Why this fish adapts quickly to lethal levels of pollution

Atlantic killifish have adapted to highly toxic levels of pollution.
Andrew Whitehead | UC Davis

Evolution is allowing a species of fish to survive in lethally polluted waters, and it could provide clues as to how genetic differences affect sensitivity to toxins in animals.

But researchers are saying the species' success may be a cautionary tale for many other animal species, including humans.

The Atlantic killifish is found in tidal marshes along the East Coast of the United States, many of which have been polluted with industrial chemicals.

Killifish are usually sensitive to chemicals and toxins, but the fish populations living in these have a secret weapon that has helped them grow tolerant to chemicals that would otherwise kill them: an extremely high level of genetic diversity, rarely seen in vertebrates, but often seen in other living things such as insects.

In fact, sheer genetic diversity is what makes it so challenging to control pests, such as insects or weeds.

This trait leaves the killifish up to 8,000 times more resistant to this level of pollution than other fish, according to a study published Thursday in the journal Science.

But there are so many different genes floating around in a population of killifish, the animal likely has the "cards in hand" needed to rapidly adapt to abrupt environmental changes, said said Andrew Whitehead, an associate professor of environmental toxicology at the University of California, Davis, and one of the study's authors.

"It is kind of a tragic irony that those species that we intend to kill with chemicals, such as insects, and weeds and viruses, are the species that tend to adapt, and the species we don't intend to kill with chemicals, like fish, birds, amphibians, and mammals, are the ones that tend to suffer. The killifish is an exception to that because it has this insect-like genetic diversity," he said.

In the study, the researchers from a number of institutions in the U.S. and U.K. sequenced genomes of nearly 400 Atlantic killifish pulled out of New Bedford Harbor in Massachusetts; Newark Bay, New Jersey; Connecticut's Bridgeport area; and Virginia's Elizabeth River — all of which have areas that have been polluted since the 1950s and 1960s by a complex mixture of industrial pollutants including dioxins, heavy metals, hydrocarbons and other chemicals.

The study was funded by the National Science Foundation's Division of Environmental Biology, and the National Institute of Environmental Health Sciences.

The team identified dozens of genes that were involved in making fish more resistant to chemicals, and a few key mutations that appeared to be important to the fish's resilience to pollution in every population.

This fact that all of the fish had a few key changes in common is striking since the waters these fish swim in are not polluted with only a few chemicals, but complex mixtures of toxins.

Whitehead said the research may help identify which genes regulate sensitivity to chemicals in humans, and help understand how humans might react to different pollutants.

First, these mutations have been tested in a wild animal in its natural habitat — unlike, say, mutations induced in lab animals. Second, fish have attributes that are in many ways similar to humans — the way fish hearts work, for example, is remarkably similar to how a human heart works, Whitehead said. Finally, chemical sensitivity in humans can be hard to study, since there are a variety of factors — such as diet and lifestyle — that can influence disease in people. The fish in their natural habitat have fewer of these confounding factors, making it easier to pick out what matters.

But the study also suggests that other animal species will not be so lucky as the killifish in adapting to rapid environmental changes, such as climate change or environmental pollution, Whitehead noted.

"People think 'oh that's great, we have these species that are evolving their way out of our environmental mess-ups,'" he said. "I think this is kind of a cautionary tale. Most of us don't carry those cards in our hands. That is going to be a problem for a lot of species, they are challenged to respond to these rapid environmental changes. It is going to be hard for many species we care about to keep up."

Whitehead is also skeptical of the idea of creating genetically engineered species that have this resistance.

"I have had a couple of people say 'gee this is great, if we know the mutations, maybe we can genetically engineer other species to have genetic tolerance,' and I think that is highly unlikely," he said.

This is in part because the full range of genetic changes that help these animals survive could be hard to reverse engineer in a lab. "In killifish, we found a number of key mutations that are important, but there are a whole bunch of other mutations that likely help resolve some of the side effects," of these key mutations, Whitehead said. "So the full set of genetic tweaks is probably highly complex and difficult to discover."

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