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Eyespot development on butterfly wings: the focal signal.

The eyespot colour pattern on butterfly wings is specified in the early pupal epidermis by signals from a central "focus." In Bicyclus anynana we show that a small square of focal epidermis, grafted to a range of distal wing sites, induces eyespot formation in surrounding host tissue. Signaling is limited to the focus, and even an adjacent (parafocal) graft does not maintain its normal fate (of contributing to the eyespot) and does not influence its surroundings. Along the wing, there is an abrupt change in the epidermis, as a focus grafted to a proximal site provokes no host response. The results of several grafting experiments demonstrate that their different response properties are autonomous to small areas of the distal and proximal epidermis and that the nonresponding proximal tissue can nonetheless transmit the focal sign. The Bicyclus dorsal forewing has a small anterior and a large posterior eyespot, and we show that this results mainly from a difference in focal signals, not in the epidermal response. A grafted posterior focus induces a large eyespot, whereas an anterior focus induces a small eyespot. Furthermore, the anterior and posterior eyespots differ in proportions, and this difference also depends on the identity of the focus, not on the responding epidermis. Eyespots are specified over many cell diameters from the focus by a mechanism which could consist of one long-range signal, such as a morphogen gradient or of a cascade of short-range interactions initiated by the focus. Focal control of the difference in size and proportion between the anterior and posterior eyespot is more readily compatible with a gradient mechanism. Neither model, however, readily explains why the pattern induced by a grafted focus is smaller, but its peripheral gold annulus is broader than in the corresponding control eyespot. Also, there is no direct evidence for long-range gradients, in the butterfly wing or any other insect epithelium.