Research Brief: Jewelweed, Gene Flow, and Fragmented Landscapes

Rachel Toczydlowski planting more than 2,000 jewelweed seedlings in the Arboretum plot, April 2015.

Rachel Toczydlowski planting more than 2,000 jewelweed seedlings in the Arboretum plot, April 2015.

As habitats become increasingly fragmented by agriculture and development, it is critical to understand if disparate populations can remain genetically connected and diverse—or if they succumb to inbreeding. Rachel Toczydlowski, a PhD student in Professor Don Waller’s lab, is using native orange jewelweed (Impatiens capensis) to study gene flow in temperate riverine systems—that is, how genes migrate between plant populations, and how the landscape facilitates or impedes genetic connectivity.

Jewelweed lives in wet nutrient-rich soil along waterways and in moist forests. Rachel’s study sites are in south-central Wisconsin floodplain forests, which tend to be small and fragmented, often left in a natural state because they are so wet. Rachel says “I was really struck by the contrast between the interior of these floodplain forest patches and the surrounding agricultural landscape; they were unlike any other natural place I had previously visited or worked. I would pull off the road next to a forest patch, surrounded by agricultural fields, walk 5–10 feet off the road, and find myself inside a totally different world.”

Although the forest patches are small and isolated, rivers flow through and connect them. Rachel wants to understand whether the rivers work to connect these separate jewelweed populations genetically, moving seeds from one to another, for instance during spring floods. She explains: “As suitable habitats become smaller and less connected to each other, populations often suffer from inbreeding, which reduces fitness and can cause local extinctions. If populations are able to exchange genes somehow, inbreeding can often be alleviated and small populations can persist.” In addition, she is asking whether populations are uniquely adapted to their local environments.

During fall 2014, Rachel measured traits on more than 1,000 plants in 12 forest sites, documenting morphological differences among the populations. She also collected leaf tissue for genetic analysis and gathered 7,200 seeds—a complicated task because jewelweed seed capsules explode when touched.

Rachel counting jewelweed seed capsules in the Arboretum plot.

To test whether the plants’ site-specific morphological differences reflect inherent genetic differences or simply variable growing conditions, she germinated the collected seeds and planted them in a plot at the Arboretum in spring 2015. Rachel tracked plant height, seed production, and stem diameter every two weeks during the growing season. She found clear differences among plants from one population to the next, even though they all grew together in the same environment, which demonstrates that the jewelweed populations differ genetically. Rachel will next use the genetic data to understand gene flow patterns and see if it tells the same population differentiation story as the morphological data.

“Each population could harbor unique adaptations,” she notes, and maintaining gene flow may be necessary to sustain the genetic variation that evolution depends on. She is also concerned how patterns of gene flow may change “if climate change is altering patterns of water flow and flooding. We need to understand how landscape and changing climate affect gene flow and the ability of this and other species to adapt.”

Rachel’s work is supported by an NSF Graduate Research Fellowship, an NSF Dimensions of Biodiversity grant on which Don Waller is a co–principal investigator, and Botany Department funding through the Flora Aeterna Research Award, Demeter Research Award, and Davis Field Travel Award. She is pursuing a career in ecological and evolutionary botany research and outreach.