Functional studies have demonstrated the role of fibro-blast growth factor (Fgf), transforming growth factor (Tgfβand Fgf signaling) and Wnt signaling in apical plate patterning. For example, Fgf signaling controls formation of the apical organ in sea anemone [14]; the apical plate of sea urchins is patterned by Tgfβsignaling [39] and fgfr1 localizes to the apical plate during apical organ formation [37]. Similarly, thePlatynereisapical organ region showed specific expression offgfr in ahighlyrestrictedpopulation of cells (Figure 2N). We also found an apically restricted domain of the Tgfβsignaling antagonistnogginin cells de-void ofsix3expression (white dashed line in Figure 2O’), which indicates that both signaling systems also play a role in apical patterning in Platynereis. Notably, ex-pression ofnogginhas also been reported for thesix3-free apical organ in sea anemone [40]