Eyespot development on butterfly wings: the epidermal response to damage.

Eyespot colour patterns decorate the wings of many butterfly species. The eyespot is specified in the early pupal epidermis by signals from a central "focus," and it has been suggested that the focus is the source of a diffusible morphogen gradient. We show that ectopic eyespots can be induced in nonfocal positions throughout the distal, but not the proximal, wing epidermis of Bicyclus anynana by mild epidermal damage inflicted at 12-18 hr (into a 6- to 7-day pupal period). Damage may lower, locally and transiently, the threshold for response to morphogen. Here, we have tested two predictions of the gradient model. As predicted, mild damage close to the focus (parafocal) locally extends the eyespot, making it markedly asymmetrical, even after early operations (1 or 6 hr), when remote epidermis is non-responsive. Early parafocal operations also have an unexpected result, however, reducing the extent of the eyespot in other directions, perhaps through a long-range "wound effect" on the signaling activity of the focal cells. The model also correctly predicts that increasing the severity of a nonfocal operation will prolong the transient damage and hence give ectopic eyespots after early (1- or 6-hr) operations. We do not, however, find the expected size increase in ectopics induced by the later (12 or 18 hr) severe damage. Similarly, we demonstrate the predicted effect of early (1 or 6 hr) mild damage in increasing the response to a subsequent (18 hr) operation at the same position. The gradient model is therefore supported by most aspects of eyespot induction in response to epidermal damage.