Butterfly wing pattern mutants: developmental heterochrony and co-ordinately regulated phenotypes

Butterfly wings are colored late in development, when pigments are synthesi zed in specialized wing scale cells in a fixed developmental succession. In this succession, colored pigments are deposited first and the remaining ar eas are later melanized black or brown. Here we studied the developmental c hanges underlying two wing pattern mutants, firstly melanic mutants of the swallowtail Papilio glaucus, in which the yellow background is turned black , and secondly a Spotty mutant of the satyrid Bicyclus anynana, which carri es two additional eyespots. Despite the Very different pattern changes in t hese two mutants, they are both associated with changes in rates of scale d evelopment and correspondingly, the final color pattern. In the melanic swa llowtail, background scales originally destined to become yellow (normally developing early and synthesizing papiliochrome) show delayed development, fail to make papiliochrome, and subsequently melanize at the same time as s cales in the wild-type black pattern. In the B. anynana eyespot. scale matu ration begins with the central white focus, then progresses to the surround ing gold ring and later finishes with melanization of the black center. Mut ants showing additional eyespots display accelerated rates of scale develop ment (corresponding to new eyespots) in wing cells not normally occupied by eyespots. Thus by either delaying or accelerating rates of scale developme nt, the final color, or position, of a wing pattern element can be changed. We propose that this heterochrony of scale development is a basic mechanis m of color pattern formation on which developmental mutants act to change l epidopteran color patterns.