An Evolutionary Rescue in Polluted Waters
How Genetics, Resources and a Long-Distant Relative Helped One Lucky Fish Species Adapt to Extreme Pollution
Most species don’t survive radically altered environments. By studying the survivors, we get insight into what it takes to be successful. In the case of the killifish, it came down to huge population sizes and luck.
– Andrew Whitehead
The combination of a big population, good genes and luck helps explain how a species of fish in Texas’ Houston Ship Channel was able to adapt to what normally would be lethal levels of toxins for most other species, according to a study to be published May 3 in the journal Science.
The exceptional survivor story of the Gulf killifish was one scientists at the University of California, Davis, Baylor University and their co-authoring colleagues wanted to unveil so they could learn more about what other species may need to adapt to drastically changed environments.
The minnow-like Gulf killifish are an important part of the food web for a number of larger fish species in coastal marsh habitats.
“Most species don’t survive radically altered environments,” said corresponding author Andrew Whitehead, a UC Davis professor of environmental toxicology. “By studying the survivors, we get insight into what it takes to be successful. In the case of the killifish, it came down to huge population sizes and luck.”
Five-day old Gulf killifish embryos, following exposure to PCB-126. The embryo on the left is
from the Smith Point reference site, whereas the embryo on the right is from the Vince Bayou pollution-tolerant population. The Vince Bayou embryo is developing normally in spite of the exposure. For the Smith Point embryo, the heart has failed to properly develop, which has dramatically reduced its growth and will ultimately prove fatal. (Cole Matson/Haley Davis)
The researchers sequenced the genomes of hundreds of Gulf killifish living across a spectrum of toxicity – from clean water, moderately polluted water and very polluted water. They were searching for the footprints of natural selection that allowed the species to rapidly transition from a fish that is highly sensitive to pollution to one extremely resistant to it.
They were surprised to find that the adaptive DNA that rescued this Gulf Coast species came from an Atlantic Coast species of killifish, which has also been known to rapidly evolve high levels of pollution resistance. But Atlantic Coast killifish live at least 1,500 miles from their Houston brethren, leaving researchers to think their transport to the Gulf was likely an accident initiated by humans.
Cole Matson throws a cast net in Galveston Bay, Texas during a Gulf killifish (Fundulus grandis) collection trip. (Leanne Baker)
Nonnative species can wreak environmental havoc on native species and habitats. But in this case, their arrival in the 1970s—right at a moment when Gulf killifish were likely beginning to decline–amounted to an “evolutionary rescue” from pollution for the Gulf killifish.
“While the vast majority of research on invasive species rightly focuses on the environmental damage they can cause, this research shows that under rare circumstances they can also contribute valuable genetic variation to a closely related native species, thus acting as a mechanism of evolutionary rescue,” said co-corresponding author Cole Matson, an associate professor at Baylor University.
A cautionary tale
Gulf killifish began with many advantages other species do not have. Species with large populations can harbor high levels of genetic diversity that can help them adapt to rapid change. Gulf killifish already had among the highest levels of genetic diversity of any species with a backbone. Then, at the moment its population was beginning to decline, a long-distant relative—the Atlantic Coast killifish—came to visit, was able to successfully mate, and injected the Gulf species with genetic resources that helped it develop resilience and resistance to toxins. Whitehead is quick to note that not all species are so lucky.
“The adaptation of these killifish is a cautionary tale,” Whitehead said. “It tells us what we need to do better for the vast majority of species that don’t have access to the kind of genetic resources killifish have. If we care about preserving biodiversity, we can’t expect evolution to be the solution. We need to reduce how much and how quickly we’re changing the environment so that species can keep up.”
UC Davis environmental toxicology Professor Andrew Whitehead carries minnow traps while collecting Gulf killifish from the Mississippi coast. (Pat Sullivan)
Humans are not only radically changing the environment, we are also fragmenting it, making it harder for animals to move throughout their range. Whitehead said a key lesson from killifish is the importance of keeping the doors to genetic diversity open. This includes connecting and preserving landscapes to allow for genetic variation to move more freely and naturally. That could help set the stage for more evolutionary “rescues” in the rapidly changing future.
“We are challenging animals by rapidly transforming their surroundings into extreme environments,” said first author Elias Oziolor, who was a postdoctoral fellow in Whitehead’s lab. “However, some, like the Gulf killifish, have been fortunate to find creative and novel ways to persist. My hope is that our work can motivate efforts in conserving our environment by better understanding and mitigating human impacts on populations.”
Additional coauthoring institutions include the University of Connecticut and Indiana University.
The work was supported by funding from a C. Gus Glasscock Endowed Research Fellowship, Baylor University, the Exxon-Valdez Oil Spill Trustees Council, National Science Foundation, National Institutes of Environmental Health Sciences and Indiana University.