Abstract
Deciphering the mechanisms of axis formation in amphioxus is a key step to understanding the evolution of chordate body plan. 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. Here, we re-examined the role of Wnt/βcatenin signaling in the dorsal/ventral patterning of amphioxus embryo. 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. Microinjection of Wnt8 and Wnt11 mRNA induced ectopic dorsal axis in neurulae and larvae. Finally, we demonstrated that Nodal and Wnt/β-catenin signaling cooperate to promote the dorsal-specific gene expression in amphioxus gastrula. Our study reveals high evolutionary conservation of dorsal organizer formation in the chordate lineage.
Highlights
One of the most intriguing queries in developmental biology is how the body plan evolved throughout the animal kingdom
The dorsal organizer was first described in amphibian embryo at the early gastrula stage as a group of cells from dorsal blastopore lip, which has the ability to induce the formation of secondary embryonic axis if grafted to the ventral side of the host embryo (Spemann and Mangold, 1924)
It was demonstrated that the dorsal organizer, which is located in the dorsal blastopore lip of gastrula embryo, is present in the cephalochordates (Le Petillon et al, 2017; Tung and Yeh, 1961), which represent an early branch of the chordate group and share with vertebrates the typical chordate features such as notochord, dorsal hollow neural tube, pharyngeal slits, and post-anal extending tail
Summary
One of the most intriguing queries in developmental biology is how the body plan evolved throughout the animal kingdom. The dorsal organizer was first described in amphibian embryo at the early gastrula stage as a group of cells from dorsal blastopore lip, which has the ability to induce the formation of secondary embryonic axis (a tween embryo) if grafted to the ventral side of the host embryo (Spemann and Mangold, 1924). The role of Wnt/ b-catenin signaling in the dorsal/ventral (DV) axis establishment was first demonstrated in Xenopus by gain-of-function experiments, in which ectopic Wnt proteins induced secondary organizer formation and duplication of the embryonic axis (McMahon and Moon, 1989; Smith and Harland, 1991; Sokol et al, 1991). Maternal Wnt/b-catenin signaling induces expression of genes encoding transcription factors and secreting proteins that are involved in the initial formation of Spemann
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