Howard Hughes Medical Institute 2010 Annual Report

Wing patterns in males of three fruit fly species: <em>Drosophila melanogaster</em> (left), <em>Drosophila biarmipes</em> (middle), and <em>Drosophila guttifera</em> (right). The patterns, controlled by the spatial distribution of a single molecule, are thought to be important in certain courtship behaviors. African fruit flies living at high elevations have dark abdomens, instead of the lighter coloring of flies that live at lower elevations. The magnificent diversity of pigment patterns on the wings of the different fly species represented here has evolved over 60 to 80 million years. The wing pigmentation patterns in insects have various biological functions, including mimicry, camouflage, body temperature regulation, and mate selection. A theme in evolutionary and developmental biology (“evo-devo”) is that patterns of gene expression in early stages of development (left column) reflect forms in later in development (right column). This theme is illustrated by a fruit fly embryo vs. a later-stage fruit fly larva (top row); and a developmental form of a butterfly wing vs. an adult butterfly wing (bottom row).

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The Art of Evolution

It’s been a banner year for Sean B. Carroll, a longtime HHMI investigator at the University of Wisconsin–Madison. He published a critically acclaimed book, Remarkable Creatures; began writing a series of thought-provoking science essays for The New York Times; was nominated as a finalist for The National Book Award; and became HHMI’s vice president for science education, succeeding Peter J. Bruns.

Carroll, a noted chronicler of Charles Darwin’s scientific legacy, has spent his career studying the genetic underpinnings of evolution and development, identifying the molecular mechanisms that lead to new traits and species. He has said he views the modern era as a second Golden Age in evolutionary science—the first occurring in the 1800s, during Darwin’s lifetime.

In contrast to the descriptive data about life on Earth collected by Victorian scientists, researchers now have at their fingertips an incredible amount of detailed genetic information from genome sequences of myriad species. And, more importantly, they now have access to powerful research tools that can parse the genetic equivalent of the fossil record in a matter of seconds.

“We have developed new ways of peering into the past that even modern biologists would have thought impossible until very recently,” Carroll says.

Carroll’s own research is helping to provide answers to questions at the heart of some of evolutionary biology’s deepest mysteries. In the past year, his group published two landmark research articles on how genes are regulated to control color changes in fruit flies and how genes work together to create new wing spots.

In the first study, which was published in Science, Carroll and collaborators traced a yellow-to-black color change in African fruit flies to five single-letter genetic mutations. The scientists also pinpointed when the mutations appeared in natural fly populations. The studies showed that the mutations do not appear within the code of the color-producing gene itself, but instead just outside of the gene, in a region that helps switch the gene on or off.

“For big evolutionary changes in a species, such as changes in body morphology or color, these regulatory regions are where we think the action is,” says Carroll.

In the group’s second major paper of the year, they examined how Drosophila guttifera—a close relative of the genetic workhorse, Drosophila melanogaster—acquired its characteristic “polka-dotted” wings. The work, which was published in Nature, showed that pigment production in the wing is patterned according to the spatial distribution of a molecule that helps sculpt the shape of the body during development.

The finding underscores the concept that evolution likes to tinker with existing genetic machinery to evolve new patterns and forms. “When nature invents, it does so by using what is readily available,” says Carroll.

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Photo Credit: Slide 1: Benjamin Prud’homme and Nicolas Gompel  Slide 2: John Pool and Sean B. Carroll  Slide 3: Benjamin Prud'homme, Nicolas Gompel and Sean B. Carroll  Slide 4: Steve Paddock, Jim Langeland, Jayne Selegue and Craig Brunetti