Is the chromatin control of Hox genes expression conserved by evolution?

The maintenance of homeotic gene expression is achieved in Drosophila through the activity of two sets of genes, the Polycomb and trithorax groups. Their mode of action through the formation of chromatin complexes is supported by multiple molecular and genetic evidence. Homologues of these genes are now known in vertebrates. Recent data from transgenesis experiments in mice bring evidence for functional evolutionary conservation of these genes in the control of Hox genes' expression. These results are discussed in the light of evolutionary and developmental processes in both arthropod and chordate lineages. Arguments support the involvement of these genes in the temporal, not only spatial, control of the expression of the Hox genes and favour the idea that the genomic structure of the Hox complexes has been preserved by the effect of this temporal regulation. It is suggested that this chromatin mode of regulation is primitive, and would act in vertebrates and primitive arthropods prior to transcriptional regulation.