The Diversification of Morphology

Transcriptomic exploration of the coleopteran wings reveals insight into the mechanisms underlying evolution of the novel elytron structure 

The acquisition of morphologically novel structures and their subsequent diversification is central to evolution, yet the molecular mechanisms that underlie this process are still elusive. The unique forewings of beetles (called elytra) can serve as a powerful model to study these mechanisms. In the past, the orthologs of genes important for Drosophila wing formation served as the starting point for studies examining the evolution of elytra (called a candidate gene approach). Although effective, candidate gene lists are finite and can only explore genes that are evolutionarily maintained across species. In an effort to move fully away from candidate genes, we used RNA sequencing (RNAseq) to explore the wing transcriptomes of Tribolium during their development. Using this approach, I have revealed genes enriched in Tribolium elytra (57 genes) as well as genes in the more 'typical' hindwings (29 genes). Impressively, over a third of the hindwing enriched genes were candidate genes, which have been analyzed previously in Tribolium. I also performed a parallel RNAseq analysis in a second beetle species (Dorcus hopei) and compared our results to Tribolium to reveal conserved coleopteran elytron and hindwing enriched genes. Although the overlap was limited, key wing candidate genes were conserved between the two species. Additionally, in both beetles, we discovered patterns in our RNAseq that suggested that the elytra and hindwing transcriptomes were highly similar at any given developmental time point, yet broadly dynamic over even short variations in developmental time. Ultimately, the function of most genes revealed by RNAseq were unknown (non-candidate genes). To go beyond in silico-based approaches we used RNA interference-based gene knockdown (RNAi) to evaluate the function of a subset of these genes in Tribolium. RNAi revealed genes with roles in forming various aspects of the elytron unique morphology, such as pigmentation, hardening, sensory structure, and vein formation. In summary, we used RNAseq and RNAi to reveal and functionally test genes enriched in coleopteran elytron tissue without relying on Drosophila­­-based candidate genes. These analyses have provided insight into the formation of a morphologically novel structure and the mechanisms underlying this critical evolutionary process.