![]() Our study reveals high evolutionary conservation of dorsal organizer formation in the chordate lineage. Finally, we demonstrated that Nodal and Wnt/β-catenin signaling cooperate to promote the dorsal-specific gene expression in amphioxus gastrula. Microinjection of Wnt8 and Wnt11 mRNA induced ectopic dorsal axis in neurulae and larvae. We demonstrated that the spatial activity of Wnt/β-catenin signaling is located in presumptive dorsal cells from cleavage to gastrula stage, and provided functional evidence that Wnt/β-catenin signaling is necessary for the specification of dorsal cell fate in a stage-dependent manner. Here, we re-examined the role of Wnt/βcatenin signaling in the dorsal/ventral patterning of amphioxus embryo. The current view is that Nodal signaling is the only factor promoting the dorsal axis specification in the amphioxus whereas Wnt/β-catenin signaling plays no role in this process. Finally, by comparing amphibian gastrulation to gastrulation of protochordates and amniotes, we discuss the gastrulation movement evolutionarily conserved among chordates.ĭeciphering the mechanisms of axis formation in amphioxus is a key step to understanding the evolution of chordate body plan. Collectively, these results are consistent with the S&Z gastrulation model and identify the embryonic region sufficient for construction of the complete dorsal structure. Furthermore, a blastocoel roof explant of the blastula, which should contain the organizer and the prospective neuroectoderm in the S&Z model, autonomously underwent gastrulation and formed the complete dorsal structure. To investigate this possibility, we conducted stepwise tissue deletions using Xenopus laevis embryos and revealed that the dorsal one-third of the marginal zone had the ability to form the complete dorsal structure by itself. According to this model, the body axis is derived from limited regions of the dorsal marginal zone at ACE. The developmental stage when contact between the head organizer and the anterior-most neuroectoderm is established is called “anterior contact establishment (ACE).” After ACE, the A-P body axis elongates posteriorly. In this model, the organizer and the prospective neuroectoderm are originally localized in the blastula’s blastocoel roof, and these embryonic regions move downward to make physical contact of their inner surfaces with each other at the dorsal marginal zone. Previously, we proposed a novel amphibian gastrulation model, the “subduction and zippering (S&Z) model”. However, the morphological movement during gastrulation appears to be divergent across species, making it difficult to discuss the evolution of the process. Gastrulation is a critical event whose molecular mechanisms are thought to be conserved among vertebrates.
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