:: By Lydialyle Gibson

Investigations ::

The long war

Botanists—and biologists of every stripe—have long visualized the interaction between pathogens and their hosts as an evolutionary arms race. A disease develops a virulent mutation, and the organism’s immune system counters with a mutation of its own, which prompts another disease mutation. It goes on and on, as each matches the other, gene for gene, with retaliatory, if short-lived, variations.

Imagine Joy Bergelson’s surprise, then, when she found Arabidopsis thaliana—a white-flowering plant commonly known as mouse-ear cress—and its soft-rot nemesis Pseudomonas viridiflava engaged in a battle that looked more like trench warfare. “We thought we would see rapid generation and turnover of alleles,” says Bergelson, an ecology and evolution professor. But after five years of studying 30 different A. thaliana resistance genes, she discovered they were “harboring old variations” and that genes have been “maintained for millions of years.” Instead of a “dynamical turnover of alleles,” the interaction is an “advance and retreat” of old alleles retained within the species.

Bergelson says farming‘s practice of monoculture—growing a single crop in a single field—bolsters the idea that arms races occur in nature by showing that pathogens can evolve rapidly to avoid detection. “In monoculture, there is strong selection,” she says. Even when a mutation handicaps a pathogen in other ways, it will persist if it helps the bug escape detection by plants. “Nature is a more complex world, with alternative hosts and alternative pathogen strains.” Bergelson’s research team was one of the first to study A. thaliana outside the greenhouse. A skeletal and stalky plant that sprouts from a rosette of coarse leaves, the plant is usually “a lab rat,” she says. Its relatively short genome and rapid regeneration make it a favorite among researchers.

Her findings have raised new questions about A. thaliana: “How does selection maintain these variations? What are the costs and benefits associated with different alleles?”