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

Abstract

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 beta-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.

Developmental gene expression provides clues to relationships between sponge and eumetazoan body plans

“…3d). We have not detected specific Wnt expression in exopinacoderm as reported for O. lobularis 44 pathway (signalling through b-catenin) is specification of the endomesoderm 19,51,52 . We thus studied expression of downstream components of the pathway (Figs 4 and 5).…”

The Evolution of the Wnt Pathway

“…During gastrulation, b-catenin is activated at the site of the future blastopore, and blocking GSK3 causes extended gastrulation movements in Nematostella (Wikramanayake et al 2003). Dsh is necessary for this blastoporal activation (Lee et al 2007). In the hydrozoan Clytia, Fz1 and maternal Wnt3 are required for oral b-catenin activation (Momose and Houliston 2007;Momose et al 2008).…”

A maternally localised Wnt ligand required for axial patterning in the cnidarianClytia hemisphaerica

“…In sea urchins, the cytoplasmic regulator Dishevelled (Dsh) localises dynamically to a vegetal cortical layer of the egg, and has been shown to mediate Wnt signalling activation in this region of the embryos; however, its global overexpression does not affect germ layer specification, implying the involvement of additional localised factors that restrict its activity (Leonard and Ettensohn, 2007;Weitzel et al, 2004). Dsh protein is likewise maternally localised and necessary for subsequent patterning events in Nematostella (Lee et al, 2007), in which the absence of detectable maternal expression of any of the genomic Wnt repertoire (Lee et al, 2006) has led to the suggestion that Dsh protein might play a primary directive role. In Xenopus embryos, in which β-catenin stabilisation is favoured on the prospective 'organiser' side of the cleavage-stage embryo, corresponding enrichments have been reported not only for Dsh but also for β-catenin, whereas an inhibitor of the regulatory kinase GSK3-β moves in the appropriate direction in the fertilised egg.…”