Expression of a homologue of the fushi tarazu (ftz) gene in a cirripede crustacean.

In Metazoa, Hox genes control the identity of the body parts along the anteroposterior axis. In addition to this homeotic function, these genes are characterized by two conserved features: They are clustered in the genome, and they contain a particular sequence, the homeobox, encoding a DNA-binding domain. Analysis of Hox homeobox sequences suggests that the Hox cluster emerged early in Metazoa and then underwent gene duplication events. In arthropods, the Hox cluster contains eight genes with a homeotic function and two other Hox-like genes, zerknullt (zen)/Hox3 and fushi tarazu (ftz). In insects, these two genes have lost their homeotic function but have acquired new functions in embryogenesis. In contrast, in chelicerates, these genes are expressed in a Hox-like pattern, which suggests that they have conserved their ancestral homeotic function. We describe here the characterization of Diva, the homologue of ftz in the cirripede crustacean Sacculina carcini. Diva is located in the Hox cluster, in the same position as the ftz genes of insects, and is not expressed in a Hox-like pattern. Instead, it is expressed exclusively in the central nervous system. Such a neurogenic expression of ftz has been also described in insects. This study, which provides the first information about the Hoxcluster in Crustacea, reveals that it may not be much smaller than the insect cluster. Study of the Diva expression pattern suggests that the arthropod ftz gene has lost its ancestral homeotic function after the divergence of the Crustacea/Hexapoda clade from other arthropod clades. In contrast, the function of ftz during neurogenesis is well conserved in insects and crustaceans.