As a graduate student, David Stern traveled to far-flung lands to study the evolutionary history of a sterile caste of plant-sucking aphids.

To gardeners, aphids are disease-carrying pests that can suck the life out of plants. The tiny, soft, green insects, known as “plant lice,” brought France's legendary industry to near collapse in the Great French Wine Blight of the mid-1800s.

To evolutionary scientists, however, aphids are a fascinating oddity that have played an “outsized role” in the history of biology, says David Stern, an HHMI investigator at Princeton University. And they were his ticket to see the world.

		Exotic Aphids See pictures of aphids and galls from David Stern's travels around the world.

Stern describes aphids' unique characteristics: “With the same genome, they can make different phenotypes, depending on the environment.” For example, they can reproduce with or without the help of a mate and can spawn offspring with wings or without them. Certain aphids provoke trees into forming galls—bizarre, sometimes beautiful, hollow growths that serve as aphid hideaways. Some aphid species breed castes of “soldiers”—fierce (but sterile) warriors. Some variants poison their predators and some stab them with sharp horns, wrote Stern in an admiring essay, while others “squeeze their enemies into submission with their fat hind legs.”

To Stern, an avid gardener since age six, aphids were merely a nuisance until, during his graduate studies, he took a five-year sojourn in Japan, Malaysia, Indonesia, and Taiwan to study aphids in the field.

The discovery of the sterile caste of aphid soldiers in the 1970s by a Japanese taxonomist had raised interesting questions about the evolution of social behavior. Intrigued, Stern packed his bags for Japan in 1989 and spent several years collecting aphids and sequencing their DNA—a time-consuming process in those days. He then used DNA evidence to determine how many times the soldiers had emerged in aphids' evolutionary history.

The fieldwork for his Ph.D. “was not as romantic as it sounds,” Stern admits. The sapsuckers were most abundant on roadside plants. “So I would walk along the road, flipping over leaves.” He also attached clippers to long poles to cut off branches where soldier-producing species had formed galls, so he could bag the aphids inside their domiciles.

Photo: Adi Lavy

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