Rapid evolution and the resilient Atlantic killifish

Rapid evolution and the resilient Atlantic killifish URI student studying how one species of fish has evolved to survive polluted waters

Survival of the fittest, a phrase by Charles Darwin, the English naturalist whose theory of evolution has helped scientists to identify changes in different species over many generations. As species evolve in a more polluted world, marine investigators are discovering that certain species of fish thrive in lethally polluted and toxic waters.

Kamila Guerra, a biological science major, is closely working with this resilient fish species under Dr. Jeffrey Markert, a researcher at Providence College.

Fundulus heteroclitus, better known as the Atlantic killifish, can be found in both clean and polluted marshes across the East Coast of the U.S. Since the 1970s, sediments in New Bedford Harbor have contained increased levels of polychlorinated biphenyls (PCBs) that are 10,000 times greater than limits set by the U.S. Environmental Protection Agency (EPA). PCBs can be released into the environment from various activities, such as improper disposal and storage of old electrical equipment. This is an important fact given that PCBs are linked with a host of health problems in both humans and animals, except for these Atlantic killifish.

Since the early 2000s, the EPA has set the stage by asking specific research questions related to the evolution of these fish. Kamila is contributing to the EPA’s discovery by focusing on the evolutionary population genetics of the killifish. “We are studying their DNA and want to know what is behind their genetics that is responsible for making them resist and flourish in polluted environments,” says Kamila, a 2021 SURF student.

2021 SURF student Kamila Guerra retrieves traps from the salt marsh at the public boat launch in Wesport, Mass. Image by Ciara French

Kamila has done both field and lab work, first gathering samples from two sites: the public boat launch in Westport, MA and Little Compton Town Beach, RI. Killifish form habitats in salt marshes and near vegetation to breed and feed on other organisms such as insect larvae, worms and phytoplankton. She places cages in various sites of the salt marshes, one for the polluted water and one for clean water. Once the fish are collected, Dr. Markert assists Kamila with preparing an anesthesia solution to then cut a small amount of their dorsal fin, which grows back quickly.

Kamila has gone through an extensive training program in order to treat the fish safely.

“We want to treat all animals appropriately," says Dr. Markert. "These guys remove a little bit of their tail fin and they grow back and wake up happy."

In the lab, most of Kamila’s days consist of converting fish fin samples to strands of DNA, a process that begins by turning the fins into a liquid with the help of a buffer solution, an aqueous solution that maintains the pH of the sample at a constant value. A polymerase chain reaction (PCR) reaction then synthesizes new strands of DNA resulting in a much more concentrated sample.

From there, Kamila separates DNA strands through electrophoresis, a technique through which electricity is sent through a gel containing the killifish DNA. The gel is then converted into a liquid where Kamila uses a nanophotometer to determine the amount of DNA in each gel. Then the sample is sent to a gene sequencing facility.

Alleles are pairs of genes on a chromosome that determine hereditary characteristics such as hair color or blood type. In particular, Kamila and Dr. Markert are closely studying the Aryl Hydrocarbon Receptor (AHR) protein gene of the killifish.

“It's in [the Atlantic killifish’s] liver and lets them detoxify different toxins. The question is what version of the AHR do these fish have and how common is that in a clean site versus a more polluted site?” says Dr. Markert. “Now the big question is how are these types of species of fish doing this genetically? Is this a genetic variant unique to the very polluted New Bedford Harbor or is it something that’s common in all these populations?”

Dr. Markert’s EPA colleagues have conducted breeding experiments to show that fish from polluted environments and their offsprings survive in polluted waters even if they were raised in clean water, demonstrating a genetic adaptation.

The question remains, though: how are these various populations connected?

Kamila faced challenges as a student new to the lab equipment required for her research. “There were a lot of things I didn’t know how to do, especially with COVID since we weren’t in person last year,” says Kamila. “We did a lot of different things that I would not have learned if it wasn’t for Dr. Markert - pipettes, DNA extraction, PCR.”

Moving forward, Kamila and Dr. Markert will do DNA “ fingerprinting” that will allow them to figure out how well each site is connected to another and if there are migration barriers for these fish.

“Do genes from this site migrate all the way to New Bedford and so on or are there real hard barriers, so if you’re in a certain bay, you or your offspring will never mate with a fish 20 or 30 miles away,” says Dr. Markert. “That becomes important because it tells you the population size. Population size determines how likely you are to have genes that tolerate things. So it could be likely that the whole East Coast is one ginormous population and if somebody here has alleles that help you survive in a novel pollutant, well theoretically those alleles can be shared up and down the coast.”

“We will want to know if [evolutionary change] happens with other types of fish," Kamila says. "We want to know more about their genetics so that we can understand their resistance to PCBs (pollutants) and contamination in general."

Beyond SURF, Kamila has plans to attend medical school with hopes of becoming a pediatrician or obstetrician.

“It depends," she says. "I want something with kids because I love kids. I think they're the happiness of the world. They're funny, spontaneous, just unique. I just love kids and making sure their healthy would be my main concern."

This story was written by Ciara French, a 2021 Summer Undergraduate Research Fellow (SURF) for the Rhode Island Consortium for Coastal Ecology Assessment, Innovation, and Modeling. She is a rising senior at the University of Rhode Island majoring in biomedical engineering.