Extended Data Figure 4. Expression of BMP components in Nemertodermatida and Acoela.

(a) In the nemertodermatid Meara stichopi, the BMP pathway antagonists twisted gastrulation (tsg), and crossveinless 2 (cv2) are expressed dorsally, while the antagonist BAMBI is broadly detected in the ventral side. The gene tolloid (tld) is expressed both dorsally and ventrally. The BMP receptors bmpR-I is expressed dorsolaterally and the bmpR-II is detected more broadly. The genes smad1 and smad4 are expressed broadly and smad6 is expressed along the dorsal nerve cords. (b) bmp2/4 is not expressed in neuronal cells (elav1⁺ cells), but in cells located medially to the nerve cords (tubulin positive). The cells expressing bmp2/4 also express tsg, and cv2 is expressed dorsally along the nerve cords. (c) In the acoel Isodiametra pulchra, the BMP antagonist tld is expressed ventrally, the bmpR-I is detected in the inner body, and bmpR-II is expressed anteriorly and posteriorly around the copulatory organ. The genes smad1 and smad4 are expressed generally, while smad6 is expressed in two bilaterally symmetrical anterior clusters. All main panels are dorsoventral views, and the insets are lateral views.