Why killifish thrives in polluted environments

(scienceinfo.net) - This article addresses a puzzle about evolution: Why can Atlantic killifish thrive in heavily polluted water?

For four decades, waste from neighboring factories has flowed into the waters of New Bedford Harbor - an 18,000-acre area and a bustling seaport. The harbor is contaminated with polychlorinated biphenyls (PCBs) and heavy metals, which is one of the largest cleanup locations of the Environmental Protection Agency (EPA) Superfund program. Superfund is an environmental program founded in 1980 to treat abandoned areas of toxic chemicals.

The New Bedford Harbor seaport is also the location of an evolutionary puzzle that researchers at their colleagues' Hole Oceanographic Institution (WHOI) are seeking to answer.

Atlantic killifish - estuarine fish that are about 3 inches long - not only endure the toxic environment in the harbor, they seem to thrive there. How can these fish adapt and live in such a heavily polluted environment? In a new paper published in Evolutionary Biology, the researchers published the discovery of changes in a receptor protein , called aryl hydrocarbon receptor 2 (AHR2), that could explain how the killifish in New Bedford Harbor has developed genetic resistance to PCBs chemicals.

Killifish is a predatory fish that does not migrate. They live their entire lives in the same area, usually within a few hundred meters around where they have hatched. Unlike other fish that can infrequently swim in and out of ports during the summer months to feed, killifish live in its area throughout the year and winter burrows take refuge. in contaminated sediments.

Usually when fish are exposed to harmful chemicals, the body promotes the production of enzymes that destroy pollutants, a process controlled by the AHR2 protein . Some PCBs chemicals are not destroyed in this way, and stimulation continues to control the AHR2 to disrupt cellular functions, leading to poisoning. In the New Bedford Harbor killifish, the AHR2 system becomes durable for this impact.

"Killifish must manage to shut down the entrance ," said Mark Hahn, a biologist at the WHOI research institute and co-author of the paper. 'This is an example of how some pollutants can be adapted to change in their environment'.

The research team is composed of colleagues from the Atlantic Ecology Department of the US Environmental Protection Agency, Boston Public Health School, and Northern Carolia University in Charloote, using an approach ' The gene instructs' sequencing the protein coding part of three antibody genes (AHR1, AHR2, AHRR) in fish samples collected from this port area and at 6 other locations, both in the clean area and the The area has been contaminated, located along the northeast coast.

To search for single nucleotide polymorphisms (SNPs) or sub-variants in the DNA sequence, they found differences in AHR2, which play an important role in mediating Detoxification in early life.

"The function of this receptor is to mediate the effects of toxic substances ," said Sibel Karchner, a co-author and biologist at Hahn's lab. 'If you do not have a functional receptor, you will not encounter toxic effects as much as the fish encountered.'

AHR2 in killifish has 951 amino acids and 9 of them differ between individuals. Different combinations of amino acid variants lead to 26 different proteins.

'We find that variations in the New Bedford Harbor killifish are much different than those in the surrounding areas, which are unexpected in normal circumstances , ' Hahn said. There are a few protein variants that are common in New Bedford Harbor killifish, but are not common elsewhere. Similarly, the most common protein variants in comparison areas are less common in New Bedford Harbor 'killifish.

Another article was presented in BMC Evolutionary Biology by colleagues at the EPA lab in Narragansett, RI, using a 'scan gene directive' approach - studying SNPs from 42 genes linked to The entrance of AHR - has also identified AHR2 as a gene that appears under selection and may participate in the fight against toxins. Research results show that the evolution of resistance in the independent populations of killifish converges on the same target gene.

'The results of these studies and the genetic methods developed in the process of implementing these studies are helping to analyze in detail how evolution occurs at a current stage (rather than with geology) and why some species, seem to adapt to a changing world more quickly, ' said Diane Nacci, a biologist at EPA and co-author of both articles. .

AHR2 is also the same gene that was identified in an article published in Science magazine in 2011, by WHOI biologists and colleagues from New York University and NOAA about PCB resistant cod from the Hudson River. . The AHR2 protein in Hudson River cod is missing two of the 1,104 amino acids commonly found in this protein.

"Although the specific molecular changes found in antibodies to cod and killifish PCBs are different, in both AHR2 species it seems to be one of the genes - possibly an important gene - that is responsible. for the fight against toxins , " Hahn said.

While the killifish itself is immune to the toxic effects of PCBs, they can still transfer contaminants into the food chain. Killifish is an important food source for bluefish, striped bass and other fish that are eaten by humans.

Despite their healthy appearance, there may be unknown negative damage to killifish related to PCBs resistance. Researchers will continue to consider how this adaptation affects how the killifish is capable of responding to other stressful good kernels in their habitats, for example. as low oxygen concentration.

"Obviously, the fact that these fish are resistant to PCBs allows them to survive in this truly polluted environment, but what will happen once this port is cleaned? Could there be loss to make it it's also suitable for fish that live there , 'Hahn said.

"It's an interesting example of how human activities can impact evolution," he added. ' The ability to adapt to changing conditions is becoming more and more important when people affect the environment, whether from acidification of ocean or rising temperatures or other global changes. takes place'.

In addition to Hahn and Karchner, WHOI researchers participating in this study include author Adam Reitzel (currently an assistant professor at the University of North Carolina at Charlotte) and Diana Franks. This research is supported by the National Institute of Environmental Health Sciences (National Insitute of Environmental Health Sciences), through the Superfund Research Program at Boston University, the Hudson River Foundation, and a grant. granted from the National Science Foundation.