Implications of the Tribolium Deformed mutant phenotype for the evolution of Hox gene function

Among insects, the genetic regulation of regional identities in the postoral head or gnathal segments (mandibular, maxillary, and labial) is best understood in the fly Drosophila melanogaster. In part, normal gnathal development depends on Deformed (Dfd) and Sex combs reduced (Scr), genes in the split Drosophila homeotic complex. The gnathal segments of Dfd and Scr mutant larvae are abnormal but not homeotically transformed. In the red flour beetle, Tribolium castaneum, we have isolated loss-of-function mutations of the Deformed ortholog, Mutant larvae display a strong transformation of mandibular appendages to antennae. The maxillary appendages, normally composed of an endite and a telopodite, develop only the telopodite in mutant larvae. We previously reported that mutations in the beetle Scr and Antennapedia orthologs cause the labial and thoracic appendages, respectively, to be transformed to antennae. Moreover, a deficiency of most of the beetle homeotic complex causes all gnathal (as well as thoracic and abdominal) segments to develop antennae. These and other observations are consistent with the hypothesis that ancestral insect homeotic gene functions have been modified considerably during the evolution of the highly specialized maggot head. One of the ancestral homeobox genes that arose close to the root of the Eumetazoa appears to have given rise to Dfd, Scr, and the Antennapedia homeobox-class homeotic genes. Evidence from both Tribolium and Drosophila suggests that this ancestral gene served to repress anterior development as well as confer a trunk-specific identity.