Figure 1

Conservation of anterior-posterior six3 / optix-, gbx / unplugged and Hox-expressing territories in Bilateria. A conserved anterior-posterior alignment of six3/optix-, otx/otd-, gbx/unplugged- and hox-expressing neuroectodermal regions in the hypothetical ancestral arthropod (a), the annelid Platynereis (b), and the hemichordate Saccoglossus (c). (a) Arrow depicts the antero-posterior neuraxis pointing at the anterior-most six3/optix-region as identified by the data presented here. Light grey in (b): developing parapodial appendages, in (c): gut. Dark grey: mouth opening. Yellow: neuroectoderm not expressing any of the mentioned genes. Purple in (a, b): six3+/otx+ regions. All animals are oriented with anterior to the top. (a, b): ventral views. (c): lateral view. ea: eye anlage. Arthropod schematic after [29, 36–38, 48–53], Platynereis and Saccoglossus schematics after references in the text.