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 s ource of a diffusible morphogen gradient. We show that ectopic eyespot s can be induced in nonfocal positions throughout the distal, but not the proximal, wing epidermis of Bicyclus anynana by mild epidermal dam age inflicted at 12-18 hr (into a 6- to 7-day pupal period). Damage ma y lower, locally and transiently, the threshold for response to morpho gen. Here, we have tested two predictions of the gradient model. As pr edicted, mild damage close to the focus (parafocal) locally extends th e eyespot, making it markedly asymmetrical, even after early operation s (1 or 6 hr), when remote epidermis is nonresponsive. Early parafocal operations also have an unexpected result, however, reducing the exte nt 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 op eration will prolong the transient damage and hence give ectopic eyesp ots 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 (1 8 hr) operation at the same position. The gradient model is therefore supported by most aspects of eyespot induction in response to epiderma l damage. (C) 1995 Academic Press, Inc.