Fig. 1

Anterior–posterior patterning and the emergence of a Hox cluster. a Bilaterians are classically defined by an anterior–posterior axis perpendicular to the dorsal ventral axis. Cnidarians are the sister taxa to bilaterians and are the only basal lineage to have a diverse cluster of Hox genes. b The common ancestor of the deuterostome lineage likely had a Hox cluster consisting of 14–15 Hox genes, closely associated with the homeobox gene Eve¹⁸. c Evidence from the protostome, Tribolium castaneum, suggests that the protostome ancestor also had an intact Hox cluster consisting of at least 10 linked Hox genes^17,70. d The cnidarian ancestor had both anterior (Hox1 and Hox2) and central/posterior (Hox9–13) class Hox genes²². e The Hox complement of the anthozoan cnidarian, Nematostella vectensis, has phylogenetically anterior (NvAx6, NvAx6a, NvAx7, and NvAx8) and central/posterior (NvAx1 and NvAx1a) Hox genes^14,15. Depiction of Hox expression along the oral–aboral axis of a cnidarian, and the anterior–posterior axis of invertebrates and vertebrates. The anterior (NvAx6) and central/posterior (NvAx1) Hox genes of Nematostella are expressed along the oral–aboral axis during larval development. Regions of anterior, central, and posterior Hox expression are designated with shades of red, green, and blue, respectively. Asterisk indicates site of mouth formation