The divergent roles of the segmentation gene hunchback.

The hunchback (hb) gene is a member of the gap class of segmentation genes first identified in the dipteran insect Drosophila melanogastor. The hb gene encodes a C(2)H(2) zinc finger transcription factor whose primary function is to regulate the expression of its target genes along the anteroposterior (AP) axis based on its distribution in the blastoderm embryo. The loss of zygotic hb in Drosophila results in a "gap" in anterior pattern elements that include the loss of labial and thoracic segments in addition to the fusion of the abdominal segments 7 and 8. The hunchback protein is also expressed in the extraembryonic epithelial tissues and the developing nervous system in the zygote. Although the role of hunchback in AP patterning is likely to be an ancestral trait to the insect clade, higher order comparisons of hunchback orthologs suggest that it is a derived trait specific to the arthropod and/or insect lineage. This view is supported by a combination of comparative gene expression data, phylogenetic analyses, and an examination of the evolution of structural domains in the hb protein isolated from annelids, nematodes, and insects. The 3 independent lines of comparative data strongly support the idea that the anterior organizing function of hb originated in the arthropod and/or insect lineage and that its roles in epithelial and CNS patterning are likely to be broadly conserved within protostomes.