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From Sensory Bristles to the Spots on a Butterfly's Wing
Evolution through co-option
Sean Carroll changed his model organism in the middle of his career. He had learned his skills in a laboratory that studied drosophila development. This is not surprising as the fruit fly had long been the chosen model for studies of molecular development. After acquiring a lab of his own, and after talking to butterfly expert Frederic Nijhout, Carroll decided to study butterflies. What better and more aesthetically appealing organism was there for studying transformation than the organism that changes from a worm-like juvenile to the colorful and delicate winged adult.
The key to the magical appearance of the butterfly is its wings. Butterfly wings often display striking patterns of vivid colors, a quality that Carroll realized made them prime models for understanding developmental variation. Carroll supposed that given the correct molecular tools, he could follow the formation of these patterns during the butterfliy's development. Even more poignant than its colors is the delicacy of a butterfly wing--a pithy realization of the vulnerability of all types of life. That fine powder that moths and butterflies leave on your fingers is actually composed of cells from their wings. Each wing of a a butterfly is covered in a series of colored scales and each of these scales is composed of a single cell. The striking patterns that one sees on butterfly wings, then, are really a complex arrangement of cells, each expressing a different type of pigment in a controlled fashion. The New York Times even noticed the importance of Carroll's choice of organism when they reported that "The spray of colors on a butterfly's wing, a vivid pattern painted with the iridescent dust of thousands of tiny scales, ranks among nature's more entrancing mysteries.” Making a scientific discovery is an important accomplishment; making a scientific discovery that evokes enduring myths about the relationship of culture to biology is entirely more important. A discovery like this brings together two types of logics, a cultural logic of transformation and a biological logic of development. It is what we've been calling a gothic moment.
This cover from Science, 1 July 1994, demonstrates this beautifully. The picture is a teaser for an article from Sean Carroll et. al., succinctly entitled "Pattern Formation and Eye Spot Determination in Butterfly Wings". In a short feature written by H. Frederik Nijhout that discusses Carroll's article (entitled "Genes on the Wing"), the famous lepidopterist explains why following the development of spots on a butterfly wing is an interesting scientific problem. "Discovering the mechanisms by which structure emerges from an initially homogeneous field of cells continues to be one of the fundamental problems in developmental biology", urges Nijhout. The next page will explaining in greater detail how Sean Carroll's key experiments understanding the fantastical transformations of butterfly wings allowed him to glimpse how some genes are implicated in development.