West Campus: The pioneers

Christopher Capozziello

Nancy Moran's lab includes a greenhouse where researches cultivate the plants that feed pea aphids. Moran has discovered the world's tiniest single-celled creatures, which live inside the insects. View full image

Redefining “viable”
by Margot Sanger-Katz ’02

Nancy Moran is studying the organisms with the world’s smallest genomes. The bacteria she examines—many of which she discovered—are at the very edge of viable life. “We are interested in what allows them to be this small—to live without genes that are considered to be essential in other organisms,” she says.

Moran, an evolutionary biologist and past MacArthur “genius” grant winner, recently came to the Microbial Diversity Institute from the University of Arizona. She came upon the tiny genomes by studying insect biology. Several insect species, she discovered, have evolved for millions of years with symbiotic bacteria that live inside insect cells and are passed from mother to offspring.

The insects and bacteria help each other survive: the hosts give the bacteria nourishment and a protected environment, and the bacteria repay their landlords. One species Moran studies helps pea aphids make crucial amino acids their diet lacks; another makes a toxin that kills parasitic wasps that can invade the aphids’ bodies.

Because the bacteria are shielded from the outside world and live in very small populations, they have evolved at a much faster rate than most similar bacteria, often by shedding inessential genes. Moran says some of the genes the bacteria have lost are surprising, since they’re generally considered beneficial. Several species, for example, are missing common genes that help repair damaged DNA.

Moran’s work has led to insights about the ways organisms can swap and shed genes. In one recent case, she found that a species of aphid had adopted entire genes from a fungus, allowing the aphids to synthesize a beneficial chemical. But the research has also changed scientists’ understanding of what is required to support life.

The bacteria genomes she studies are not just tinier than any other known organisms’—but many times tinier. (Still smaller genomes exist, but only in viruses, which are not considered “organisms” because they’re incapable of independent metabolism and reproduction.) The smallest common bacteria genomes have well over a million base pairs; E. Coli genomes have more than 4 million. Moran’s team has discovered a bacteria genome with only 145,000 base pairs, the smallest known. This bacterium, Hodgekinia cicadicola, makes many fewer proteins and performs many fewer functions than any known species.

“When you look at that, it’s amazing that they’re functional,” she says. After years of examining similar organisms, Moran says, she’s still excited about what these outliers can teach her.