When you pass the Grey Poupon, you're probably not thinking about nature's defense systems. But mustard's kick is not just for seasoning your sandwich, it's a plant's way of biting back. Recent research has found one reason why certain mustard plants carry these chemical compounds: in some environments a spicy taste may deter pests.
Molecular and field biologists from Duke University, the Max Planck Institute for Chemical Ecology in Germany and the University of Illinois at Urbana–Champaign have investigated why two populations of mustard plants from the same species produce distinctly different spicy flavors. They detail their findings in the August 30 issue of Science.
The researchers studied the plant species Boechera stricta, a member of the mustard family. They looked at two populations of B. stricta growing in the Rocky Mountains, one in Montana and another in Colorado. Each population tastes spicy but in a slightly different way, suggesting a regionally distinct chemical composition.
The researchers selected these populations for their pristine natural environment, habitats that have seen little change in 3,000 years. Co-author and Duke University evolutionary geneticist Thomas Mitchell-Olds explains that this kind of environment allows biologists to answer questions about a species's evolution. In this case, it allowed the scientists to both consider the genetic variation of mustard plants and consider the still extant environmental factors that pushed this variation.
The researchers first analyzed specimens from the Colorado and Montana populations in the lab. They identified the chemical compounds that produced the differing mustard plants' spicy flavor. They also discovered how the plants produced these chemicals. Through careful molecular analysis, they pinpointed a single gene, BCMA, that encodes for an enzyme's activity. This enzyme is responsible for beginning the production of the compounds that give each variety of mustard its distinctive taste. Depending on the variant of the BCMA gene, the enzymes will produce the distinct flavor of either a Montana or Colorado mustard plant.
With the chemical and genetic differences identified, Mitchell-Olds and colleagues next went into the field to see how these plants survived in their respective habitats. The researchers planted thousands of Colorado and Montana mustard plants together on field sites in each state. They returned in the spring to count the leaves of each plant and quantify the damage done by hungry insects. As expected, the Colorado and Montana plants produced their distinctive Colorado and Montana spices, regardless of location. What was more intriguing, however, was the way insect populations responded to each.
Montana insects stayed away from Montana plants but devoured the Colorado variety. Their aversion suggests that the Montana mustard's spice is specially formulated to deter local pests. It's possible therefore that many generations ago, a mutation on the BCMA gene created a family of plants with the Montana spice that so successfully deflected bugs that this gene became common in the population.
The Colorado site told a slightly different story. These bugs had a less discerning palate, consuming the local and foreign mustards with similar gusto. The researchers need to investigate further to understand this difference, but they did observe that certain related plants fared better than others, suggesting that there are other heritable defense traits that serve in the Colorado environment as well. It is also possible that the difference reveals something about the Colorado insect population, such as a high spice tolerance. It could also suggest that the environment is more competitive, and ravenous pests must stomach a spice-induced tummy ache or face starvation.
But a third experimental step adds additional nuance to mustard's BCMA variation. The researchers returned to the lab for an experiment with one of B. stricta's closest cousins, Arabidopsis, a small flowering plant. Arabidopsis is biology's best-understood bloomer—particularly on a genetic level. The scientists engineered Arabidopsis to express the BCMA genes, producing either the Colorado or Montana spice variants. Once again, they found the chemical composition of the plant's spice came from genetic variation. When the researchers exposed their spicy Arabidopsis plants to pests, they identified a possible trade-off of BCMA variation. Although the spicy compounds deter certain insects and pathogens, they can increase susceptibility to others. With further analysis, the researchers hope to better understand how this trade-off affected the regional evolution of B. stricta's flavor.
Although many questions remain, Mitchell-Olds and colleagues have presented a first step in understanding how natural selection has shaped a mustard species's variation over evolutionary time. "This paper helps set the standard for studies of adaptation in the wild," says Harvard University zoologist Hopi Hoekstra. "This research demonstrates beautifully how field experiments and molecular techniques can be integrated to tell a more complete story of adaptation than either one alone."
Jack Schultz, who studies the chemical ecology of Arabidopsis at the University of Missouri–Columbia, also praises the methods, observing the need for more studies integrating genomics with field biology. "The approach may be as significant as the result." Combining molecular and fieldwork is a relatively recent effort, but Schultz and others believe it is crucial to furthering the study of ecology.
Mitchell-Olds, for his part, remains amazed at how neatly their findings have demonstrated evolution in action. He says, "We found that environment matters, rapid evolution happens, we can localize changes down to a particular amino acid… The fact that it works, our ideas from evolutionary ecology could be verified—that's delightful."