MADISON - How the elephant got its trunk, the deer its antlers and the rattlesnake its rattles may seem like disparate questions of developmental biology, but the origins of these novelties, according to the genes of butterflies, may have much in common.
Writing in this week's edition (Jan. 22) of the journal Science, scientists from the Howard Hughes Medical Institute at the University of Wisconsin-Madison describe a genetic trick that helps explain the staggering diversity of patterning and color exhibited on butterfly wings. The same trick, the scientists suggest, is widely used among animals and may be one of the underlying mechanisms that helps explain how new morphological characteristics -- from teeth and tortoise shells to fur and feathers -- arise through the course of evolution.
"The origin of new morphological characters is a long-standing problem in evolutionary biology," write molecular biologists David N. Keys, David L. Lewis and Sean B. Carroll in a paper that may help explain how unique bits of body architecture are added long after an animal's basic body has evolved.
The new insight into how animals acquire "novelties" was derived from comparisons of the wing-making genes of fruit flies and two very different species of butterflies, one from North America and one from East Africa. By tracing the genetic circuits that govern the development of wing shape and coloration, the Wisconsin scientists discovered that butterflies, instead of inventing new genes for color, simply reuse a part of the wing-building genetic circuit to turn on enzymes that produce the pigments responsible for wing decoration.
In the big scheme of animal evolution, according to Keys, a graduate student in Carroll's lab, the decoration of a butterfly's wing is a much more recent invention than the building of the wing itself: "Evolution, somewhere along the line, took this system and came up with a way to reuse it in an entirely new context while maintaining its original function."
"To our thinking, the reuse of genes makes innovation easier," said Carroll, a biologist whose work has helped resolve the role genes play in making such things as arms, legs and wings .
"Evolution," he said, "is working by integrating sets of things it already has. You're using a circuit over and over again."
How butterflies co-opt existing genetic programs to create eyespot patterns that help them fool predators, said Carroll, may in fact be a general mechanism animals use to create "luxury items -- teeth, antlers, shells, hair, coloration -- in the course of evolution."
"The invention of these novelties is an important facet of animal evolution," said Lewis, a UW-Madison post-doctoral fellow. "They impact hugely the lifestyles of the organisms."
Moreover, according to Carroll, since these luxuries are genetically wired to a specific developmental output, they can be further tinkered with at the genetic level to create new variants. Think, for example, of the enormous diversity of pattern and color in butterflies, or the array of antlers sported by different members of the deer family such as elk and moose.
"Once you make a new connection, it can independently evolve" through the interplay of development and genetic change, Carroll said. "Evolution can muck with that by changing things in the circuit. At the ends of these (genetic) pathways, the output can be very different."
"One of the amazing things about butterflies is that these genetic programs result in a tremendous variety of color patterns, not structures," said Lewis.
"Few would have guessed," said Keys, "that those beautiful color patterns evolved from the same genetic processes which all insects use to shape their wings."
Co-authors of the paper include Jane E. Selegue and Bret J. Pearson, also of UW-Madison; Lisa V. Goodrich, Ronald L. Johnson and Matthew P. Scott of the Howard Hughes Medical Research Institute at the Stanford University Medical Center; and Julie Gates of the Howard Hughes Medical Institute at the University of Utah.
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