Key Concepts:

Figure 1. A consensus metazoan phylogenetic tree. The relationships of all major clades are shown. The Bilateria are subdivided into two major groups, the Acoelomorpha and the Nephrozoa (containing the superclades: Deuterostomia, Lophotrochozoa and Ecdysozoa). The sister group to the Bilateria contains the Cnidaria. Ctenophora and Cnidaria form the paraphyletic group Coelenterata, the name of which refers to the presence of a single sac-like body space, the coelenteron, which communicates with the surrounding medium through the mouth. The relationships between the most basal metazoan groups are still hotly debated (thus, the polytomy in the tree).

Figure 2. Model for the evolution of the HOX cluster in metazoans based on recent phylogenetic studies in Cnidaria, which combines the most parsimonious hypothesis for Hox evolution. A unique ANTP ProtoHox gene duplicated several times in tandem in the lineage leading to C-BLCA after the divergence of the Porifera, giving rise to an ancestral HOX gene cluster. Three Hox genes were present in the C-BLCA (PG1–PG2–PPHox). In the lineage leading to the LCBA, another tandem duplication gave rise to the PG5 gene. Two sister groups formed from the LCBA: that leading to the present day acoelomorphs, and another giving rise to the P-DLCA. In the acoel lineage, the original cluster might have disintegrated (at least in Symsagittifera roscoffensis) and some genes could have been lost (possibly, but not proven, the PG2 genes), here represented by an empty box. A similar process could have occurred in nemertodermatids, although the absence of anterior genes (empty boxes) is more likely the result of limited sampling. A further tandem duplication in the Nemertodermatida lineage gave rise to a second PG5 gene. Finally, a series of tandem duplications, namely those involving the central Hox class, gave rise to the extended HOX cluster present in the P-DLCA. As the orthology between posterior Hox genes in cnidarians (CPHox) and bilaterians (BPHox) remains unclear, it is fair to suppose that cnidarian and bilaterian posterior Hox genes were produced independently in both lineages from the proto-posterior Hox gene (PPHox) present in the C-BLCA. C-BLCA, cnidarian–bilaterian last common ancestor; LCBA, last common bilaterian ancestor and P-DLCA, protostome–deuterostome last common ancestor.

Figure 3. Model for the evolution of the HOX cluster in metazoans based on the data on Hox genes in the Cnidaria constructed using nontree-based methodologies. Four Hox genes were present in the C-BLCA (PG1–PG3–PG5–CentralPPHox). The complement in the LCBA is the same, but these genes acquired a new role patterning the AP body axis in the Bilateria, with the ‘Hox code’ being established in this lineage. The absence of this gene in acoelomorphs could be due to limited sampling or its specific loss. Finally, a series of tandem duplications involving the central Hox class gave rise to the extended HOX cluster present in the P-DLCA. C-BLCA, cnidarian–bilaterian last common ancestor; LCBA, last common bilaterian ancestor and P-DLCA, protostome–deuterostome last common ancestor.