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Sunday, March 3, 2024

The Epic Saga Of How A Single two-inch Fish Saved A Population

New research explores how the Gulf Killifish adapted to deadly pollution in the Houston Ship Channel

In the 1970s, toxic chemicals in the Houston ship channel off the coast of Texas were causing catastrophic heart problems in a population of Gulf Killifish, leading to population shrinkage. Yet today, 50 years later, Gulf Killifish are thriving in the area. The fish have adapted a resistance to the pollutants. It seemed like a classic example of natural selection: a random mutation had given some killifish the ability to survive the toxic chemicals, and the advantaged killifish passed down their genes, while disadvantaged fish died off before they could procreate. However, when a group of scientists from UC Davis and Baylor investigated the origins of the advantageous mutation in a study published in Science on May 3rd, the story became a whole lot more interesting.

Gulf Killifish are not the only species of Killifish to have adapted to increased pollution. Several populations of Atlantic Killifish on the East Coast, hundreds of miles away from Gulf Killifish, have adapted to and currently thrive in heavily polluted water. When Andrew Whitehead, a professor of Environmental Toxicology at UC Davis, found out about these resilient populations of Atlantic Killifish, he was immediately intrigued. How had these fish adapted so quickly to the conditions that were killing them?

So Whitehead and his team painstakingly sequenced the full genomes of almost 400 fish. They compared the DNA of the adapted fish population who lived in polluted water to the genome of nearby populations who had not developed the pollution resistance. The found that in resistant Killifish, a deletion mutation had disabled a pathway that was previously being activated by the toxic chemicals in Killifish embryo’s and causing serious developmental problems.

“In resistant fish the signaling pathway is essentially desensitized,” Whitehead said. “It’s kind of broken, it doesn’t get turned on by these chemicals during development.”

The key genetic mutation was also identified in a small percentage of the non-resistant fish populations. To researchers, this indicated that the mutation had come from the local gene pool and, due to its highly advantageous nature, had spread through populations of Killifish exposed to the deadly polluted waters.

When the researchers turned their attention to a population of Gulf Killifish in the Houston Ship Channel that had also miraculously adapted to damaging pollutants, they expected to uncover a similar story. After Whitehead and his lab group, this time led by Ph.D. candidate Elias M. Oziolor, sequenced the genomes of Gulf Killifish off the coast of Texas, they found that resistant Houston Ship Channel Gulf Killifish had a mutation in the genes which controlled the same signalling pathway as the resistant Atlantic Killifish hundreds of miles away.

“We were able to confirm that Gulf killifish had indeed adapted to pollutants in the Houston Ship Channel, and that it was through a similar genetic mechanism as observed in Atlantic killifish, a recalcitrant aryl hydrocarbon receptor pathway,” said Cole Matson, a professor of Environmental Science at Baylor University and a co-author on the study.

The story of the resistant Gulf Killifish and the resistant Atlantic Killifish seemed similar. However there was a problem. The Gulf Killifish mutation did not come from local genetic variation.

If local genetic variation provided the mutation, that would mean that the mutation already occured in a small percentage of the fish, and then spread through the whole population when it provided a selective advantage. If that was the case, one would expect to find the same mutation occurring in a low percentage of populations of nearby non-resistant fish. Yet when the research team sequenced the genomes of neighboring non-resistant Gulf Killifish they could not find the mutation at any percentage. Where had the genetic variation that allowed the Houston Ship Channel Killifish to survive come from?

To answer that question the scientists dug even deeper into the genome. They knew that when a mutation provides selective advantage, DNA located nearby is passed down along with the mutation. This hitchhiking DNA is a sort of genetic calling card. The researchers examined the “calling card” in the Gulf Killifish and found that it was genetically identical to DNA from the Atlantic Killifish. The genetic variation that allowed the Houston Ship Channel Killifish to survive had come from a different species hundreds of miles away.

“About forty years ago, some Atlantic Killifish which live on the Atlantic coast were brought into the Houston Ship Channel, likely by accident,” said Elias Oziolooor. “They successfully mated with Gulf Killifish and they conferred genes which allowed for these fish to adapt and persist in this environment.”

This biological story is incredible because killifish are not migratory.

“You catch a killifish and it was probably born within 100 meters of where you caught it,” Whitehead said.

Yet, somehow, in a ship, or in the styrofoam cooler of a fisherman, an Atlantic Killifish traveled hundreds of miles and successfully mated with a different species.

“We’ve discovered something that is really unique and lucky,” Whitehead said. “But we can’t all be Killifish”

Moreover, genetic variation, the key ingredient in this survival story, is at risk. Across the globe, biodiversity is rapidly declining due to human destruction.

Yet, the story also demonstrates the great power of natural selection. Life on earth will outlast human pollution and destruction. The open question is how life on earth will be changed and whether humans will be a part of that future.

Written by:  Peter Smith—science@theaggie.com


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