Models for the early evolution of Hox and ParaHox genes. The predicted homology groups for each phylogenetic group are indicated with colors in the table. The uncertainty of the phylogenetic position of placozoans is indicated by a dashed line [64, 65]. The cnidarian/bilaterian ancestor inferred Hox-ParaHox repertoire is depicted in a double box. Posterior genes are depicted with two colors to reflect the uncertainty of predictions into Central and Posterior groups. This repertoire would result from three equally parsimonious scenarios: a two-gene ProtoHox cluster composed of ancestral Anterior/PG3 and Central (or Central/Posterior) genes, undergoing two or three duplications; a three-gene ProtoHox cluster composed of ancestral Anterior/PG1, Anterior/PG3 and Central (or Central/Posterior) genes undergoing one gene loss, and one or two duplication; or a four-gene ProtoHox cluster composed of ancestral Anterior/PG1, Anterior/PG3, Central and Posterior genes, undergoing two gene losses and a possible duplication.