Hypothetical scenarios for the evolution and diversification of homeodomain classes relative to the cnidarian-bilaterian divergence. The timing of the cnidarian-bilaterian divergence is indicated by an arrow and a dashed vertical line. Cnidarian homeobox genes are indicated by red lines. Protostome (for example, Drosophila) homeobox genes are indicated by green lines. Deuterostome (for example, human) homeobox genes are indicated by blue lines. (a) Cnidaria diverges from Bilateria prior to origin of the major homeodomain classes (ANTP, PRD, LIM, POU, SINE, TALE). (b) Cnidaria diverges from Bilateria after the origin of homeodomain classes but before their diversification. (c) Cnidaria diverges from Bilateria after the diversification of homeobox classes. (d) At the time of the cnidarian-bilaterian divergence, some homeobox classes have not yet originated (ANTP, PRD) whereas others have diversified extensively (POU, SINE).
Ryan et al. Genome Biology 2006 7:R64 doi:10.1186/gb-2006-7-7-r64