Kansas State University

Researchers finds toxin-adapted fish pass down epigenetic mutations to freshwater offspring

Submitted by Division of Biology

You can take a fish out of toxic water, but its epigenetic mutations will remain for at least two generations.

A team of researchers from Washington State University and Kansas State University analyzed the epigenetics — molecular factors that determine whether genes are turned on or off — of a group of live-bearing fish, the Atlantic molly, that lives in springs naturally high in hydrogen sulfide, which is toxic to most organisms.

The researchers removed fish from the toxic water and bred them in normal freshwater. They found that the grandchildren of the sulfide-adapted fish had more epigenetic marks in common with their wild, toxic-water-living grandparents than other Atlantic mollies that had always lived in freshwater.

"After two generations in laboratory conditions, the fish generally retained the same epigenetic marks, which was really unexpected," said Joanna Kelley, WSU associate professor of evolutionary genomics and a corresponding author on the study published in Proceedings of the National Academy of Sciences. "In an evolutionary context, the study shows that these epigenetic marks are fairly stable."

Hydrogen sulfide is found in nature and as a by-product generated by many human activities, such as paper manufacturing, sewage treatment and gas exploration. For most animals, including humans, it is highly toxic at relatively low concentrations and fatal at high levels. Yet some populations of Atlantic molly have adapted to living in springs with high levels of hydrogen sulfide, while other groups of the same species have remained in freshwater. These fish present a natural experiment to help address questions of how evolutionary adaptations occur.

"For the better part of the past decade, we have worked toward identifying changes in the DNA sequence of sulfide spring fish to learn exactly how they manage to survive in the toxic conditions," said Michi Tobler, professor of biology at K-State. "But it turns out, the answer to the puzzle may not only be in how the DNA has changed, but how products from different genes are used."

The researchers raised fish captured in sulfidic and non-sulfidic habitats in freshwater conditions in the laboratory. When the fish produced two generations of offspring, they measured their epigenetic similarities, specifically regions of DNA methylation, a type of chemical modification that can turn a gene on or off without changing the DNA sequence string itself. They found that the grand-offspring of the sulfidic fish had an 80% overlap in DNA methylation regions with the grandparent fish — even though they had been raised in freshwater. This compares to only 20% overlap with the non-sulfidic fish that had always lived in freshwater.

"A big challenge when we try to understand how organisms evolve the most intricate adaptations to the peculiar environments they find themselves in is to disentangle the complexity of life," Tobler said. "Organisms, as we can observe them, are the product of the interactions of thousands of genes and their environment, and finding the genes that matter can be really tricky. The fact that changes in organisms may not be a mere consequence of changes in the DNA sequence adds another layer of complexity… or perhaps it is the key to the puzzle we have been looking for all along."