Meeting Abstract
Understanding the origins of new characters is crucial to explaining the diversity of life. In the insects, new body parts often arise as projections of the body wall that are then molded by natural selection, e.g. beetles’ horns. A stunning example of novelty is found in the hemipteran family Membracidae (treehoppers), sap-sucking insects with enlarged, elaborate projections of the pronotum (dorsal body wall) termed helmets. Treehopper helmets have been sculpted by natural selection into an array of structures aiding predator defense via crypsis, mimicry, and aposematism. Members of their sister group, the leafhoppers (Cicadellidae), exhibit the plesiomorphic condition, a flat, shield-like pronotum. We tested three hypotheses for the developmental origin of the treehopper helmet using comparative transcriptomics in four tissues of nymphal Entylia carinata (a treehopper) and Homalodisca vitripennis (a leafhopper). Differential gene expression analysis indicates that in the leafhopper, the pronotum and mesonotum are most similar, as would be predicted of serial homologs. In treehoppers, however, gene expression in the developing pronotum/helmet is most similar to that of wings. Many transcripts upregulated in both wings and helmet of the treehopper are known for their roles in Drosophila wing development, including nubbin, vestigial, and wingless. This preliminary evidence supports a wing-cooption scenario for the origin of the treehopper helmet, wherein elements of the wing-patterning network are redeployed in the novel context of developing pronotal tissue. We are now evaluating the functional significance of these expression similarities with comparative RNAi in E. carinata and milkweed bugs (Oncopeltus).
