Nancy Moran on a rooftop with boxes of her research

Nancy Moran keeps honey bees on a university building rooftop so she can study their microbiomes.

Julia Robinson

A love of insects and their microbial partners helped this biologist reveal secrets of symbiosis

Nancy Moran has found clues to evolution in some unlikely places. Some 20 years ago, living in Arizona, she would frequent a Mexican restaurant in Tucson for more than its food. She regularly climbed the fire escape behind it to visit the upper branches of a hackberry tree—along with all the insects lurking there. One night, she reached into the foliage and scooped up a nondescript bug that helped change the way she and other biologists think about the evolution of complex life.

The sesame seed–size bug she nabbed—a psyllid, which causes the plant stems or leaves it feeds on to form hard nodules called galls around the insect—harbored symbiotic bacteria that appear to capture a key stage in the evolution of the cell. Their genomes are so shrunken, Moran found when she returned to her lab and analyzed the bug’s microbial cargo, that they seem to be losing their ability to live on their own. They may be on their way to turning into organelles, like mitochondria and chloroplasts, which originated as symbiotic microbes early in the history of life but ultimately became dependent wards of the cell.

Moran, an evolutionary biologist now at the University of Texas (UT) in Austin, has built a career from groundbreaking findings made in plant-dwelling insects. Her work on psyllids, aphids, and other sap-sucking insects has uncovered intricate, intertwined relationships with internal bacteria, which help them survive on a meager diet of plant juices. Moran is “one of the people who pioneered symbiosis as a field and did so with rigorous work and creativity,” says John McCutcheon, a former postdoc and now an evolutionary biologist at the University of Montana in Missoula.

Today, such symbioses are widely recognized for creating life as we know it. Energy-producing mitochondria power all complex cells; chloroplasts, where photosynthesis takes place, make plant life possible. The cementing of other host-microbial alliances enabled animals to expand what they could eat, diversify into new species, and conquer almost all parts of the planet. We humans are increasingly aware that communities of microbes in our guts, on our skin, and elsewhere—our microbiome—shape our physical and perhaps even mental well-being.

Moran, who received a MacArthur “genius grant” early in her career and was elected to the National Academy of Sciences in 2004, has developed her own vital partnership. She has teamed up with Howard Ochman, another UT biologist, for more than 20 years, both personally—they married in 1998—and professionally. She has dedicated her career to symbiosis; he has ranged more widely but has contributed fundamental principles about how microbes evolve. “This is quite the power couple,” says biotechnologist Andrew Ellington, a UT colleague.

After decades uncovering the evolutionary roots of symbiosis, Moran now looks to microbial communities for ways to address today’s challenges. She’s studying the gut bacteria in bees, which depend on microbial guests to thrive. That new system, she hopes, will suggest ways to stop the decline of the bees and other pollinators and perhaps yield a simple model for exploring the roles of gut microbes in people.

Honey bees need their gut bacteria to thrive and keep their hives healthy.

Julia Robinson

While playing outside with her seven siblings or hanging out at the Dallas, Texas, drive-in theater her father ran, the young Moran would collect bugs, leaves, and flowers wherever she could. “I was known as the kid who liked plants and insects,” she recalls. Her favorites were the tarantulas. (Yes, the entomology Ph.D. knows they are spiders, not insects.) She kept them in jars and fed them crickets. Her family accepted her hobbies, fretting only when, at age 9, she convinced a friend they should test whether the poison ivy next to the school playground really could cause a rash. “That was a horrible disaster,” Moran recalls.

Yet she was slow to realize that she could make a career of biology. At UT, she majored first in art and then in philosophy. But an introductory biology class, a university requirement, had an enduring impact. “Once I learned about evolution and natural selection, I decided this was the most interesting thing to spend time on,” Moran says.

As a graduate student at the University of Michigan in Ann Arbor, Moran trained with the famous 20th century theoretical evolutionary biologist W. D. Hamilton, and they became close friends. “We talked about everything … big ideas and what kinds of science make a difference in understanding the evolution of life,” Moran says. Entomology remained her first love, however. Every free moment she wriggled into bushes, looked under leaves, and peered into flowers to see what new insect species she could find.

After she took a faculty job at the University of Arizona in Tucson in 1986, a phone call from Paul Baumann, a microbiologist at the University of California (UC), Davis, helped her link her two scientific passions. Baumann was studying Buchnera, a once free-living bacterium now found solely inside aphids. In the 1960s, a German biologist named Paul Buchner had cataloged these endosymbionts and written a tome with intricate illustrations of where they lived in the aphids, as well as in lice, beetles, and other insects. Buchner suggested those symbioses were essential, life-long relationships that had existed for millions of years.

If so, the microbes and the insects must have evolved together—and their DNA should tell the tale. To test the idea, Baumann needed Moran’s aphid expertise. By sequencing the genomes of various aphid species and their Buchnera, Baumann and Moran built family trees for both organisms, and found that the microbes had diversified in step with the insects. Using various aphid fossils to date the trees, they found that the partnership began some 200 million years ago. Since then, Buchnera has passed from one aphid generation to the next, coevolving with its host.

Can’t live without you

Aphids dine on sap they suck from a plant’s phloem, or circulatory system, but that diet lacks key nutrients. The insects rely on internal bacteria called Buchnera to convert amino acids in sap, such as glutamate, into ones they are missing. The bacteria, in turn, benefit from other nutrients and shelter provided by the aphid.