RNAi analysis of Deformed, proboscipedia and Sex combs reduced in the milkweed bug Oncopeltus fasciatus: novel roles for Hox genes in the Hemipteran head

Insects have evolved a large variety of specialized feeding strategies, wit h a corresponding variability in mouthpart morphology, We have, however, li ttle understanding of the developmental mechanisms that underlie this diver sity. Until recently it was difficult to perform any analysis of gene funct ion outside of the genetic model insects Drosophila melanogaster and Tribol ium castaneum. In this paper, we report the use of dsRNA-mediated interfere nce (RNAi) to dissect gene function in the development of the milkweed bug Oncopeltus fasciatus, which has specialized suctorial mouthparts, The Hox g enes Deformed (Dfd), proboscipedia (pb) and Sex combs reduced (Scr) have pr eviously been shown to be expressed in the gnathal appendages of this speci es. Strikingly, the milkweed bug was found to have an unusual expression pa ttern of pb, Here, by analyzing single and combination RNAi depletions, we find that Dfd, pb and Scr are used in the milkweed bug to specify the ident ity of the mouthparts, The exact roles of the genes, however, are different from what is known in the two genetic model insects. The maxillary appenda ges in the bug are determined by the activities of the genes Dfd and Scr, r ather than Dfd and pb as in the fly and beetle. The mandibular appendages a re specified by Dfd, but their unique morphology in Oncopeltus suggests tha t Dfd's target genes are different. As in flies and beetles, the labium is specified by the combined activities of pb and Scr, but again, the function of pb appears to be different. Additionally, the regulatory control of pb by the other two genes seems to be different in the bug than in either of t he other species. These novelties in Hox function, expression pattern and r egulatory relationships may have been important for the evolution of the un ique Hemipteran head.