Minimal ProtoHox cluster inferred from bilaterian and cnidarian Hox complements

Dividing line for Hox genesThe main body axis of bilaterian animals, from insects to humans, is shaped under the instruction of the same group of genes named Hox. Understanding the evolution of Hox and the related ParaHox genes requires knowledge of the Hox complement in still more ancient lineages. Chourrout et al. have investigated Hox genes in two of these, the cnidarians Nematostella and Hydra, which arose over 600 million years ago. Although these organisms have genes that correspond with anterior-group Hox genes in bilateria, the remainder of the Hox genes show signs of an independent origin. The research also demonstrates a simple origin of the putative ancestral ProtoHox cluster, which began with just two genes. After extensive gene duplication, the Hox clusters of cnidarians and bilaterians followed independent trajectories and reached distinct fates.