Investigating the Evolution of Morphology

Posted February 26th, 2009 at 2:03 pm.

Gregory Davis

As a graduate student at the University of Chicago, Gregory Davis pursued what might now be considered more traditional evolutionary developmental biology research, focusing on broad phylogenetic comparisons. He investigated differences in expression patterns among arthropods in a group of genes involved at different levels of segmentation in Drosophila.

“What I really wanted to do, though, was study how evolving development actually affects morphology—how we get such incredible diversity in the biological world,” says Davis, now an assistant professor of biology at Bryn Mawr College. In the late ’90s, Davis notes, technology was not yet advanced enough to enable researchers to determine whether gene-expression differences were functionally significant. “I was looking for a system in which I could do that,” Davis says.

As a postdoctoral researcher in the laboratory of David Stern at Princeton University, Davis focused on identifying changes in the regulation of developmental genes responsible for the evolution of morphology between closely related Drosophila species. Because the species are related, Davis explains, it is possible to cross them to demonstrate the genetic basis of morphological change. “A lot of these evolved differences are due to differences in the timing and spatial expansion of genes,” Davis says. “The genome sequences are so closely related that you can align them and see which parts of the genome in one species correspond to the parts in the other. This is a huge advantage.” His postdoctoral research aimed to identify the enhancers that control gene expression and understand how those enhancers have evolved to alter morphology.

Environmentally Cued Phenotypes

Davis’s current research interest involves polyphenism (a type of plasticity resulting in different phenotypes) in aphids, which develop as either winged or unwinged, depending on environmental cues. “The environment sends them down one track or the other as they’re developing,” he explains. “It’s very discrete—there’s not a continuous range of them; it’s just the winged and unwinged forms. Their bodies are different, and their behaviors are totally different.”

Wings develop in aphids “as a dispersal mechanism when the mother is exposed to a crowded environmental situation,” Davis explains. “Aphids adapt to an unstable environment without modifying their genome, which would require more time and commitment than they can afford. They have amazing flexibility.” Interestingly, he notes, aphids also exhibit a reproductive polyphenism; female aphids, which can reproduce asexually, develop as either sexual or asexual. Not only does the morphology of asexual and sexual females differ, but so does the development of their progeny, Davis notes. Asexual reproduction involves yolkless eggs that develop within the mother; in sexual reproduction, the mother lays an egg that is specially adapted to survive the winter.

“In the summer months, they are exclusively asexual,” Davis says. Offspring of asexual reproduction, which are all females, are genetically identical to each other. In the winter, when the nights get longer, sexual females and males are produced. “In aphids, sex is really about getting through the winter,” Davis explains. “In the summer, they can dispense with males.”

Just as he did as a postdoc, Davis says, he is comparing gene expression and inferring function, “but in this case it’s two different modes of the same species, all directed by the same genome—not different species.”

With the help of undergraduates at Bryn Mawr, Davis will study polyphenism. He will investigate how aphid embryos choose a particular developmental pathway, and how the molecular mechanisms of pattern formation change to accommodate changes in embryology.

Diverse Background

As an undergraduate at Duke University, Davis double-majored in biology and philosophy. “Most of the philosophy that I studied was epistemological,” he says, “I wanted to become a philosophically informed scientist; my interests were broad at that point.”

Davis also earned a certificate in women’s studies at Duke. “Feminism, to me, seemed to be some of the most exciting work going on,” he says. He joined Men Acting for Change, an undergraduate group that discussed gender relations.

After graduating from Duke, Davis earned a master’s degree in history and philosophy of science from the University of Pittsburgh, where he studied early efforts to integrate studies of evolution and development. “I came out of the program with a much deeper appreciation of history, and perhaps more skeptical about continuing in philosophy,” he says.

Davis’ master’s studies also led him to a targeted research interest. “I got hooked on evolutionary developmental biology, or ‘evo-devo.’ I wanted to be a part of this in contemporary science.”

Broad Horizons

In the spring semester, Davis is teaching a seminar-based course on the history of biology. “I’m really enjoying the flexibility at Bryn Mawr,” he says. “In the context of teaching, I can explore all of these areas with students, which is engaging and exciting.

“As a postdoc, concentrating on a very focused project can be intellectually isolating. At Bryn Mawr, I have the opportunity to pursue a more eclectic menu of interests.”

Davis says he is enjoying his interactions with new faculty from Bryn Mawr and Haverford Colleges in the Teaching and Learning Initiative. “It’s a very nice forum to exchange ideas and share new methods.”

Davis is enjoying his students, as well. “They are amazingly engaged,” he says. “I have a small group of students, and I feel that I know them.”

Barbara Spector writes on science and technology as well as business topics. She is the editor-in-chief of Family Business magazine and former editor of The Scientist.

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