| Understanding the processes which govern morphological evolution has been one of the fundamental challenges in biology over the last century and a half. A new approach to this problem is to assay the conservation and diversification of developmental processes involved in the evolution of body structure. Lepidoptera are an excellent model system for such research as they have undergone extensive diversification, and, due to their relatively close phylogenetic proximity to Drosophila melanogaster, are amenable to genetic and developmental study. To identify the molecular process underlying the generation of wing color patterns, several cognates of Drosophila appendage patterning genes have been cloned and their expression patterns analyzed in multiple species, as well as selected lines, mutant lines, and polyphenic lines which produce different color pattern phenotypes. The results of these studies indicate that the development of the general shape and morphology of Lepidoptera wings involves two inductive organizing centers, the anterior--posterior boundary and the distal margin, which are shared with Drosophila. In addition, many of the genes which have been shown to be patterned in relation to these two organizers also have secondary Lepidopteran specific expression patterns whose domains are indicative of involvement with color pattern formation. The developmental pathway that leads to eyespots is shown to be a four step process which progressively determines the presence, positions, number, sizes and colors of the eyespots. A model showing how this developmental pathway is flexible to modulations at each of these stages suggests that the evolution of eyespot patterns can occur rapidly, and requires only single, or very few changes in the regulatory genes. The association of one of the stages of eyespot development with components of the Hedgehog signaling pathway leads to the proposal that eyespot development reutilizes regulatory components of the anterior/posterior boundary organizing center. I conclude from these observations that the origin and diversification of Lepidopteran color patterns has involved various levels of changes and redeployments of preexisting regulatory circuits, and that this may be a general mechanisms underlying morphological evolution. |
