By ecoRI News staff
NEW BEDFORD, Mass. — Evolution is working under pressure to rescue some coastal fish from a lethal, human-altered environment, and a new study has revealed the complex genetic basis for the Atlantic killifish’s remarkable resilience.
New findings, published last month in the journal Science, build on decades of research into the killifish’s ability to survive industrial contamination. The Woods Hole Oceanographic Institution (WHOI) collaborated on the new multi-institutional study, which was led by the University of California, Davis.
While environmental change is outpacing the rate of evolution for many other species, according to the recent study, killifish living in four polluted East Coast estuaries turn out to be remarkably resilient. These fish have adapted to survive levels of toxic industrial pollutants that would normally kill them, tolerating concentrations up to 8,000 times higher than sensitive fish.
Although killifish aren’t commercially valuable, they are an important food for other species and an indicator of environmental degradation.
“There’s this whole enzyme system that has evolved to protect animals against pollutants,” Hahn said. “But that only works for chemicals that are readily metabolized by those enzymes.”
Polychlorinated biphenyls (PCBs) and other industrial chemicals aren’t readily broken down, so the enzyme system and hundreds of associated genes go into overdrive, ultimately harming killifish and killing their embryos.
“The constant stimulation of this genetic response pathway is bad for them, like an overactive immune response in people,” Hahn said.
In PCB-resistant killifish, however, the response pathway stays turned off, counterintuitively allowing the fish to survive what would usually be a fatal exposure.
“These are really strong selective pressures,” Hahn said. “We’ve introduced chemicals that are going to kill the embryos. So the population will be wiped out, unless there’s a subset of embryos that can tolerate the high concentrations, and unless the genetic characteristics that help them tolerate those concentrations can be passed on.”
To better understand the genetic basis for this adaptation, the study’s research team sequenced the genomes of nearly 400 individual killifish from four non-polluted sites and from four polluted ones: New Bedford Harbor; Connecticut’s Bridgeport area; Newark Bay, N.J.; and Virginia’s Elizabeth River. Since the 1950s and ’60s, these sites have been contaminated by industrial chemicals, including PCBs, dioxins and hydrocarbons.
Genetic analysis identified hundreds of “hot spots” — regions of the genome that appeared to have undergone natural selection in pollution-resistant killifish. Several of the strongest hot spots appeared in all four resistant populations; many of those included genes involved in the previously identified, potentially hyperactive response pathway.
“So the interesting thing is, it doesn’t seem to be just one gene that appears to be responsible for the resistance,” Hahn said. “It’s a group of genes that are all part of the same pathway.”
Killifish are known to have extremely high levels of genetic variation — higher than that of any other vertebrate measured so far, including humans. The more genetic diversity, the faster evolution can act. That’s one reason why insects and weeds can quickly adapt and evolve to resist pesticides, and why pathogens can evolve quickly to resist drugs created to destroy them, according to WHOI researchers.
The new findings suggest that killifish genetic diversity makes them unusually well positioned to quickly adapt and survive in radically altered habitats.
“We have four independent, geographically separated populations of fish, each of which has evolved resistance to industrial chemicals,” Hahn said. “And in each of these four populations, selection has targeted the same sets of genes from the same molecular pathway, but in slightly different ways.”
This result suggests that before being exposed to pollution, killifish already carried the genetic variation that allowed them to adapt, and that there may only be a limited number of evolutionary solutions to pollution exposure.
The recent study lays the groundwork for future research that could explore which genes confer tolerance of specific chemicals. Such work could help better explain how genetic differences among humans and other species may contribute to differences in sensitivity to environmental contaminants, according to Hahn.
Along with U.C Davis and WHOI, the study’s co-authoring institutions include the U.S. Department of Agriculture, the Environmental Protection Agency, Washington University School of Medicine, University of Birmingham, Indiana University and University of Miami.