![]() We propose a two-step model for the evolution of an archenteron and the evolution of endodermal germ layer segregation. Our results demonstrate that Wnt/PCP-mediated initial archenteron invagination can be uncoupled from Wnt/ß-catenin-mediated endoderm cell fate specification in Nematostella, and provides evidence that these two processes could have evolved independently during metazoan evolution. Conversely, selectively blocking Wnt/ß-catenin signaling inhibits endoderm cell fate specification but does not affect bottle cell formation and initial archenteron invagination. 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. 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. Here, we show that bottle cell formation and initial gut invagination in Nematostella requires NvStrabismus (NvStbm), a maternally-expressed core component of the Wnt/Planar Cell Polarity (PCP) pathway. In the anthozoan cnidarian Nematostella vectensis, initial archenteron formation begins with bottle cell-induced buckling of the blastula epithelium at the animal pole. 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. The developmental mechanisms that induced archenteron formation and led to the segregation of germ layers during metazoan evolution are unknown. 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 process of gastrulation involves two tightly coupled events during embryogenesis of most metazoans. Gastrulation is a uniquely metazoan character, and its genesis was arguably the key step that enabled the remarkable diversification within this clade. ![]()
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