Evolution of a novel wing pigmentation pattern in Drosophila : when engrailed crosses the line. Héloïse D. Dufour, Cédric Finet, Shigeyuki Koshikawa, Jane E. Selegue, Sean B. Carroll. HHMI/UW Madison, Madison, WI.

   Color patterns in animal play crucial ecological, physiological and behavioral roles. However, the molecular mechanisms underlying the evolution of complex color patterns are still largely unknown. Here, we investigate the generation and evolution of a complex white and black spot pattern in the wing of the fruit fly Samoaia leonensis. We show that the white spots correlate with engrailed pupal expression. This is an unexpected result as engrailed, which encodes a homeodomain transcription factor, plays a crucial role in the establishment of the posterior identity of Arthropod segments, where it is expressed. This role and expression pattern has been highly constrained for the last 500 million years. Earlier in development though, in the imaginal disc, the engrailed expression is restrained to its posterior canonical pattern in S. leonensis. This suggests that the engrailed role in establishing the posterior identity is maintained, while the gene is later recruited and redeployed to repress dark pigmentation. Transgenesis was established in this species to test this hypothesis. Furthermore, by collecting closely related species and reconstructing their phylogenetic relationships, we show that the spotty engrailed pattern likely evolved on a black wing background, thereby providing a visible phenotype for selection to act on. Those results make an example of how, once their role in body plan establishment is accomplished, crucial develomental genes can acquire discrete new functions.