How An Invasive Species Helped A Threatened Fish Survive Polluted Waters

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Atlantic killifish, the species that contributed key adaptive genetic variation that enabled evolutionary rescue from toxic pollution in the Gulf killifish.

Andrew Whitehead/UC Davis

The Gulf of Mexico is a heavily polluted body of water – in addition to the notorious Deepwater Horizon oil spill that sreleased 210 million gallons of crude oil, it is also where runoff from agricultural and industrial activity is often deposited. While you might expect that this would make the Gulf of Mexico mostly uninhabitable, it actually supports a wide diversity of life. Not only does it serve as an important way-station for migrating seabirds and whales, but it also harbors coral reefs, oysters (both natural and farmed), invertebrates, and fish. Among them is the Gulf Killifish, a tiny minnow-like fish that scientists recently discovered is capable of tolerating chemicals that normally cause cardiac issues courtesy of some DNA from the closely-related Atlantic Killifish.*

“There are all sorts of ways pollutants might affect coastal animals, ” says Irvin Huang, a PhD candidate studying ecotoxicology at Stony Brook University’s School of Marine and Atmospheric Sciences who was not involved in this research, “[They] rarely occur alone, but rather are found

mixed with other pollutants, such as heavy metals, excess nutrients, endocrine disruptors, pharmaceuticals, etc. Environmental scientists are still struggling to understand how these complex mixtures interact and create a toxic effect.”

The Gulf Killifish live in the Houston Ship Channel, which is in close proximity to the northwest coast of the Gulf of Mexico. From several decades of industrial activity, the Channel is contaminated with two kinds of aromatic hydrocarbons (“polycyclic” and “halogenated”), which can cause the hearts of developing animals to become deformed. Yet, Gulf Killifish appear to be resistant to these lethal chemicals. To understand why this was the case, the scientists collected killifish from 12 sites that consisted of areas with virtually no pollution, moderate amounts of these chemicals, and high quantities of the contaminants. The scientists then allowed the killifish to spawn and examined the genomes of the resulting embryos.

Embryos from resistant (left) and sensitive (right) populations ofGulf 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.

Elias Oziolor/UC Davis

When examining the portion of the genome related to how the killifish embryos’ developing hearts respond to the toxic chemicals, the scientists found that the genes were most similar to that of an invasive species – the Gulf Killifish’s Atlantic cousins. The Atlantic Killifishes (sometimes called ‘mummichogs’) are also known for their ability to tolerate heavily polluted east coast rivers. The Atlantic Killifish, whose home range is nearly 1,500 miles away from the Houston Ship Channel, were likely brought there by shipping activities.

“Killifish being able to adapt to pollution gradients gives researchers some sort of predictive power in determining how other similar species may fair in similar conditions, ” says Frederick Nelson, a PhD student who studies how organisms are coping in rapidly changing environments at the University of California, Davis and was not part of the study, “It [gives] researchers a framework for understanding which species are likely to cope in times of rapid environmental change and for how long.”

Specifically, it seems that the Atlantic Killifishes’ genes entered the Gulf Killifishes’ after hybrids of the two species continued to reproduce with non-hybridized members of the Atlantic and Gulf Killifishes’ populations over the course of multiple generations. This process, known as adaptive introgression, is what has allowed the Gulf Killifish to survive in these highly polluted environments.

“This research gives us insights into at least one way the killifish have been so adaptable, which is essentially like Batman showing us one of the tools hidden in his utility belt, ” says Huang, “Now we know what is possible with these fish and their genomes in response to pollution stress, we have a possible lead on where to look for other stressors like climate change or ocean acidification.”

*An audio/visual narration of the manuscript and figures by co-author Dr. Andrew Whitehead is available here.

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