The Developmental Origin of Novel Complex Morphological Traits in Lepidoptera.

Novel traits in the order Lepidoptera include prolegs in the abdomen of larvae, scales, and eyespot and band color patterns in the wings of adults. We review recent work that investigates the developmental origin and diversification of these four traits from a gene-regulatory network (GRN) perspective. While prolegs and eyespots appear to derive from distinct ancestral GRNs co-opted to novel body regions, scales derive from in situ modifications of a sensory bristle GRN. The origin of the basal and central symmetry systems of bands on the wing is associated with the expression of the WntA gene in those regions, whereas the more marginal bands depend on two other genes, Distal-less and spalt. Finally, several genes have been discovered that play important roles in regulating background wing color, via the regulation of pigmentation GRNs. The identification of shared and novel cis-regulatory elements of genes belonging to these distinct GRNs helps trace the developmental and evolutionary history of these traits. Future work should examine the extent to which ancestral GRNs are co-opted/modified to produce the novel traits and how these GRNs map to specific cell types in ancestral and derived traits.