Concerted evolution and developmental integration in modular butterfly wing patterns.

Developing organisms are thought to be modular in organization so that traits in different modules evolve independently whereas traits within a module change in a concerted manner. The eyespot pattern in Bicyclus anynana butterflies provides an ideal system where morphological modularity can be dissected and different levels of genetic integration analyzed. Several lines of evidence show that all eyespots in an individual butterfly are genetically integrated, suggesting that the whole pattern, rather than the separate eyespots, should be considered as a single character. However, despite the strong genetic correlations between the two eyespots on the dorsal forewing of B. anynana, there is great potential for independent changes. Here we use laboratory lines selected in different directions for the size of those eyespots to study correlated responses in the whole eyespot pattern. We show clear changes in eyespot size across all wing surfaces, which depend on eyespot position along the anterior-posterior axis. There are also changes in the number of extra eyespots and in eyespot color composition but no changes in eyespot position relative to wing margin. Our analysis of eyespot pattern modularity is discussed in the light of what is known about the cellular and genetic mechanisms of eyespot formation and the great potential for evolutionary diversification in butterfly wing patterns.