‘Resurrection’ of Dormant Eggs Reveals
Unexpected Evolutionary Response to Toxins

Studying the longterm effects of pollution in three Connecticut lakes, recent F&ES graduate Mary Rogalski expected to find that animal populations had slowly adapted to the stress. But the long-dormant eggs of a tiny crustacean told a different story.
Human impacts on the environment have been shown to trigger strong and rapid evolutionary changes for some animal species, driving the animals to evolve fitness advantages that make them better able to cope with those changes.
 
But that’s not always the case.
mary rogalski lake sediment
Photo by Phoebe Zarnetske
Scuba divers in a Connecticut lake hand a coring device containing lake-bottom sediments to Mary Rogalski.
Using “resurrection ecology” and lake sediment archives, a recent graduate of the Yale School of Forestry & Environmental Studies was able to determine that a tiny freshwater crustacean, known as Daphnia, actually became more sensitive to heavy metals as concentrations increased over time.
 
In a long-term study, Mary Rogalski ’15 Ph.D. used dated lake sediment from three Connecticut lakes to quantify pollution levels of copper and cadmium over the past 50 to 75 years. She then hatched long-lived dormant Daphnia eggs found from multiple time periods and tracked their sensitivity to low and high concentrations of the metals through time.
 
To her surprise, the Daphnia exposed to increasing concentrations of the pollutants have evolved to become more sensitive to the contaminants — or, the opposite of what might have been expected.
 
“We might assume that populations exposed to pollution over long time periods will adapt to this stress,” she said. “But my results suggest that the opposite response could be happening — these animals have become more sensitive to pollution following decades of exposure.”
ephippia rogalski study
Photo by Eric Lazo-Wasem
The outer shell of long-lived Daphnia resting eggs, which were hatched as part of Rogalski's research.
The study, published in the journal The American Naturalist, was part of Rogalski’s dissertation research at Yale. She is now a postdoctoral fellow at the University of Michigan’s Department of Ecology and Evolutionary Biology.
 
Given the widespread occurrences of toxic pollution, including heavy metals, the results illustrate the importance of better understanding the drivers and implications of such maladaptation patterns, Rogalski said.
 
“If this increase in sensitivity to metals over time affects the ability of Daphnia to survive and reproduce in natural (contaminated) environments, this would mean that toxicity is increasing following long-term exposure,” Rogalski said. “When making regulatory decisions, we consider toxicity responses to be static and uniform; however, populations in different locations vary in their sensitivity to contaminants, and this study shows that sensitivity can evolve through time.
 
“This is something we should consider when we estimate the risk associated with the release of toxic chemicals into the environment.”

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– Kevin Dennehy    kevin.dennehy@yale.edu    203 436-4842
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PUBLISHED: February 22, 2017
 

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