Origin of Bilaterian Hox Patterning System

Abstract Hox genes (homeobox‐containing genes) encode regulatory transcription factors involved in the regionalisation of the antero‐posterior body axis during the early embryonic development of bilateral animals. These genes are arranged in evolutionarily conserved clusters in Bilateria, which have been originated by means of several gene tandem duplications from an original ProtoHox gene. On the contrary, in the phylogenetic sister group of Bilateria, the Cnidaria, Hox genes do not seem to play a similar role for patterning the oral–aboral axis. Recently, new insights on the origin and evolution of the Hox gene patterning system have been obtained from the study of the Acoelomorpha, a group of flatworms that branched before the protostome‐deuterostome divergence. On the basis of the more recent data available from Cnidaria, Acoelomorpha and the rest of the Bilateria, we analyse some plausible scenarios for the origin and evolution of the Hox system in the metazoans. Key Concepts: Hox patterning system is associated to the origin of bilaterian animals. Hox genes are used as vectorial systems to pattern the major body axis. Cnidarians have Hox genes but are not used, as such, to organise their oral‐aboral axis.