Transcriptomic exploration of the Coleopteran wings reveals insight into the evolution of novel structures associated with the beetle elytron

Abstract

The acquisition of novel traits is central to organismal evolution, yet the molecular mechanisms underlying this process are elusive. The beetle forewings (elytra) are evolutionarily modified to serve as a protective shield, providing a unique opportunity to study these mechanisms. In the past, the orthologs of genes within the wing gene network from Drosophila studies served as the starting point when studying the evolution of elytra (candidate genes). Although effective, candidate gene lists are finite and only explore genes conserved across species. To go beyond candidate genes, we used RNA sequencing and explored the wing transcriptomes of two Coleopteran species, the red flour beetle (Tribolium castaneum) and the Japanese stag beetle (Dorcus hopei). Our analysis revealed sets of genes enriched in Tribolium elytra (57 genes) and genes unique to the hindwings, which possess more "typical" insect wing morphologies (29 genes). Over a third of the hindwing-enriched genes were "candidate genes" whose functions were previously analyzed in Tribolium, demonstrating the robustness of our sequencing. Although the overlap was limited, transcriptomic comparison between the beetle species found a common set of genes, including key wing genes, enriched in either elytra or hindwings. Our RNA interference analysis for elytron-enriched genes in Tribolium uncovered novel genes with roles in forming various aspects of morphology that are unique to elytra, such as pigmentation, hardening, sensory development, and vein formation. Our analyses deepen our understanding of how gene network evolution facilitated the emergence of the elytron, a unique structure critical to the evolutionary success of beetles.

Beetle elytra: evolution, modifications and biological functions

“…The formation of elytra occurred together with other modifications, including longitudinal and transverse hind wing folding which were explored by Haas [ 31 ] (see also [ 32 ]), enlargement of the metathorax, simplification of the pterothoracic musculature [ 33 ] and formation of elytral sensory structures [ 6 , 34 ].…”

“…The co-option took place in at least three stages: dorsal ‘exoskeletalization’ regulated by the apterous genes ( ap ), pre-vein ‘exoskeletalization’ (genetic modulator unknown), and the ‘exoskeletalization’ of adjacent sensory bristles (modulated by the single beetle homologue of ac and sc ). A recent study based on the exploration of wing transcriptomes has uncovered novel genes involved in the process of elytra formation, especially in hardening and pigmentation— Tc-hr38 (TC013146), formation of elytral sensory structures— Tc-hr38 (TC013146) and formation of elytral venation— Tc-chemosensory10 (TC008682) [ 34 ]. Shape transition resulting in closely fitting of wing cases was achieved through the neo-functionalization of the wing gene, abrupt [ 38 ].…”