Morphological novelty can be seen in many groups of organisms and can lead to substantial evolutionary radiations. This is often achieved by modification of primitive structures and anatomy. But how are existing developmental mechanisms modified to produce novel structures? In the Kaufman Lab, I have been exploring how high-level developmental regulatory genes in the arthropods change in function and regulation with morphological diversity. My primary research project is a comparative study of mouthpart and appendage development among insects.

The Hemiptera are a successful and ecologically diverse order of insects, which share a distinctive, highly derived mouthpart morphology. This group encompasses species as varied as leafhoppers and water striders, but also include serious crop pests such as cicadas, aphids and scale, as well as the Chagas disease vector, Triatoma protracta. What makes the Hemiptera unique is the modification of their mandibular and maxillary mouthparts, which exist as long tubular stylets, supported by a completely fused, jointed labium. The homology of hemipteran mouthpart structures to those of primitive, chewing insects remains controversial, despite more than a century of classical anatomical study.

I am investigating the roll of conserved appendage-patterning genes in the development of one hemipteran, the milkweed bug Oncopeltus fasciatus. In Hemiptera, as in all hemimetabolous insects, the appendages develop from limb buds rather than imaginal discs. The expression patterns and function of the genes Distal-less (Dll), dachshund (dac), and homothorax (hth) define distal-to-proximal domains along the appendages in Drosophila, and diverse arthropods have been shown to express orthologues of these genes in similar patterns. However, function of these genes has not been reported in a hemimetabolous insect.

Our data show that Oncopeltus orthologues perform functions in the legs and antennae similar to those described for Drosophila. However, suppression of Dll by RNA interference (RNAi) does not cause antenna-to-leg transformation, as in Drosophila. The specification of the labium requires hth for identity, but dac has no labial phenotype. The stylets require hth and dac for proper development, but Dll is not required in the maxillary stylets, despite the presence of Dll protein. The long-standing questions of hemipteran homologies may be reconsidered in the light of these molecular developmental data. Thus, we find that gene expression and function does not support the prevailing theory, proposed by Snodgrass, that the maxillary stylets are homologous to the lacinia of primitive insects.