Evolutionary redeployment of a biosynthetic module: Expression of eye pigment genes vermilion, cinnabar, and white in butterfly wing development

Ommochromes are common among insects as visual pigments; however, in some insect lineages ommochromes have evolved novel functions such as integument coloration and tryptophan secretion. One role of ommochromes, as butterfly wing pigments, can apparently be traced to a single origin in the family Nymphalidae. The synthesis and storage of ommochrome pigments is a complex process that requires the concerted activity of multiple enzyme and transporter molecules. To help understand how this subcellular process appeared in a novel context during evolution, we explored aspects of ommochrome pigment development in the wings of the nymphalid butterfly Vanessa cardui. Using chromatography and radiolabeled precursor incorporation studies we identified the ommochrome xanthommatin as a V. cardui wing pigment. We cloned fragments of two ommochrome enzyme genes, vermilion and cinnabar, and an ommochrome precursor transporter gene, white, and found that these genes were transcribed in wing tissue at relatively high levels during wing scale development. Unexpectedly, however, the spatial patterns of transcription were not associated in a simple way with adult pigment patterns. Although our results suggest that the evolution of ommochrome synthesis in butterfly wings likely arose in part through novel regulation of vermilion, cinnabar, and white transcription, they also point to a complex relationship between transcriptional prepatterns and pigment synthesis in V. cardui.