Wnt and BMP signaling pathways pattern the anterior neuroectoderm of the indirect-developing hemichordate Ptychodera flava.

The chordate gastrula organizer establishes Wnt and BMP signaling gradients that are essential for generating the chordate body plan. In non-chordate deuterostomes, including echinoderms and hemichordates, these two signals also contribute to body axial patterning, although it remains debatable whether there is a bona fide equivalent tissue to the organizer. One consequence of these patterning mechanisms is the formation of a neuron population originating from the anterior neuroectoderm (ANE). Wnt signaling is essential for restricting the ANEs in sea urchins with planktonic larval stages as well as direct-developing hemichordates (without larval stages). However, it is difficult to infer the ancestral mechanisms in deuterostomes since information is lacking regarding the mechanisms of ANE formation in indirect-developing hemichordates (with larval stages). Here, we delineate the deployment of Wnt signaling components as well as the roles of Wnt and BMP signals in ANE development in the indirect-developing hemichordate Ptychodera flava. We show that posterior Wnt signaling functions to pattern the anteroposterior axis and restrict the ANE during gastrulation. Meanwhile, BMP signaling promotes ANE development and regeneration during gastrulation, although it initially represses the formation of neural tissues. Our findings thus support a conserved role for Wnt signaling in ANE restriction and suggest a biphasic function of BMP signaling during ANE formation, providing insights into the patterning mechanisms within the common ancestor of deuterostomes.