Homeotic evidence for the appendicular origin of the labrum in Tribolium castaneum

The ontogeny of the insect labrum, or upper lip, has been debated for nearl y a century. Recent molecular data suggest a segmental appendage origin of this structure. Here we report the first arthropod mutation associated with a homeotic transformation of the labrum. Antennagalea-5 (Ag-5) transforms both antennal and labral structures to resemble those of gnathal appendages in Tribolium castaneum. This labral transformation suggests that the labru m is a fused structure composed of two pairs of appendage endites, and is s erially homologous to the gnathal appendages.