The generation and diversification of butterfly eyespot color patterns

Background: A fundamental challenge of evolutionary and developmental biolo gy is understanding how new characters arise and change. The recently deriv ed eyespots on butterfly wings vary extensively in number and pattern betwe en species and play important roles in predator avoidance. Eyespots form th rough the activity of inductive organizers (foci) at the center of developi ng eyespot fields. Foci are the proposed source of a morphogen, the levels of which determine the color of surrounding wing scale cells. However, it i s unknown how reception of the focal signal translates into rings of differ ent-colored scales, nor how different color schemes arise in different spec ies. Results: We have identified several transcription factors, including butter fly homologs of the Drosophila Engrailed/Invected and Spalt proteins, that are deployed in concentric territories corresponding to the future rings of pigmented scales that compose the adult eyespot. We have isolated a new Bi cyclus anynana wing pattern mutant, Goldeneye, in which the scales of one i nner color ring become the color of a different ring. These changes correla te with shifts in transcription factor expression, suggesting that Goldeney e affects an early regulatory step in eyespot color patterning. In differen t butterfly species, the same transcription factors are expressed in eyespo t fields, but in different relative spatial domains that correlate with div ergent eyespot color schemes. Conclusions: Our results suggest that signaling from the focus induces nest ed rings of regulatory gene expression that subsequently control the final color pattern. Furthermore, the remarkably plastic regulatory interactions downstream of focal signaling have facilitated the evolution of eyespot div ersity.