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Selection pressure induces changes in the genetic composition of a population. It works like this: if insects inflict too much damage on plants, the plants can’t reproduce as successfully, setting up a situation in which any plants that, by chance, have inherited insect-deterring traits are at an advantage.

Because of that advantage, such traits are likely to spread through the population, urged on by “pressure” from the insects.

Researchers investigated whether different genetic “families” of the common milkweed from a single population in Northern Michigan would respond differently to increasing carbon dioxide levels in the atmosphere and if so, how those responses might affect the plants’ chances of being eaten by insects.

Milkweed is one of many plants that produce toxic or bitter chemical compounds to protect themselves from being eaten by insects. These chemical defenses are the result of a long history of interactions between the plants and insects such as monarch caterpillars that feed on them.

“Specifically, we examined the response of milkweed plants to elevated carbon dioxide in terms of plant growth, asexual reproduction, and the production of chemical and physical defenses,” says Rachel Vannette, a graduate student in Mark Hunter’s research group. Hunter is a professor of ecology and evolutionary biology at the University of Michigan.

Details of the research are published in Global Change Biology.

Although all plants grew larger in response to elevated carbon dioxide, and all plant families showed similar growth and reproductive responses, plant families responded differently to elevated carbon dioxide in their production of chemical and physical defenses against plant-eating insects.

In particular, their production of heart poisons called cardenolides differed. While some plant families responded to elevated carbon dioxide by increasing cardenolide production, most decreased production—by as much as 50 percent.

“That’s a big difference if you’re a caterpillar,” says Vannette. Because the insects that consume milkweed, including monarch caterpillars, choose their host plants carefully and select specific plants based on the plants’ concentration of toxic compounds, the specialist insects act as agents of selection on milkweed plants.

Countering the shift away from chemical defenses was a shift toward physical defenses and resistance.

“The plants had tougher leaves, and they were better at tolerating herbivory by caterpillars—they grew back faster,” Vannette says.

A common milkweed (Asclepias syriaca) in flower. (Credit: Rachel Vannette)

Taken together, the results provide evidence that in response to elevated carbon dioxide, genetically-based differences in plant defense mechanisms and the changing plant-insect interactions that result may influence how plants adapt to changing climate.

Will the plants’ changing defense strategies help or hinder monarchs? “We don’t know yet,” Vannette says, “but that’s a question we’re investigating.”

The research was funded by the National Science Foundation.

More news from University of Michigan: www.ns.umich.edu/