Fish rapidly adapt to pollution thousands of times lethal levels

Polluted waters are leading to fast adaptation

Joel Sartore, National Geographic Photo Ark/Getty

By Michael Le Page

It’s evolution in action seen in unprecedented detail. Genome sequencing of hundreds of killifish in the eastern US has reveal­­ed dozens of the evolutionary changes that allow them to survive in extremely polluted waters that would normally kill such fish.

“They can survive thousands of times the usual lethal levels,” says team member Andrew Whitehead at the University of California, Davis.

Another striking thing is that they managed to evolve this extraordinary ability in just half a century or so, since the estuaries they live in started getting polluted.

Advertisement

Although many people think evolution is a slow process, it can in fact happen extremely fast. There are thousands of examples of evolution in action, from the famous peppered moths that turned black to camouflage themselves on soot-covered trees to the ever-growing numbers of antibiotic-resistant bacteria.

In most cases of contemporary evolution, the genetic changes involved have never been identified. The mutation that turned the first few peppered moths black in about 1819 was identified only earlier this year, for instance.

With DNA sequencing getting ever cheaper, biologists in the US have now been able to sequence the genomes of nearly 400 individual Atlantic killifish (Fundulus heteroclitus), a small fish also known as the mummichog that lives in estuaries along the east coast. They compared the genomes of killifish in four highly polluted areas with those from four unspoilt sites.

“I have never seen an ecological study in which people sequenced the genomes of so many individuals,” says Isaac Wirgin at New York University School of Medicine, who studies how another fish, the tomcod, has adapted to pollution. “We really are going through a revolution.”

Many changes

The team found dozens of changes in the fish that have evolved to help them tolerate pollutants such as dioxins and PCBs. “We have a picture of the entire adaptive genetic type,” says Whitehead.

Many of these changes are in genes involved in a signalling pathway that has several roles in normal killifish, including in the immune system and for oestrogen signalling.

This so-called AHR pathway also kicks in to deal with natural toxins. But if the pathway is switched on by pollutants during early development, it causes havoc. “It really messes things up,” says Whitehead.

All the pollution-tolerant fish have several changes in AHR-related genes that stop it kicking in at the wrong time. However, evolution has not taken the same route in each population. Pollution-tolerant fish from different areas have different mutations in these genes.

In addition, there are many changes in other genes to compensate for those in the AHR pathway. Because of its multiple roles, mutations that stop it being activated have many knock-on effects.

The areas in which the killifish live started becoming polluted in the 1950s, so all these evolutionary changes arose over just dozens of generations. It could happen so quickly, says Whitehead, because there was already a lot of variation in the species. “Killifish have insect-like levels of genetic diversity,” he says.

Wirgin thinks the fact that killifish are territorial also helps them evolve fast, because it means that those from polluted areas are less likely to dilute their genes by interbreeding with those from pristine ones.

Although killifish have evolved to thrive even in polluted areas, not all animals are so adaptable. In particular, there is much less diversity in large animals with smaller populations, meaning they may not be able to evolve fast enough to cope with the abrupt changes resulting from human activities. This is why it is vital to conserve diversity, says Whitehead.

Journal reference: Science, DOI: 10.1126/science.aal3211