WntA expression and wing transcriptomics illuminate the evolution of stripe patterns in skipper butterflies.

Skippers (Hesperiidae) form a distinct lineage of butterflies where the developmental mechanisms of color patterning have seldom been studied. Skipper wing patterns often consist of median stripes, and studies from the mid-twentieth century suggested these elements are homologous to the central symmetry system (CSS) found in nymphalid butterflies. Here we examined the expression of the signaling ligand gene WntA, known to mark the presumptive CSS patterns in nymphalids, in the silver-spotted skipper Epargyreus clarus, and found support for the homology of the CSS across 95 MY of evolutionary divergence. We generated an annotated genome for E. clarus and used RNAseq to profile gene expression along the wing proximo-distal (P-D) axis. These data suggest that the transcription factor genes lobe, u-shaped, and odd-paired are expressed in restricted P-D sections of the wing similarly to WntA, indicating potential roles in CSS patterning. In addition, developmental genes involved in wing P-D patterning in Drosophila - dachsous, four-jointed, homothorax, tiptop/teashirt, vestigial, scalloped - reveal similar expressions between Diptera and Lepidoptera on the wing P-D axis, suggesting a deep conservation of P-D patterning in insect wings. This work expands our understanding of the mechanisms shaping wing pattern evolution in butterflies.