Hidden within delicate blue petals and a green slender stalk, the American bellflower offers more than just aesthetic value. Biologists at the University of Virginia and Washington State University published a joint study Aug. 29 focusing on the effects of historic climate change and range expansion on the development of the American bellflower — Campanula americana. With climate change occurring on a wide scale, researchers used the American bellflower as a model to provide insight on plant and animal reactions to historic and current climate change and to predict responses to future climate change.
“With contemporary climate change, there have been lots of predictions about how organisms will respond to those changes,” said Matthew Koski, a former postdoctoral researcher at the University, an author of the study and now an assistant professor of biology at Clemson University. “They could adapt to their conditions, they could migrate or they could unfortunately go extinct. We are using historical climate change to think about how populations would respond today.”
The American bellflower is a common flower in the United States, spanning the entire length of the country from the Great Lakes to the Gulf Coast and from the East Coast to the Midwest. Despite its name, the bellflower’s flat petals extend outward and curl slightly at the edges. Laura Galloway, Commonwealth Professor of Biology and one of the authors of the article, has studied the American bellflower for some time.
“I’ve worked on the American bellflower for many years, and it’s one of these things as I keep working on it, I uncover the answers to some questions, and then new questions pop up,” Galloway said. “It’s a wonderful plant to work with in that respect. It’s always giving me questions to work on.”
Her latest research is the result of one of these questions. Several years ago, while observing samples from diverse American bellflower populations in a greenhouse at the University, Galloway observed that some plants produced fruit without the aid of pollinators. Wondering why, Galloway’s lab explored this discrepancy and noticed a pattern — in the northern limits of the bellflower’s range, self-pollination was a common method of reproduction.
A hypothesis sprouted from this analysis — perhaps the pattern stemmed from the American bellflower’s past. The species experienced significant changes in its environment approximately 15,000 years ago. At this time, glaciers covered much of North America, confining multiple species of plants and animals to restricted havens that supported life. However, as the glaciers melted and began to retreat, these species, including American bellflowers, traveled northward.
“I was trying to figure out why would it be that plants in the northwestern part of the distribution [self-pollinate] and the plants in the southeastern part don’t,” Galloway said. “I started to think that maybe it had to do with the increase in its geographic range after the last glaciation.”
In order to pinpoint the initial habitat of the flowers during this time period, Galloway joined with researchers from Washington State University, who sampled individuals from 24 populations within the bellflower’s range and sequenced their genomes. According to characteristic mutations in different populations, they determined that the plants congregated in what is known as a glacial refugium in the Appalachian Mountains in present-day eastern Kentucky. Not only that, but the progression of genetic mutations served as a reflection of the American bellflower’s geographic dissemination. Distinct genetic patterns indicated small populations migrated from the refugium and spread to the exposed terrain the melting glaciers left in their wake.
As these small populations capitalized on available land, they faced different selective pressures or environmental conditions that favored certain traits. Galloway and Koski investigated these adaptations in the same 24 bellflower populations in their greenhouse at the University. Ultimately, they confirmed Galloway’s previous discovery that populations further from the historic epicenter of the bellflower had an increased tendency to self — individuals were more likely to undergo self-fertilization in order to produce offspring rather than rely on pollinators.
However, bellflower communities at the leading edge of its geographic range did not simply gain advantages that promoted survival and reproduction. When a subset of a larger population establishes a new settlement, the amount of genetic diversity in this nascent population is invariably reduced. As a result, the relatively homogenous population has a lower chance of survival in the case of extreme changes in climate — they no longer have the luxury of a large gene pool that could potentially provide beneficial mutations in the case of habitat disruption.
“Our work tells us a lot about evolution in natural populations as they spread across the landscape and as they migrate in response to climate change,” Koski said. “We looked at the impact of historic climate change, but populations are continually spreading northward and upward in elevation due to contemporary climate change, which has a major impact on how populations reproduce.”
Findings from the study indicate that there are trade-offs to subsisting at the extremes of a species’ range. For northern offshoots of the original American bellflower habitat, self-pollination confers a selective advantage. On the other hand, these individuals also experience diminished fitness in the face of deleterious genetic mutations. Yet, the story does not end here. With climate change once again occurring on a wide scale, questions linger about whether or not plant and animal species will adapt at the rate necessary to survive.
“Historic climate change was more gradual than contemporary climate change, so it’s a good model for what could happen, but we do not really know if populations are going to be able to migrate as quickly as they need to under contemporary climate change,” Koski said.
In order to better understand how the American bellflower sustained itself during the last ice age, and if it has the capacity to weather a new wave of climate change, additional research must be done, Galloway and Koski said. For example, it has not been definitively established whether American bellflower self-pollination is the cause or effect of dispersal, a fact that may aid in predicting responses to shifting environmental conditions in other organisms.
“We can look at how plants and animals responded to the last climate change and use that to frame our expectations and our response to ongoing climate change,” Galloway said. “That’s really important because it says we have information we can use to make good predictions to test.”