for the evolution of gastrulation

<p>Abstract</p> <p>Background</p> <p>Gastrulation is a uniquely metazoan character, and its genesis was arguably the key step that enabled the remarkable diversification within this clade. The process of gastrulation involves two tightly coupled events during embryogenesis of most metazoans. Morphogenesis produces a distinct internal epithelial layer in the embryo, and this epithelium becomes segregated as an endoderm/endomesodermal germ layer through the activation of a specific gene regulatory program. The developmental mechanisms that induced archenteron formation and led to the segregation of germ layers during metazoan evolution are unknown. But an increased understanding of development in early diverging taxa at the base of the metazoan tree may provide insights into the origins of these developmental mechanisms.</p> <p>Results</p> <p>In the anthozoan cnidarian <it>Nematostella vectensis</it>, initial archenteron formation begins with bottle cell-induced buckling of the blastula epithelium at the animal pole. Here, we show that bottle cell formation and initial gut invagination in <it>Nematostella </it>requires NvStrabismus (NvStbm), a maternally-expressed core component of the Wnt/Planar Cell Polarity (PCP) pathway. The NvStbm protein is localized to the animal pole of the zygote, remains asymmetrically expressed through the cleavage stages, and becomes restricted to the apical side of invaginating bottle cells at the blastopore. Antisense morpholino-mediated NvStbm-knockdown blocks bottle cell formation and initial archenteron invagination, but it has no effect on Wnt/ß-catenin signaling-mediated endoderm cell fate specification. Conversely, selectively blocking Wnt/ß-catenin signaling inhibits endoderm cell fate specification but does not affect bottle cell formation and initial archenteron invagination.</p> <p>Conclusions</p> <p>Our results demonstrate that Wnt/PCP-mediated initial archenteron invagination can be uncoupled from Wnt/ß-catenin-mediated endoderm cell fate specification in <it>Nematostella</it>, and provides evidence that these two processes could have evolved independently during metazoan evolution. We propose a two-step model for the evolution of an archenteron and the evolution of endodermal germ layer segregation. Asymmetric accumulation and activation of Wnt/PCP components at the animal pole of the last common ancestor to the eumetazoa may have induced the cell shape changes that led to the initial formation of an archenteron. Activation of Wnt/ß-catenin signaling at the animal pole may have led to the activation of a gene regulatory network that specified an endodermal cell fate in the archenteron.</p

Distinct Frizzled receptors independently mediate endomesoderm specification and primary archenteron invagination during gastrulation in Nematostella

Endomesodermal cell fate specification and archenteron formation during gastrulation are tightly linked developmental processes in most metazoans. However, studies have shown that in the anthozoan cnidarian Nematostella vectensis, Wnt/β-catenin (cWnt) signalling-mediated endomesodermal cell fate specification can be experimentally uncoupled from Wnt/Planar Cell Polarity (PCP) signalling-mediated primary archenteron invagination. The upstream signalling mechanisms regulating cWnt signalling-dependent endomesoderm cell fate specification and Wnt/PCP signalling-mediated primary archenteron invagination in Nematostella embryos are not well understood. By screening for potential upstream mediators of cWnt and Wnt/PCP signalling, we identified two Nematostella Frizzled homologs that are expressed early in development. NvFzd1 is expressed maternally and in a broad pattern during early development while NvFzd10 is zygotically expressed at the animal pole in blastula stage embryos and is restricted to the invaginating cells of the presumptive endomesoderm. Molecular and morphological characterization of NvFzd1 and NvFzd10 knock-down phenotypes provide evidence for distinct regulatory roles for the two receptors in endomesoderm cell fate specification and primary archenteron invagination. These results provide further experimental evidence for the independent regulation of endomesodermal cell fate specification and primary archenteron invagination during gastrulation in Nematostella. Moreover, these results provide additional support for the previously proposed two-step model for the independent evolution of cWnt-mediated cell fate specification and Wnt/PCP-mediated primary archenteron invagination. [Display omitted] •Endomesoderm specification and gastrulation are tightly linked events in Bilateria.•These processes can be experimentally uncoupled in the cnidarian N. vectensis.•Nematostella primary gut invagination can occur without endomesoderm specification.•NvFzd1 regulates endomesoderm specification and NvFzd10 regulates gut invagination.•These results suggest that the two processes arose independently during evolution

Asymmetric developmental potential along the animal–vegetal axis in the anthozoan cnidarian, Nematostella vectensis, is mediated by Dishevelled

The relationship between egg polarity and the adult body plan is well understood in many bilaterians . However, the evolutionary origins of embryonic polarity are not known. Insight into the evolution of polarity will come from understanding the ontogeny of polarity in non-bilaterian forms, such as cnidarians . We examined how the axial properties of the starlet sea anemone, Nematostella vectensis (Anthozoa, Cnidaria), are established during embryogenesis . Egg-cutting experiments and sperm localization show that Nematostella eggs are only fertilized at the animal pole . Vital marking experiments demonstrate that the egg animal pole corresponds to the sites of first cleavage and gastrulation, and the oral pole of the adult . Embryo separation experiments demonstrate an asymmetric segregation of developmental potential along the animal–vegetal axis prior to the 8-cell stage . We demonstrate that Dishevelled (Dsh) plays an important role in mediating this asymmetric segregation of developmental fate . Although NvDsh mRNA is ubiquitously expressed during embryogenesis, the protein is associated with the female pronucleus at the animal pole in the unfertilized egg, becomes associated with the unipolar first cleavage furrow, and remains enriched in animal pole blastomeres. Our results suggest that at least one mechanism for Dsh enrichment at the animal pole is through its degradation at the vegetal pole. Functional studies reveal that NvDsh is required for specifying embryonic polarity and endoderm by stabilizing β-catenin in the canonical Wnt signaling pathway . The localization of Dsh to the animal pole in Nematostella and two other anthozoan cnidarians (scleractinian corals) provides a possible explanation for how the site of gastrulation has changed in bilaterian evolution while other axial components of development have remained the same and demonstrates that modifications of the Wnt signaling pathway have been used to pattern a wide variety of metazoan embryos