I remember this older article about reusing gene circuitry for novel evolutionary/developmental features. It's a very interesting topic - in case Fang or anyone wants write on it.

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Science 22 January 1999:
Vol. 283. no. 5401, pp. 532 - 534
DOI: 10.1126/science.283.5401.532

Recruitment of a hedgehog Regulatory Circuit in Butterfly Eyespot Evolution

David N. Keys, * David L. Lewis, * Jane E. Selegue, Bret J. Pearson, Lisa V. Goodrich, Ronald L. Johnson, dagger Julie Gates, ddagger Matthew P. Scott, Sean B. Carroll §

The origin of new morphological characters is a long-standing problem in evolutionary biology. Novelties arise through changes in development, but the nature of these changes is largely unknown. In butterflies, eyespots have evolved as new pattern elements that develop from special organizers called foci. Formation of these foci is associated with novel expression patterns of the Hedgehog signaling protein, its receptor Patched, the transcription factor Cubitus interruptus, and the engrailed target gene that break the conserved compartmental restrictions on this regulatory circuit in insect wings. Redeployment of preexisting regulatory circuits may be a general mechanism underlying the evolution of novelties.