The evolution of pattern formation in butterfly wings

Abstract

In this dissertation I employ a comparative gene expression approach to address the evolution of butterfly wing pattern formation at several levels, with emphasis on early pattern determination and pigment gene regulation during late development. Expression analysis of the receptor molecule Notch suggested previously unknown roles for Notch signaling in butterfly wing patterning. Notch upregulation was found to precede the activation of the transcription factor Distal-less during early eyespot color pattern determination. A phylogenetic comparison of expression time series from multiple moth and butterfly species suggested that changes in a Notch/Distal-less temporal pattern formation process were associated with the gain and loss of both eyespot and midline color patterns during wing pattern evolution. Additionally, Notch expression was found to occur in a grid-like pattern in the butterfly wing epithelium shortly after pupation. This observation, together with previous expression and simulation studies, support a Notch-mediated lateral inhibition model of wing scale organization. Tryptophan-derived ommochrome pigments are a derived feature of nymphalid butterfly wings. I found that multiple genes in the ommochrome biosynthetic pathway were expressed in the wings of selected nymphalid butterflies. Additionally, transcriptional regulation of genes encoding the ommochrome synthesis enzymes vermilion and cinnabar was found to be temporally and spatially associated with the polymorphism and development of forewing band patterns in the mimetic butterfly Heliconius erato. These findings provide evidence that changes in ommochrome gene regulation underlie the evolution and development of major nymphalid wing pattern elements.