These little fish adapted to survive lethal toxins

A five-day old Gulf killifish embryo from the Vince Bayou pollution-tolerant population. (Cole Matson/Haley Davis)

A big population, good genes, and a little luck help explain how a species of fish adapted to what would normally be lethal levels of toxins, say researchers.

They looked to the exceptional survivor story of the minnow-like Gulf killifish to learn more about what other species may need to adapt to drastically changed environments. This species is an important part of the food web for a number of larger fish species in coastal marsh habitats that hangs out in the Houston Ship Channel.

“Most species don’t survive radically altered environments,” says corresponding author Andrew Whitehead, a professor of environmental toxicology at the University of California, Davis. “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.”

two fish embryos side by side on white
Embryos from resistant (left) and sensitive (right) populations of Gulf killifish dosed at the same concentration of industrial contaminants. Resistant population embryo develops a normal, two chambered, heart with proper blood flow, while sensitive embryo develops a string heart with no blood flow. Right embryo is unlikely to survive to hatch. (Credit: Elias Oliozor/UC Davis)

Invasive, but helpful

As reported in Science, researchers sequenced the genomes of hundreds of Gulf killifish living across a spectrum of toxicity—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 the Gulf Coast species came from an Atlantic Coast species of killifish, also known to rapidly evolve high levels of pollution resistance. But Atlantic Coast killifish live at least 1,500 miles from their Houston brethren. That led researchers to think their transport to the Gulf was likely an accident initiated by humans.

Nonnative species can wreak environmental havoc on native species and habitats. But in this case, the Atlantic species’ 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,” says co-corresponding author Cole Matson, an associate professor at Baylor University.

Not all species are so lucky

Gulf killifish began with many advantages other species don’t 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, that long-distant relative—the Atlantic Coast killifish—came to visit, could successfully mate, and injected the Gulf species with genetic resources that helped it develop resilience and resistance to toxins.

“The adaptation of these killifish is a cautionary tale. 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.”

Humans are not only radically changing the environment, we are also fragmenting it, making it harder for animals to move throughout their range, Whitehead says. 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.

Additional coauthors are from the University of Connecticut and Indiana University. A C. Gus Glasscock Endowed Research Fellowship, Baylor University, the Exxon-Valdez Oil Spill Trustees Council, the National Science Foundation, the National Institutes of Environmental Health Sciences, and Indiana University funded the work.

Source: UC Davis