Hox Homeotic Selector Genes: Key Regulators of Embryogenesis

The Hox genes are major regulators of the animal bodyplan. They determine the identities of sequential zones along the main body axis. The Hox genes show collinearity– spatial and temporal ordering of their expression corresponding to their genomic ordera spectacular phenomenon that has excited life scientists since its discovery in 1978. Three mechanisms have been proposed to explain this spatially sequential axial pattern of Hox gene expression in animal embryonic development: 1) That it is secondary and regulated by other spatially ordered genes. 2) That it is regulated by interactions among Hox genes, or 3) It is regulated by the progressive opening of chromatin in the Hox clusters, from 3’ to 5’. A review of the evidence across different species and developmental stages points to the universal involvement of trans-acting factors and cell–cell interactions. The evidence focuses attention on interactions between Hox genes and on the vertebrate somitogenesis clock. These novel conclusions open new perspectives for the field.